diff --git a/index.html b/index.html index faf3de8..9ae954c 100644 --- a/index.html +++ b/index.html @@ -85,18 +85,15 @@
  15.1 Interrupt Handlers
  15.2 Detecting button presses
  15.3 Bottom Half -
 16 Crypto -
  16.1 Hash functions -
  16.2 Symmetric key encryption -
 17 Virtual Input Device Driver -
 18 Standardizing the interfaces: The Device Model -
 19 Optimizations -
  19.1 Likely and Unlikely conditions -
  19.2 Static keys -
 20 Common Pitfalls -
  20.1 Using standard libraries -
  20.2 Disabling interrupts -
 21 Where To Go From Here? +
 16 Virtual Input Device Driver +
 17 Standardizing the interfaces: The Device Model +
 18 Optimizations +
  18.1 Likely and Unlikely conditions +
  18.2 Static keys +
 19 Common Pitfalls +
  19.1 Using standard libraries +
  19.2 Disabling interrupts +
 20 Where To Go From Here?

1 Introduction

The Linux Kernel Module Programming Guide is a free book; you may reproduce @@ -111,14 +108,14 @@ the provisions of the Open Sof distribute this book free of charge or for a profit. No explicit permission is required from the author for reproduction of this book in any medium, physical or electronic. - - -

Derivative works and translations of this document must be placed under the Open Software License, and the original copyright notice must remain intact. If you have contributed new material to this book, you must make the material and source code available for your revisions. Please make revisions and updates available directly to the document maintainer, Jim Huang <jserv@ccns.ncku.edu.tw>. This will allow + + + for the merging of updates and provide consistent revisions to the Linux community.

If you publish or distribute this book commercially, donations, royalties, and/or @@ -153,13 +150,13 @@ Scarpazza, David Porter, demonsome, Dimo Velev, Ekang Monyet, Ethan Chan, fennecJ, Francois Audeon, Gilad Reti, heartofrain, Horst Schirmeier, Hsin-Hsiang Peng, Ignacio Martin, Iûnn Kiàn-îng, Jian-Xing Wu, Johan Calle, keytouch, Kohei Otsuka, Kuan-Wei Chiu, manbing, Marconi Jiang, - - - mengxinayan, Peter Lin, Roman Lakeev, Sam Erickson, Shao-Tse Hung, Shih-Sheng Yang, Stacy Prowell, Steven Lung, Tristan Lelong, Tse-Wei Lin, Tucker Polomik, Tyler Fanelli, VxTeemo, Wei-Lun Tsai, Xatierlike Lee, Yin-Chiuan Chen, Yi-Wei Lin, Ylowy, Yu-Hsiang Tseng.

+ + +

1.3 What Is A Kernel Module?

@@ -192,11 +189,11 @@ desired.

On Arch Linux: 

1sudo pacman -S gcc kmod
+

+

-

-

1.5 What Modules are in my Kernel?

To discover what modules are already loaded within your current kernel use the command lsmod @@ -5403,314 +5400,8 @@ when an interrupt is triggered. 168MODULE_DESCRIPTION("Interrupt with top and bottom half");

-

16 Crypto

-

At the dawn of the internet, everybody trusted everybody completely…but that did -not work out so well. When this guide was originally written, it was a more innocent -era in which almost nobody actually gave a damn about crypto - least of all kernel -developers. That is certainly no longer the case now. To handle crypto stuff, the -kernel has its own API enabling common methods of encryption, decryption and your -favourite hash functions. -

-

-

16.1 Hash functions

-

Calculating and checking the hashes of things is a common operation. -Here is a demonstration of how to calculate a sha256 hash within a -kernel module. To provide the sha256 algorithm support, make sure - CONFIG_CRYPTO_SHA256 - is enabled in kernel. -

-

- 
1/* 
-2 * cryptosha256.c 
-3 */ 
-4#include <crypto/internal/hash.h> 
-5#include <linux/module.h> 
-6 
-7#define SHA256_LENGTH 32 
-8 
-9static void show_hash_result(char *plaintext, char *hash_sha256) 
-10{ 
-11    int i; 
-12    char str[SHA256_LENGTH * 2 + 1]; 
-13 
-14    pr_info("sha256 test for string: \"%s\"\n", plaintext); 
-15    for (i = 0; i < SHA256_LENGTH; i++) 
-16        sprintf(&str[i * 2], "%02x", (unsigned char)hash_sha256[i]); 
-17    str[i * 2] = 0; 
-18    pr_info("%s\n", str); 
-19} 
-20 
-21static int __init cryptosha256_init(void) 
-22{ 
-23    char *plaintext = "This is a test"; 
-24    char hash_sha256[SHA256_LENGTH]; 
-25    struct crypto_shash *sha256; 
-26    struct shash_desc *shash; 
-27 
-28    sha256 = crypto_alloc_shash("sha256", 0, 0); 
-29    if (IS_ERR(sha256)) { 
-30        pr_err( 
-31            "%s(): Failed to allocate sha256 algorithm, enable CONFIG_CRYPTO_SHA256 and try again.\n", 
-32            __func__); 
-33        return -1; 
-34    } 
-35 
-36    shash = kmalloc(sizeof(struct shash_desc) + crypto_shash_descsize(sha256), 
-37                    GFP_KERNEL); 
-38    if (!shash) 
-39        return -ENOMEM; 
-40 
-41    shash->tfm = sha256; 
-42 
-43    if (crypto_shash_init(shash)) 
-44        return -1; 
-45 
-46    if (crypto_shash_update(shash, plaintext, strlen(plaintext))) 
-47        return -1; 
-48 
-49    if (crypto_shash_final(shash, hash_sha256)) 
-50        return -1; 
-51 
-52    kfree(shash); 
-53    crypto_free_shash(sha256); 
-54 
-55    show_hash_result(plaintext, hash_sha256); 
-56 
-57    return 0; 
-58} 
-59 
-60static void __exit cryptosha256_exit(void) 
-61{ 
-62} 
-63 
-64module_init(cryptosha256_init); 
-65module_exit(cryptosha256_exit); 
-66 
-67MODULE_DESCRIPTION("sha256 hash test"); 
-68MODULE_LICENSE("GPL");
-

Install the module: -

-

- 
1sudo insmod cryptosha256.ko 
-2sudo dmesg
-

And you should see that the hash was calculated for the test string. -

Finally, remove the test module: -

- - - -

- 
1sudo rmmod cryptosha256
-

-

-

16.2 Symmetric key encryption

-

Here is an example of symmetrically encrypting a string using the AES algorithm -and a password. -

-

- 
1/* 
-2 * cryptosk.c 
-3 */ 
-4#include <crypto/internal/skcipher.h> 
-5#include <linux/crypto.h> 
-6#include <linux/module.h> 
-7#include <linux/random.h> 
-8#include <linux/scatterlist.h> 
-9 
-10#define SYMMETRIC_KEY_LENGTH 32 
-11#define CIPHER_BLOCK_SIZE 16 
-12 
-13struct tcrypt_result { 
-14    struct completion completion; 
-15    int err; 
-16}; 
-17 
-18struct skcipher_def { 
-19    struct scatterlist sg; 
-20    struct crypto_skcipher *tfm; 
-21    struct skcipher_request *req; 
-22    struct tcrypt_result result; 
-23    char *scratchpad; 
-24    char *ciphertext; 
-25    char *ivdata; 
-26}; 
-27 
-28static struct skcipher_def sk; 
-29 
-30static void test_skcipher_finish(struct skcipher_def *sk) 
-31{ 
-32    if (sk->tfm) 
-33        crypto_free_skcipher(sk->tfm); 
-34    if (sk->req) 
-35        skcipher_request_free(sk->req); 
-36    if (sk->ivdata) 
-37        kfree(sk->ivdata); 
-38    if (sk->scratchpad) 
-39        kfree(sk->scratchpad); 
-40    if (sk->ciphertext) 
-41        kfree(sk->ciphertext); 
-42} 
-43 
-44static int test_skcipher_result(struct skcipher_def *sk, int rc) 
-45{ 
-46    switch (rc) { 
-47    case 0: 
-48        break; 
-49    case -EINPROGRESS: 
-50    case -EBUSY: 
-51        rc = wait_for_completion_interruptible(&sk->result.completion); 
-52        if (!rc && !sk->result.err) { 
-53            reinit_completion(&sk->result.completion); 
-54            break; 
-55        } 
-56        pr_info("skcipher encrypt returned with %d result %d\n", rc, 
-57                sk->result.err); 
-58        break; 
-59    default: 
-60        pr_info("skcipher encrypt returned with %d result %d\n", rc, 
-61                sk->result.err); 
-62        break; 
-63    } 
-64 
-65    init_completion(&sk->result.completion); 
-66 
-67    return rc; 
-68} 
-69 
-70static void test_skcipher_callback(struct crypto_async_request *req, int error) 
-71{ 
-72    struct tcrypt_result *result = req->data; 
-73 
-74    if (error == -EINPROGRESS) 
-75        return; 
-76 
-77    result->err = error; 
-78    complete(&result->completion); 
-79    pr_info("Encryption finished successfully\n"); 
-80 
-81    /* decrypt data */ 
-82#if 0 
-83    memset((void*)sk.scratchpad, '-', CIPHER_BLOCK_SIZE); 
-84    ret = crypto_skcipher_decrypt(sk.req); 
-85    ret = test_skcipher_result(&sk, ret); 
-86    if (ret) 
-87        return; 
-88 
-89    sg_copy_from_buffer(&sk.sg, 1, sk.scratchpad, CIPHER_BLOCK_SIZE); 
-90    sk.scratchpad[CIPHER_BLOCK_SIZE-1] = 0; 
-91 
-92    pr_info("Decryption request successful\n"); 
-93    pr_info("Decrypted: %s\n", sk.scratchpad); 
-94#endif 
-95} 
-96 
-97static int test_skcipher_encrypt(char *plaintext, char *password, 
-98                                 struct skcipher_def *sk) 
-99{ 
-100    int ret = -EFAULT; 
-101    unsigned char key[SYMMETRIC_KEY_LENGTH]; 
-102 
-103    if (!sk->tfm) { 
-104        sk->tfm = crypto_alloc_skcipher("cbc-aes-aesni", 0, 0); 
-105        if (IS_ERR(sk->tfm)) { 
-106            pr_info("could not allocate skcipher handle\n"); 
-107            return PTR_ERR(sk->tfm); 
-108        } 
-109    } 
-110 
-111    if (!sk->req) { 
-112        sk->req = skcipher_request_alloc(sk->tfm, GFP_KERNEL); 
-113        if (!sk->req) { 
-114            pr_info("could not allocate skcipher request\n"); 
-115            ret = -ENOMEM; 
-116            goto out; 
-117        } 
-118    } 
-119 
-120    skcipher_request_set_callback(sk->req, CRYPTO_TFM_REQ_MAY_BACKLOG, 
-121                                  test_skcipher_callback, &sk->result); 
-122 
-123    /* clear the key */ 
-124    memset((void *)key, '\0', SYMMETRIC_KEY_LENGTH); 
-125 
-126    /* Use the world's favourite password */ 
-127    sprintf((char *)key, "%s", password); 
-128 
-129    /* AES 256 with given symmetric key */ 
-130    if (crypto_skcipher_setkey(sk->tfm, key, SYMMETRIC_KEY_LENGTH)) { 
-131        pr_info("key could not be set\n"); 
-132        ret = -EAGAIN; 
-133        goto out; 
-134    } 
-135    pr_info("Symmetric key: %s\n", key); 
-136    pr_info("Plaintext: %s\n", plaintext); 
-137 
-138    if (!sk->ivdata) { 
-139        /* see https://en.wikipedia.org/wiki/Initialization_vector */ 
-140        sk->ivdata = kmalloc(CIPHER_BLOCK_SIZE, GFP_KERNEL); 
-141        if (!sk->ivdata) { 
-142            pr_info("could not allocate ivdata\n"); 
-143            goto out; 
-144        } 
-145        get_random_bytes(sk->ivdata, CIPHER_BLOCK_SIZE); 
-146    } 
-147 
-148    if (!sk->scratchpad) { 
-149        /* The text to be encrypted */ 
-150        sk->scratchpad = kmalloc(CIPHER_BLOCK_SIZE, GFP_KERNEL); 
-151        if (!sk->scratchpad) { 
-152            pr_info("could not allocate scratchpad\n"); 
-153            goto out; 
-154        } 
-155    } 
-156    sprintf((char *)sk->scratchpad, "%s", plaintext); 
-157 
-158    sg_init_one(&sk->sg, sk->scratchpad, CIPHER_BLOCK_SIZE); 
-159    skcipher_request_set_crypt(sk->req, &sk->sg, &sk->sg, CIPHER_BLOCK_SIZE, 
-160                               sk->ivdata); 
-161    init_completion(&sk->result.completion); 
-162 
-163    /* encrypt data */ 
-164    ret = crypto_skcipher_encrypt(sk->req); 
-165    ret = test_skcipher_result(sk, ret); 
-166    if (ret) 
-167        goto out; 
-168 
-169    pr_info("Encryption request successful\n"); 
-170 
-171out: 
-172    return ret; 
-173} 
-174 
-175static int __init cryptoapi_init(void) 
-176{ 
-177    /* The world's favorite password */ 
-178    char *password = "password123"; 
-179 
-180    sk.tfm = NULL; 
-181    sk.req = NULL; 
-182    sk.scratchpad = NULL; 
-183    sk.ciphertext = NULL; 
-184    sk.ivdata = NULL; 
-185 
-186    test_skcipher_encrypt("Testing", password, &sk); 
-187    return 0; 
-188} 
-189 
-190static void __exit cryptoapi_exit(void) 
-191{ 
-192    test_skcipher_finish(&sk); 
-193} 
-194 
-195module_init(cryptoapi_init); 
-196module_exit(cryptoapi_exit); 
-197 
-198MODULE_DESCRIPTION("Symmetric key encryption example"); 
-199MODULE_LICENSE("GPL");
-

-

-

17 Virtual Input Device Driver

-

The input device driver is a module that provides a way to communicate +

16 Virtual Input Device Driver

+

The input device driver is a module that provides a way to communicate with the interaction device via the event. For example, the keyboard can send the press or release event to tell the kernel what we want to do. The input device driver will allocate a new input structure with @@ -5718,7 +5409,7 @@ do. The input device driver will allocate a new input structure with and sets up input bitfields, device id, version, etc. After that, registers it by calling input_register_device() . -

Here is an example, vinput, It is an API to allow easy +

Here is an example, vinput, It is an API to allow easy development of virtual input drivers. The drivers needs to export a vinput_device() that contains the virtual device name and @@ -5731,527 +5422,530 @@ development of virtual input drivers. The drivers needs to export a

  • the input event injection function: send() + + +
  • the readback function: read()
    • -

    Then using vinput_register_device() +

    Then using vinput_register_device() and vinput_unregister_device() will add a new device to the list of support virtual input devices.

    - 
    1int init(struct vinput *);
    -

    This function is passed a struct vinput - already initialized with an allocated struct input_dev + 

    1int init(struct vinput *);
    +

    This function is passed a struct vinput + already initialized with an allocated struct input_dev . The init() function is responsible for initializing the capabilities of the input device and register it.

    - 
    1int send(struct vinput *, char *, int);
    -

    This function will receive a user string to interpret and inject the event using the + 

    1int send(struct vinput *, char *, int);
    +

    This function will receive a user string to interpret and inject the event using the input_report_XXXX or input_event call. The string is already copied from user.

    - 
    1int read(struct vinput *, char *, int);
    -

    This function is used for debugging and should fill the buffer parameter with the + 

    1int read(struct vinput *, char *, int);
    +

    This function is used for debugging and should fill the buffer parameter with the last event sent in the virtual input device format. The buffer will then be copied to user. -

    vinput devices are created and destroyed using sysfs. And, event injection is done +

    vinput devices are created and destroyed using sysfs. And, event injection is done through a /dev node. The device name will be used by the userland to export a new virtual input device. -

    The class_attribute +

    The class_attribute structure is similar to other attribute types we talked about in section 8:

    - 
    1struct class_attribute { 
-2    struct attribute attr; 
-3    ssize_t (*show)(struct class *class, struct class_attribute *attr, 
-4                    char *buf); 
-5    ssize_t (*store)(struct class *class, struct class_attribute *attr, 
-6                    const char *buf, size_t count); 
-7};
    - - - -

    In vinput.c, the macro CLASS_ATTR_WO(export/unexport) + 

    1struct class_attribute { 
+2    struct attribute attr; 
+3    ssize_t (*show)(struct class *class, struct class_attribute *attr, 
+4                    char *buf); 
+5    ssize_t (*store)(struct class *class, struct class_attribute *attr, 
+6                    const char *buf, size_t count); 
+7};
    +

    In vinput.c, the macro CLASS_ATTR_WO(export/unexport) defined in include/linux/device.h (in this case, device.h is included in include/linux/input.h) will generate the class_attribute structures which are named class_attr_export/unexport. Then, put them into + + + vinput_class_attrs array and the macro ATTRIBUTE_GROUPS(vinput_class) - will generate the struct attribute_group vinput_class_group + will generate the struct attribute_group vinput_class_group that should be assigned in vinput_class . Finally, call class_register(&vinput_class) to create attributes in sysfs. -

    To create a vinputX sysfs entry and /dev node. +

    To create a vinputX sysfs entry and /dev node.

    - 
    1echo "vkbd" | sudo tee /sys/class/vinput/export
    -

    To unexport the device, just echo its id in unexport: + 

    1echo "vkbd" | sudo tee /sys/class/vinput/export
    +

    To unexport the device, just echo its id in unexport:

    - 
    1echo "0" | sudo tee /sys/class/vinput/unexport
    + 
    1echo "0" | sudo tee /sys/class/vinput/unexport

    - 
    1/* 
-2 * vinput.h 
-3 */ 
-4 
-5#ifndef VINPUT_H 
-6#define VINPUT_H 
-7 
-8#include <linux/input.h> 
-9#include <linux/spinlock.h> 
-10 
-11#define VINPUT_MAX_LEN 128 
-12#define MAX_VINPUT 32 
-13#define VINPUT_MINORS MAX_VINPUT 
-14 
-15#define dev_to_vinput(dev) container_of(dev, struct vinput, dev) 
-16 
-17struct vinput_device; 
-18 
-19struct vinput { 
-20    long id; 
-21    long devno; 
-22    long last_entry; 
-23    spinlock_t lock; 
-24 
-25    void *priv_data; 
-26 
-27    struct device dev; 
-28    struct list_head list; 
-29    struct input_dev *input; 
-30    struct vinput_device *type; 
-31}; 
-32 
-33struct vinput_ops { 
-34    int (*init)(struct vinput *); 
-35    int (*kill)(struct vinput *); 
-36    int (*send)(struct vinput *, char *, int); 
-37    int (*read)(struct vinput *, char *, int); 
-38}; 
-39 
-40struct vinput_device { 
-41    char name[16]; 
-42    struct list_head list; 
-43    struct vinput_ops *ops; 
-44}; 
-45 
-46int vinput_register(struct vinput_device *dev); 
-47void vinput_unregister(struct vinput_device *dev); 
-48 
-49#endif
    + 
    1/* 
+2 * vinput.h 
+3 */ 
+4 
+5#ifndef VINPUT_H 
+6#define VINPUT_H 
+7 
+8#include <linux/input.h> 
+9#include <linux/spinlock.h> 
+10 
+11#define VINPUT_MAX_LEN 128 
+12#define MAX_VINPUT 32 
+13#define VINPUT_MINORS MAX_VINPUT 
+14 
+15#define dev_to_vinput(dev) container_of(dev, struct vinput, dev) 
+16 
+17struct vinput_device; 
+18 
+19struct vinput { 
+20    long id; 
+21    long devno; 
+22    long last_entry; 
+23    spinlock_t lock; 
+24 
+25    void *priv_data; 
+26 
+27    struct device dev; 
+28    struct list_head list; 
+29    struct input_dev *input; 
+30    struct vinput_device *type; 
+31}; 
+32 
+33struct vinput_ops { 
+34    int (*init)(struct vinput *); 
+35    int (*kill)(struct vinput *); 
+36    int (*send)(struct vinput *, char *, int); 
+37    int (*read)(struct vinput *, char *, int); 
+38}; 
+39 
+40struct vinput_device { 
+41    char name[16]; 
+42    struct list_head list; 
+43    struct vinput_ops *ops; 
+44}; 
+45 
+46int vinput_register(struct vinput_device *dev); 
+47void vinput_unregister(struct vinput_device *dev); 
+48 
+49#endif

    - 
    1/* 
-2 * vinput.c 
-3 */ 
-4 
-5#include <linux/cdev.h> 
-6#include <linux/input.h> 
-7#include <linux/module.h> 
-8#include <linux/slab.h> 
-9#include <linux/spinlock.h> 
-10 
-11#include <asm/uaccess.h> 
-12 
-13#include "vinput.h" 
-14 
-15#define DRIVER_NAME "vinput" 
-16 
-17#define dev_to_vinput(dev) container_of(dev, struct vinput, dev) 
-18 
-19static DECLARE_BITMAP(vinput_ids, VINPUT_MINORS); 
-20 
-21static LIST_HEAD(vinput_devices); 
-22static LIST_HEAD(vinput_vdevices); 
-23 
-24static int vinput_dev; 
-25static struct spinlock vinput_lock; 
-26static struct class vinput_class; 
-27 
-28/* Search the name of vinput device in the vinput_devices linked list, 
-29 * which added at vinput_register(). 
-30 */ 
-31static struct vinput_device *vinput_get_device_by_type(const char *type) 
-32{ 
-33    int found = 0; 
-34    struct vinput_device *vinput; 
-35    struct list_head *curr; 
-36 
-37    spin_lock(&vinput_lock); 
-38    list_for_each (curr, &vinput_devices) { 
-39        vinput = list_entry(curr, struct vinput_device, list); 
-40        if (vinput && strncmp(type, vinput->name, strlen(vinput->name)) == 0) { 
-41            found = 1; 
-42            break; 
-43        } 
-44    } 
-45    spin_unlock(&vinput_lock); 
-46 
-47    if (found) 
-48        return vinput; 
-49    return ERR_PTR(-ENODEV); 
-50} 
-51 
-52/* Search the id of virtual device in the vinput_vdevices linked list, 
-53 * which added at vinput_alloc_vdevice(). 
-54 */ 
-55static struct vinput *vinput_get_vdevice_by_id(long id) 
-56{ 
-57    struct vinput *vinput = NULL; 
-58    struct list_head *curr; 
-59 
-60    spin_lock(&vinput_lock); 
-61    list_for_each (curr, &vinput_vdevices) { 
-62        vinput = list_entry(curr, struct vinput, list); 
-63        if (vinput && vinput->id == id) 
-64            break; 
-65    } 
-66    spin_unlock(&vinput_lock); 
-67 
-68    if (vinput && vinput->id == id) 
-69        return vinput; 
-70    return ERR_PTR(-ENODEV); 
-71} 
-72 
-73static int vinput_open(struct inode *inode, struct file *file) 
-74{ 
-75    int err = 0; 
-76    struct vinput *vinput = NULL; 
-77 
-78    vinput = vinput_get_vdevice_by_id(iminor(inode)); 
-79 
-80    if (IS_ERR(vinput)) 
-81        err = PTR_ERR(vinput); 
-82    else 
-83        file->private_data = vinput; 
-84 
-85    return err; 
-86} 
-87 
-88static int vinput_release(struct inode *inode, struct file *file) 
-89{ 
-90    return 0; 
-91} 
-92 
-93static ssize_t vinput_read(struct file *file, char __user *buffer, size_t count, 
-94                           loff_t *offset) 
-95{ 
-96    int len; 
-97    char buff[VINPUT_MAX_LEN + 1]; 
-98    struct vinput *vinput = file->private_data; 
-99 
-100    len = vinput->type->ops->read(vinput, buff, count); 
-101 
-102    if (*offset > len) 
-103        count = 0; 
-104    else if (count + *offset > VINPUT_MAX_LEN) 
-105        count = len - *offset; 
-106 
-107    if (raw_copy_to_user(buffer, buff + *offset, count)) 
-108        count = -EFAULT; 
-109 
-110    *offset += count; 
-111 
-112    return count; 
-113} 
-114 
-115static ssize_t vinput_write(struct file *file, const char __user *buffer, 
-116                            size_t count, loff_t *offset) 
-117{ 
-118    char buff[VINPUT_MAX_LEN + 1]; 
-119    struct vinput *vinput = file->private_data; 
-120 
-121    memset(buff, 0, sizeof(char) * (VINPUT_MAX_LEN + 1)); 
-122 
-123    if (count > VINPUT_MAX_LEN) { 
-124        dev_warn(&vinput->dev, "Too long. %d bytes allowed\n", VINPUT_MAX_LEN); 
-125        return -EINVAL; 
-126    } 
-127 
-128    if (raw_copy_from_user(buff, buffer, count)) 
-129        return -EFAULT; 
-130 
-131    return vinput->type->ops->send(vinput, buff, count); 
-132} 
-133 
-134static const struct file_operations vinput_fops = { 
-135    .owner = THIS_MODULE, 
-136    .open = vinput_open, 
-137    .release = vinput_release, 
-138    .read = vinput_read, 
-139    .write = vinput_write, 
-140}; 
-141 
-142static void vinput_unregister_vdevice(struct vinput *vinput) 
-143{ 
-144    input_unregister_device(vinput->input); 
-145    if (vinput->type->ops->kill) 
-146        vinput->type->ops->kill(vinput); 
-147} 
-148 
-149static void vinput_destroy_vdevice(struct vinput *vinput) 
-150{ 
-151    /* Remove from the list first */ 
-152    spin_lock(&vinput_lock); 
-153    list_del(&vinput->list); 
-154    clear_bit(vinput->id, vinput_ids); 
-155    spin_unlock(&vinput_lock); 
-156 
-157    module_put(THIS_MODULE); 
-158 
-159    kfree(vinput); 
-160} 
-161 
-162static void vinput_release_dev(struct device *dev) 
-163{ 
-164    struct vinput *vinput = dev_to_vinput(dev); 
-165    int id = vinput->id; 
-166 
-167    vinput_destroy_vdevice(vinput); 
-168 
-169    pr_debug("released vinput%d.\n", id); 
-170} 
-171 
-172static struct vinput *vinput_alloc_vdevice(void) 
-173{ 
-174    int err; 
-175    struct vinput *vinput = kzalloc(sizeof(struct vinput), GFP_KERNEL); 
-176 
-177    try_module_get(THIS_MODULE); 
-178 
-179    memset(vinput, 0, sizeof(struct vinput)); 
-180 
-181    spin_lock_init(&vinput->lock); 
-182 
-183    spin_lock(&vinput_lock); 
-184    vinput->id = find_first_zero_bit(vinput_ids, VINPUT_MINORS); 
-185    if (vinput->id >= VINPUT_MINORS) { 
-186        err = -ENOBUFS; 
-187        goto fail_id; 
-188    } 
-189    set_bit(vinput->id, vinput_ids); 
-190    list_add(&vinput->list, &vinput_vdevices); 
-191    spin_unlock(&vinput_lock); 
-192 
-193    /* allocate the input device */ 
-194    vinput->input = input_allocate_device(); 
-195    if (vinput->input == NULL) { 
-196        pr_err("vinput: Cannot allocate vinput input device\n"); 
-197        err = -ENOMEM; 
-198        goto fail_input_dev; 
-199    } 
-200 
-201    /* initialize device */ 
-202    vinput->dev.class = &vinput_class; 
-203    vinput->dev.release = vinput_release_dev; 
-204    vinput->dev.devt = MKDEV(vinput_dev, vinput->id); 
-205    dev_set_name(&vinput->dev, DRIVER_NAME "%lu", vinput->id); 
-206 
-207    return vinput; 
-208 
-209fail_input_dev: 
-210    spin_lock(&vinput_lock); 
-211    list_del(&vinput->list); 
-212fail_id: 
-213    spin_unlock(&vinput_lock); 
-214    module_put(THIS_MODULE); 
-215    kfree(vinput); 
-216 
-217    return ERR_PTR(err); 
-218} 
-219 
-220static int vinput_register_vdevice(struct vinput *vinput) 
-221{ 
-222    int err = 0; 
-223 
-224    /* register the input device */ 
-225    vinput->input->name = vinput->type->name; 
-226    vinput->input->phys = "vinput"; 
-227    vinput->input->dev.parent = &vinput->dev; 
-228 
-229    vinput->input->id.bustype = BUS_VIRTUAL; 
-230    vinput->input->id.product = 0x0000; 
-231    vinput->input->id.vendor = 0x0000; 
-232    vinput->input->id.version = 0x0000; 
-233 
-234    err = vinput->type->ops->init(vinput); 
-235 
-236    if (err == 0) 
-237        dev_info(&vinput->dev, "Registered virtual input %s %ld\n", 
-238                 vinput->type->name, vinput->id); 
-239 
-240    return err; 
-241} 
-242 
-243static ssize_t export_store(struct class *class, struct class_attribute *attr, 
-244                            const char *buf, size_t len) 
-245{ 
-246    int err; 
-247    struct vinput *vinput; 
-248    struct vinput_device *device; 
-249 
-250    device = vinput_get_device_by_type(buf); 
-251    if (IS_ERR(device)) { 
-252        pr_info("vinput: This virtual device isn't registered\n"); 
-253        err = PTR_ERR(device); 
-254        goto fail; 
-255    } 
-256 
-257    vinput = vinput_alloc_vdevice(); 
-258    if (IS_ERR(vinput)) { 
-259        err = PTR_ERR(vinput); 
-260        goto fail; 
-261    } 
-262 
-263    vinput->type = device; 
-264    err = device_register(&vinput->dev); 
-265    if (err < 0) 
-266        goto fail_register; 
-267 
-268    err = vinput_register_vdevice(vinput); 
-269    if (err < 0) 
-270        goto fail_register_vinput; 
-271 
-272    return len; 
-273 
-274fail_register_vinput: 
-275    device_unregister(&vinput->dev); 
-276fail_register: 
-277    vinput_destroy_vdevice(vinput); 
-278fail: 
-279    return err; 
-280} 
-281/* This macro generates class_attr_export structure and export_store() */ 
-282static CLASS_ATTR_WO(export); 
-283 
-284static ssize_t unexport_store(struct class *class, struct class_attribute *attr, 
-285                              const char *buf, size_t len) 
-286{ 
-287    int err; 
-288    unsigned long id; 
-289    struct vinput *vinput; 
-290 
-291    err = kstrtol(buf, 10, &id); 
-292    if (err) { 
-293        err = -EINVAL; 
-294        goto failed; 
-295    } 
-296 
-297    vinput = vinput_get_vdevice_by_id(id); 
-298    if (IS_ERR(vinput)) { 
-299        pr_err("vinput: No such vinput device %ld\n", id); 
-300        err = PTR_ERR(vinput); 
-301        goto failed; 
-302    } 
-303 
-304    vinput_unregister_vdevice(vinput); 
-305    device_unregister(&vinput->dev); 
-306 
-307    return len; 
-308failed: 
-309    return err; 
-310} 
-311/* This macro generates class_attr_unexport structure and unexport_store() */ 
-312static CLASS_ATTR_WO(unexport); 
-313 
-314static struct attribute *vinput_class_attrs[] = { 
-315    &class_attr_export.attr, 
-316    &class_attr_unexport.attr, 
-317    NULL, 
-318}; 
-319 
-320/* This macro generates vinput_class_groups structure */ 
-321ATTRIBUTE_GROUPS(vinput_class); 
-322 
-323static struct class vinput_class = { 
-324    .name = "vinput", 
-325    .owner = THIS_MODULE, 
-326    .class_groups = vinput_class_groups, 
-327}; 
-328 
-329int vinput_register(struct vinput_device *dev) 
-330{ 
-331    spin_lock(&vinput_lock); 
-332    list_add(&dev->list, &vinput_devices); 
-333    spin_unlock(&vinput_lock); 
-334 
-335    pr_info("vinput: registered new virtual input device '%s'\n", dev->name); 
-336 
-337    return 0; 
-338} 
-339EXPORT_SYMBOL(vinput_register); 
-340 
-341void vinput_unregister(struct vinput_device *dev) 
-342{ 
-343    struct list_head *curr, *next; 
-344 
-345    /* Remove from the list first */ 
-346    spin_lock(&vinput_lock); 
-347    list_del(&dev->list); 
-348    spin_unlock(&vinput_lock); 
-349 
-350    /* unregister all devices of this type */ 
-351    list_for_each_safe (curr, next, &vinput_vdevices) { 
-352        struct vinput *vinput = list_entry(curr, struct vinput, list); 
-353        if (vinput && vinput->type == dev) { 
-354            vinput_unregister_vdevice(vinput); 
-355            device_unregister(&vinput->dev); 
-356        } 
-357    } 
-358 
-359    pr_info("vinput: unregistered virtual input device '%s'\n", dev->name); 
-360} 
-361EXPORT_SYMBOL(vinput_unregister); 
-362 
-363static int __init vinput_init(void) 
-364{ 
-365    int err = 0; 
-366 
-367    pr_info("vinput: Loading virtual input driver\n"); 
-368 
-369    vinput_dev = register_chrdev(0, DRIVER_NAME, &vinput_fops); 
-370    if (vinput_dev < 0) { 
-371        pr_err("vinput: Unable to allocate char dev region\n"); 
-372        err = vinput_dev; 
-373        goto failed_alloc; 
-374    } 
-375 
-376    spin_lock_init(&vinput_lock); 
-377 
-378    err = class_register(&vinput_class); 
-379    if (err < 0) { 
-380        pr_err("vinput: Unable to register vinput class\n"); 
-381        goto failed_class; 
-382    } 
-383 
-384    return 0; 
-385failed_class: 
-386    class_unregister(&vinput_class); 
-387failed_alloc: 
-388    return err; 
-389} 
-390 
-391static void __exit vinput_end(void) 
-392{ 
-393    pr_info("vinput: Unloading virtual input driver\n"); 
-394 
-395    unregister_chrdev(vinput_dev, DRIVER_NAME); 
-396    class_unregister(&vinput_class); 
-397} 
-398 
-399module_init(vinput_init); 
-400module_exit(vinput_end); 
-401 
-402MODULE_LICENSE("GPL"); 
-403MODULE_DESCRIPTION("Emulate input events");
    -

    Here the virtual keyboard is one of example to use vinput. It supports all + 

    1/* 
+2 * vinput.c 
+3 */ 
+4 
+5#include <linux/cdev.h> 
+6#include <linux/input.h> 
+7#include <linux/module.h> 
+8#include <linux/slab.h> 
+9#include <linux/spinlock.h> 
+10 
+11#include <asm/uaccess.h> 
+12 
+13#include "vinput.h" 
+14 
+15#define DRIVER_NAME "vinput" 
+16 
+17#define dev_to_vinput(dev) container_of(dev, struct vinput, dev) 
+18 
+19static DECLARE_BITMAP(vinput_ids, VINPUT_MINORS); 
+20 
+21static LIST_HEAD(vinput_devices); 
+22static LIST_HEAD(vinput_vdevices); 
+23 
+24static int vinput_dev; 
+25static struct spinlock vinput_lock; 
+26static struct class vinput_class; 
+27 
+28/* Search the name of vinput device in the vinput_devices linked list, 
+29 * which added at vinput_register(). 
+30 */ 
+31static struct vinput_device *vinput_get_device_by_type(const char *type) 
+32{ 
+33    int found = 0; 
+34    struct vinput_device *vinput; 
+35    struct list_head *curr; 
+36 
+37    spin_lock(&vinput_lock); 
+38    list_for_each (curr, &vinput_devices) { 
+39        vinput = list_entry(curr, struct vinput_device, list); 
+40        if (vinput && strncmp(type, vinput->name, strlen(vinput->name)) == 0) { 
+41            found = 1; 
+42            break; 
+43        } 
+44    } 
+45    spin_unlock(&vinput_lock); 
+46 
+47    if (found) 
+48        return vinput; 
+49    return ERR_PTR(-ENODEV); 
+50} 
+51 
+52/* Search the id of virtual device in the vinput_vdevices linked list, 
+53 * which added at vinput_alloc_vdevice(). 
+54 */ 
+55static struct vinput *vinput_get_vdevice_by_id(long id) 
+56{ 
+57    struct vinput *vinput = NULL; 
+58    struct list_head *curr; 
+59 
+60    spin_lock(&vinput_lock); 
+61    list_for_each (curr, &vinput_vdevices) { 
+62        vinput = list_entry(curr, struct vinput, list); 
+63        if (vinput && vinput->id == id) 
+64            break; 
+65    } 
+66    spin_unlock(&vinput_lock); 
+67 
+68    if (vinput && vinput->id == id) 
+69        return vinput; 
+70    return ERR_PTR(-ENODEV); 
+71} 
+72 
+73static int vinput_open(struct inode *inode, struct file *file) 
+74{ 
+75    int err = 0; 
+76    struct vinput *vinput = NULL; 
+77 
+78    vinput = vinput_get_vdevice_by_id(iminor(inode)); 
+79 
+80    if (IS_ERR(vinput)) 
+81        err = PTR_ERR(vinput); 
+82    else 
+83        file->private_data = vinput; 
+84 
+85    return err; 
+86} 
+87 
+88static int vinput_release(struct inode *inode, struct file *file) 
+89{ 
+90    return 0; 
+91} 
+92 
+93static ssize_t vinput_read(struct file *file, char __user *buffer, size_t count, 
+94                           loff_t *offset) 
+95{ 
+96    int len; 
+97    char buff[VINPUT_MAX_LEN + 1]; 
+98    struct vinput *vinput = file->private_data; 
+99 
+100    len = vinput->type->ops->read(vinput, buff, count); 
+101 
+102    if (*offset > len) 
+103        count = 0; 
+104    else if (count + *offset > VINPUT_MAX_LEN) 
+105        count = len - *offset; 
+106 
+107    if (raw_copy_to_user(buffer, buff + *offset, count)) 
+108        count = -EFAULT; 
+109 
+110    *offset += count; 
+111 
+112    return count; 
+113} 
+114 
+115static ssize_t vinput_write(struct file *file, const char __user *buffer, 
+116                            size_t count, loff_t *offset) 
+117{ 
+118    char buff[VINPUT_MAX_LEN + 1]; 
+119    struct vinput *vinput = file->private_data; 
+120 
+121    memset(buff, 0, sizeof(char) * (VINPUT_MAX_LEN + 1)); 
+122 
+123    if (count > VINPUT_MAX_LEN) { 
+124        dev_warn(&vinput->dev, "Too long. %d bytes allowed\n", VINPUT_MAX_LEN); 
+125        return -EINVAL; 
+126    } 
+127 
+128    if (raw_copy_from_user(buff, buffer, count)) 
+129        return -EFAULT; 
+130 
+131    return vinput->type->ops->send(vinput, buff, count); 
+132} 
+133 
+134static const struct file_operations vinput_fops = { 
+135    .owner = THIS_MODULE, 
+136    .open = vinput_open, 
+137    .release = vinput_release, 
+138    .read = vinput_read, 
+139    .write = vinput_write, 
+140}; 
+141 
+142static void vinput_unregister_vdevice(struct vinput *vinput) 
+143{ 
+144    input_unregister_device(vinput->input); 
+145    if (vinput->type->ops->kill) 
+146        vinput->type->ops->kill(vinput); 
+147} 
+148 
+149static void vinput_destroy_vdevice(struct vinput *vinput) 
+150{ 
+151    /* Remove from the list first */ 
+152    spin_lock(&vinput_lock); 
+153    list_del(&vinput->list); 
+154    clear_bit(vinput->id, vinput_ids); 
+155    spin_unlock(&vinput_lock); 
+156 
+157    module_put(THIS_MODULE); 
+158 
+159    kfree(vinput); 
+160} 
+161 
+162static void vinput_release_dev(struct device *dev) 
+163{ 
+164    struct vinput *vinput = dev_to_vinput(dev); 
+165    int id = vinput->id; 
+166 
+167    vinput_destroy_vdevice(vinput); 
+168 
+169    pr_debug("released vinput%d.\n", id); 
+170} 
+171 
+172static struct vinput *vinput_alloc_vdevice(void) 
+173{ 
+174    int err; 
+175    struct vinput *vinput = kzalloc(sizeof(struct vinput), GFP_KERNEL); 
+176 
+177    try_module_get(THIS_MODULE); 
+178 
+179    memset(vinput, 0, sizeof(struct vinput)); 
+180 
+181    spin_lock_init(&vinput->lock); 
+182 
+183    spin_lock(&vinput_lock); 
+184    vinput->id = find_first_zero_bit(vinput_ids, VINPUT_MINORS); 
+185    if (vinput->id >= VINPUT_MINORS) { 
+186        err = -ENOBUFS; 
+187        goto fail_id; 
+188    } 
+189    set_bit(vinput->id, vinput_ids); 
+190    list_add(&vinput->list, &vinput_vdevices); 
+191    spin_unlock(&vinput_lock); 
+192 
+193    /* allocate the input device */ 
+194    vinput->input = input_allocate_device(); 
+195    if (vinput->input == NULL) { 
+196        pr_err("vinput: Cannot allocate vinput input device\n"); 
+197        err = -ENOMEM; 
+198        goto fail_input_dev; 
+199    } 
+200 
+201    /* initialize device */ 
+202    vinput->dev.class = &vinput_class; 
+203    vinput->dev.release = vinput_release_dev; 
+204    vinput->dev.devt = MKDEV(vinput_dev, vinput->id); 
+205    dev_set_name(&vinput->dev, DRIVER_NAME "%lu", vinput->id); 
+206 
+207    return vinput; 
+208 
+209fail_input_dev: 
+210    spin_lock(&vinput_lock); 
+211    list_del(&vinput->list); 
+212fail_id: 
+213    spin_unlock(&vinput_lock); 
+214    module_put(THIS_MODULE); 
+215    kfree(vinput); 
+216 
+217    return ERR_PTR(err); 
+218} 
+219 
+220static int vinput_register_vdevice(struct vinput *vinput) 
+221{ 
+222    int err = 0; 
+223 
+224    /* register the input device */ 
+225    vinput->input->name = vinput->type->name; 
+226    vinput->input->phys = "vinput"; 
+227    vinput->input->dev.parent = &vinput->dev; 
+228 
+229    vinput->input->id.bustype = BUS_VIRTUAL; 
+230    vinput->input->id.product = 0x0000; 
+231    vinput->input->id.vendor = 0x0000; 
+232    vinput->input->id.version = 0x0000; 
+233 
+234    err = vinput->type->ops->init(vinput); 
+235 
+236    if (err == 0) 
+237        dev_info(&vinput->dev, "Registered virtual input %s %ld\n", 
+238                 vinput->type->name, vinput->id); 
+239 
+240    return err; 
+241} 
+242 
+243static ssize_t export_store(struct class *class, struct class_attribute *attr, 
+244                            const char *buf, size_t len) 
+245{ 
+246    int err; 
+247    struct vinput *vinput; 
+248    struct vinput_device *device; 
+249 
+250    device = vinput_get_device_by_type(buf); 
+251    if (IS_ERR(device)) { 
+252        pr_info("vinput: This virtual device isn't registered\n"); 
+253        err = PTR_ERR(device); 
+254        goto fail; 
+255    } 
+256 
+257    vinput = vinput_alloc_vdevice(); 
+258    if (IS_ERR(vinput)) { 
+259        err = PTR_ERR(vinput); 
+260        goto fail; 
+261    } 
+262 
+263    vinput->type = device; 
+264    err = device_register(&vinput->dev); 
+265    if (err < 0) 
+266        goto fail_register; 
+267 
+268    err = vinput_register_vdevice(vinput); 
+269    if (err < 0) 
+270        goto fail_register_vinput; 
+271 
+272    return len; 
+273 
+274fail_register_vinput: 
+275    device_unregister(&vinput->dev); 
+276fail_register: 
+277    vinput_destroy_vdevice(vinput); 
+278fail: 
+279    return err; 
+280} 
+281/* This macro generates class_attr_export structure and export_store() */ 
+282static CLASS_ATTR_WO(export); 
+283 
+284static ssize_t unexport_store(struct class *class, struct class_attribute *attr, 
+285                              const char *buf, size_t len) 
+286{ 
+287    int err; 
+288    unsigned long id; 
+289    struct vinput *vinput; 
+290 
+291    err = kstrtol(buf, 10, &id); 
+292    if (err) { 
+293        err = -EINVAL; 
+294        goto failed; 
+295    } 
+296 
+297    vinput = vinput_get_vdevice_by_id(id); 
+298    if (IS_ERR(vinput)) { 
+299        pr_err("vinput: No such vinput device %ld\n", id); 
+300        err = PTR_ERR(vinput); 
+301        goto failed; 
+302    } 
+303 
+304    vinput_unregister_vdevice(vinput); 
+305    device_unregister(&vinput->dev); 
+306 
+307    return len; 
+308failed: 
+309    return err; 
+310} 
+311/* This macro generates class_attr_unexport structure and unexport_store() */ 
+312static CLASS_ATTR_WO(unexport); 
+313 
+314static struct attribute *vinput_class_attrs[] = { 
+315    &class_attr_export.attr, 
+316    &class_attr_unexport.attr, 
+317    NULL, 
+318}; 
+319 
+320/* This macro generates vinput_class_groups structure */ 
+321ATTRIBUTE_GROUPS(vinput_class); 
+322 
+323static struct class vinput_class = { 
+324    .name = "vinput", 
+325    .owner = THIS_MODULE, 
+326    .class_groups = vinput_class_groups, 
+327}; 
+328 
+329int vinput_register(struct vinput_device *dev) 
+330{ 
+331    spin_lock(&vinput_lock); 
+332    list_add(&dev->list, &vinput_devices); 
+333    spin_unlock(&vinput_lock); 
+334 
+335    pr_info("vinput: registered new virtual input device '%s'\n", dev->name); 
+336 
+337    return 0; 
+338} 
+339EXPORT_SYMBOL(vinput_register); 
+340 
+341void vinput_unregister(struct vinput_device *dev) 
+342{ 
+343    struct list_head *curr, *next; 
+344 
+345    /* Remove from the list first */ 
+346    spin_lock(&vinput_lock); 
+347    list_del(&dev->list); 
+348    spin_unlock(&vinput_lock); 
+349 
+350    /* unregister all devices of this type */ 
+351    list_for_each_safe (curr, next, &vinput_vdevices) { 
+352        struct vinput *vinput = list_entry(curr, struct vinput, list); 
+353        if (vinput && vinput->type == dev) { 
+354            vinput_unregister_vdevice(vinput); 
+355            device_unregister(&vinput->dev); 
+356        } 
+357    } 
+358 
+359    pr_info("vinput: unregistered virtual input device '%s'\n", dev->name); 
+360} 
+361EXPORT_SYMBOL(vinput_unregister); 
+362 
+363static int __init vinput_init(void) 
+364{ 
+365    int err = 0; 
+366 
+367    pr_info("vinput: Loading virtual input driver\n"); 
+368 
+369    vinput_dev = register_chrdev(0, DRIVER_NAME, &vinput_fops); 
+370    if (vinput_dev < 0) { 
+371        pr_err("vinput: Unable to allocate char dev region\n"); 
+372        err = vinput_dev; 
+373        goto failed_alloc; 
+374    } 
+375 
+376    spin_lock_init(&vinput_lock); 
+377 
+378    err = class_register(&vinput_class); 
+379    if (err < 0) { 
+380        pr_err("vinput: Unable to register vinput class\n"); 
+381        goto failed_class; 
+382    } 
+383 
+384    return 0; 
+385failed_class: 
+386    class_unregister(&vinput_class); 
+387failed_alloc: 
+388    return err; 
+389} 
+390 
+391static void __exit vinput_end(void) 
+392{ 
+393    pr_info("vinput: Unloading virtual input driver\n"); 
+394 
+395    unregister_chrdev(vinput_dev, DRIVER_NAME); 
+396    class_unregister(&vinput_class); 
+397} 
+398 
+399module_init(vinput_init); 
+400module_exit(vinput_end); 
+401 
+402MODULE_LICENSE("GPL"); 
+403MODULE_DESCRIPTION("Emulate input events");
    +

    Here the virtual keyboard is one of example to use vinput. It supports all KEY_MAX keycodes. The injection format is the KEY_CODE such as defined in include/linux/input.h. A positive value means @@ -6259,136 +5953,136 @@ will generate the class_attribute while a negative value is a KEY_RELEASE . The keyboard supports repetition when the key stays pressed for too long. The following demonstrates how simulation work. -

    Simulate a key press on "g" ( KEY_G +

    Simulate a key press on "g" ( KEY_G = 34):

    - 
    1echo "+34" | sudo tee /dev/vinput0
    -

    Simulate a key release on "g" ( KEY_G + 

    1echo "+34" | sudo tee /dev/vinput0
    +

    Simulate a key release on "g" ( KEY_G = 34):

    +

    + 
    1echo "-34" | sudo tee /dev/vinput0
    -

    - 
    1echo "-34" | sudo tee /dev/vinput0

    - 
    1/* 
-2 * vkbd.c 
-3 */ 
-4 
-5#include <linux/init.h> 
-6#include <linux/input.h> 
-7#include <linux/module.h> 
-8#include <linux/spinlock.h> 
-9 
-10#include "vinput.h" 
-11 
-12#define VINPUT_KBD "vkbd" 
-13#define VINPUT_RELEASE 0 
-14#define VINPUT_PRESS 1 
-15 
-16static unsigned short vkeymap[KEY_MAX]; 
-17 
-18static int vinput_vkbd_init(struct vinput *vinput) 
-19{ 
-20    int i; 
-21 
-22    /* Set up the input bitfield */ 
-23    vinput->input->evbit[0] = BIT_MASK(EV_KEY) | BIT_MASK(EV_REP); 
-24    vinput->input->keycodesize = sizeof(unsigned short); 
-25    vinput->input->keycodemax = KEY_MAX; 
-26    vinput->input->keycode = vkeymap; 
-27 
-28    for (i = 0; i < KEY_MAX; i++) 
-29        set_bit(vkeymap[i], vinput->input->keybit); 
-30 
-31    /* vinput will help us allocate new input device structure via 
-32     * input_allocate_device(). So, we can register it straightforwardly. 
-33     */ 
-34    return input_register_device(vinput->input); 
-35} 
-36 
-37static int vinput_vkbd_read(struct vinput *vinput, char *buff, int len) 
-38{ 
-39    spin_lock(&vinput->lock); 
-40    len = snprintf(buff, len, "%+ld\n", vinput->last_entry); 
-41    spin_unlock(&vinput->lock); 
-42 
-43    return len; 
-44} 
-45 
-46static int vinput_vkbd_send(struct vinput *vinput, char *buff, int len) 
-47{ 
-48    int ret; 
-49    long key = 0; 
-50    short type = VINPUT_PRESS; 
-51 
-52    /* Determine which event was received (press or release) 
-53     * and store the state. 
-54     */ 
-55    if (buff[0] == '+') 
-56        ret = kstrtol(buff + 1, 10, &key); 
-57    else 
-58        ret = kstrtol(buff, 10, &key); 
-59    if (ret) 
-60        dev_err(&vinput->dev, "error during kstrtol: -%d\n", ret); 
-61    spin_lock(&vinput->lock); 
-62    vinput->last_entry = key; 
-63    spin_unlock(&vinput->lock); 
-64 
-65    if (key < 0) { 
-66        type = VINPUT_RELEASE; 
-67        key = -key; 
-68    } 
-69 
-70    dev_info(&vinput->dev, "Event %s code %ld\n", 
-71             (type == VINPUT_RELEASE) ? "VINPUT_RELEASE" : "VINPUT_PRESS", key); 
-72 
-73    /* Report the state received to input subsystem. */ 
-74    input_report_key(vinput->input, key, type); 
-75    /* Tell input subsystem that it finished the report. */ 
-76    input_sync(vinput->input); 
-77 
-78    return len; 
-79} 
-80 
-81static struct vinput_ops vkbd_ops = { 
-82    .init = vinput_vkbd_init, 
-83    .send = vinput_vkbd_send, 
-84    .read = vinput_vkbd_read, 
-85}; 
-86 
-87static struct vinput_device vkbd_dev = { 
-88    .name = VINPUT_KBD, 
-89    .ops = &vkbd_ops, 
-90}; 
-91 
-92static int __init vkbd_init(void) 
-93{ 
-94    int i; 
-95 
-96    for (i = 0; i < KEY_MAX; i++) 
-97        vkeymap[i] = i; 
-98    return vinput_register(&vkbd_dev); 
-99} 
-100 
-101static void __exit vkbd_end(void) 
-102{ 
-103    vinput_unregister(&vkbd_dev); 
-104} 
-105 
-106module_init(vkbd_init); 
-107module_exit(vkbd_end); 
-108 
-109MODULE_LICENSE("GPL"); 
-110MODULE_DESCRIPTION("Emulate keyboard input events through /dev/vinput");
    -

    + 

    1/* 
+2 * vkbd.c 
+3 */ 
+4 
+5#include <linux/init.h> 
+6#include <linux/input.h> 
+7#include <linux/module.h> 
+8#include <linux/spinlock.h> 
+9 
+10#include "vinput.h" 
+11 
+12#define VINPUT_KBD "vkbd" 
+13#define VINPUT_RELEASE 0 
+14#define VINPUT_PRESS 1 
+15 
+16static unsigned short vkeymap[KEY_MAX]; 
+17 
+18static int vinput_vkbd_init(struct vinput *vinput) 
+19{ 
+20    int i; 
+21 
+22    /* Set up the input bitfield */ 
+23    vinput->input->evbit[0] = BIT_MASK(EV_KEY) | BIT_MASK(EV_REP); 
+24    vinput->input->keycodesize = sizeof(unsigned short); 
+25    vinput->input->keycodemax = KEY_MAX; 
+26    vinput->input->keycode = vkeymap; 
+27 
+28    for (i = 0; i < KEY_MAX; i++) 
+29        set_bit(vkeymap[i], vinput->input->keybit); 
+30 
+31    /* vinput will help us allocate new input device structure via 
+32     * input_allocate_device(). So, we can register it straightforwardly. 
+33     */ 
+34    return input_register_device(vinput->input); 
+35} 
+36 
+37static int vinput_vkbd_read(struct vinput *vinput, char *buff, int len) 
+38{ 
+39    spin_lock(&vinput->lock); 
+40    len = snprintf(buff, len, "%+ld\n", vinput->last_entry); 
+41    spin_unlock(&vinput->lock); 
+42 
+43    return len; 
+44} 
+45 
+46static int vinput_vkbd_send(struct vinput *vinput, char *buff, int len) 
+47{ 
+48    int ret; 
+49    long key = 0; 
+50    short type = VINPUT_PRESS; 
+51 
+52    /* Determine which event was received (press or release) 
+53     * and store the state. 
+54     */ 
+55    if (buff[0] == '+') 
+56        ret = kstrtol(buff + 1, 10, &key); 
+57    else 
+58        ret = kstrtol(buff, 10, &key); 
+59    if (ret) 
+60        dev_err(&vinput->dev, "error during kstrtol: -%d\n", ret); 
+61    spin_lock(&vinput->lock); 
+62    vinput->last_entry = key; 
+63    spin_unlock(&vinput->lock); 
+64 
+65    if (key < 0) { 
+66        type = VINPUT_RELEASE; 
+67        key = -key; 
+68    } 
+69 
+70    dev_info(&vinput->dev, "Event %s code %ld\n", 
+71             (type == VINPUT_RELEASE) ? "VINPUT_RELEASE" : "VINPUT_PRESS", key); 
+72 
+73    /* Report the state received to input subsystem. */ 
+74    input_report_key(vinput->input, key, type); 
+75    /* Tell input subsystem that it finished the report. */ 
+76    input_sync(vinput->input); 
+77 
+78    return len; 
+79} 
+80 
+81static struct vinput_ops vkbd_ops = { 
+82    .init = vinput_vkbd_init, 
+83    .send = vinput_vkbd_send, 
+84    .read = vinput_vkbd_read, 
+85}; 
+86 
+87static struct vinput_device vkbd_dev = { 
+88    .name = VINPUT_KBD, 
+89    .ops = &vkbd_ops, 
+90}; 
+91 
+92static int __init vkbd_init(void) 
+93{ 
+94    int i; 
+95 
+96    for (i = 0; i < KEY_MAX; i++) 
+97        vkeymap[i] = i; 
+98    return vinput_register(&vkbd_dev); 
+99} 
+100 
+101static void __exit vkbd_end(void) 
+102{ 
+103    vinput_unregister(&vkbd_dev); 
+104} 
+105 
+106module_init(vkbd_init); 
+107module_exit(vkbd_end); 
+108 
+109MODULE_LICENSE("GPL"); 
+110MODULE_DESCRIPTION("Emulate keyboard input events through /dev/vinput");
    +

    -

    18 Standardizing the interfaces: The Device Model

    -

    Up to this point we have seen all kinds of modules doing all kinds of things, but there +

    17 Standardizing the interfaces: The Device Model

    +

    Up to this point we have seen all kinds of modules doing all kinds of things, but there was no consistency in their interfaces with the rest of the kernel. To impose some consistency such that there is at minimum a standardized way to start, suspend and resume a device model was added. An example is shown below, and you can @@ -6396,111 +6090,111 @@ use this as a template to add your own suspend, resume or other interface functions.

    - 
    1/* 
-2 * devicemodel.c 
-3 */ 
-4#include <linux/kernel.h> 
-5#include <linux/module.h> 
-6#include <linux/platform_device.h> 
-7 
-8struct devicemodel_data { 
-9    char *greeting; 
-10    int number; 
-11}; 
-12 
-13static int devicemodel_probe(struct platform_device *dev) 
-14{ 
-15    struct devicemodel_data *pd = 
-16        (struct devicemodel_data *)(dev->dev.platform_data); 
-17 
-18    pr_info("devicemodel probe\n"); 
-19    pr_info("devicemodel greeting: %s; %d\n", pd->greeting, pd->number); 
-20 
-21    /* Your device initialization code */ 
-22 
-23    return 0; 
-24} 
-25 
-26static int devicemodel_remove(struct platform_device *dev) 
-27{ 
-28    pr_info("devicemodel example removed\n"); 
-29 
-30    /* Your device removal code */ 
-31 
-32    return 0; 
-33} 
-34 
-35static int devicemodel_suspend(struct device *dev) 
-36{ 
-37    pr_info("devicemodel example suspend\n"); 
-38 
-39    /* Your device suspend code */ 
-40 
-41    return 0; 
-42} 
-43 
-44static int devicemodel_resume(struct device *dev) 
-45{ 
-46    pr_info("devicemodel example resume\n"); 
-47 
-48    /* Your device resume code */ 
-49 
-50    return 0; 
-51} 
-52 
-53static const struct dev_pm_ops devicemodel_pm_ops = { 
-54    .suspend = devicemodel_suspend, 
-55    .resume = devicemodel_resume, 
-56    .poweroff = devicemodel_suspend, 
-57    .freeze = devicemodel_suspend, 
-58    .thaw = devicemodel_resume, 
-59    .restore = devicemodel_resume, 
-60}; 
-61 
-62static struct platform_driver devicemodel_driver = { 
-63    .driver = 
-64        { 
-65            .name = "devicemodel_example", 
-66            .pm = &devicemodel_pm_ops, 
-67        }, 
-68    .probe = devicemodel_probe, 
-69    .remove = devicemodel_remove, 
-70}; 
-71 
-72static int __init devicemodel_init(void) 
-73{ 
-74    int ret; 
-75 
-76    pr_info("devicemodel init\n"); 
-77 
-78    ret = platform_driver_register(&devicemodel_driver); 
-79 
-80    if (ret) { 
-81        pr_err("Unable to register driver\n"); 
-82        return ret; 
-83    } 
-84 
-85    return 0; 
-86} 
-87 
-88static void __exit devicemodel_exit(void) 
-89{ 
-90    pr_info("devicemodel exit\n"); 
-91    platform_driver_unregister(&devicemodel_driver); 
-92} 
-93 
-94module_init(devicemodel_init); 
-95module_exit(devicemodel_exit); 
-96 
-97MODULE_LICENSE("GPL"); 
-98MODULE_DESCRIPTION("Linux Device Model example");
    -

    + 

    1/* 
+2 * devicemodel.c 
+3 */ 
+4#include <linux/kernel.h> 
+5#include <linux/module.h> 
+6#include <linux/platform_device.h> 
+7 
+8struct devicemodel_data { 
+9    char *greeting; 
+10    int number; 
+11}; 
+12 
+13static int devicemodel_probe(struct platform_device *dev) 
+14{ 
+15    struct devicemodel_data *pd = 
+16        (struct devicemodel_data *)(dev->dev.platform_data); 
+17 
+18    pr_info("devicemodel probe\n"); 
+19    pr_info("devicemodel greeting: %s; %d\n", pd->greeting, pd->number); 
+20 
+21    /* Your device initialization code */ 
+22 
+23    return 0; 
+24} 
+25 
+26static int devicemodel_remove(struct platform_device *dev) 
+27{ 
+28    pr_info("devicemodel example removed\n"); 
+29 
+30    /* Your device removal code */ 
+31 
+32    return 0; 
+33} 
+34 
+35static int devicemodel_suspend(struct device *dev) 
+36{ 
+37    pr_info("devicemodel example suspend\n"); 
+38 
+39    /* Your device suspend code */ 
+40 
+41    return 0; 
+42} 
+43 
+44static int devicemodel_resume(struct device *dev) 
+45{ 
+46    pr_info("devicemodel example resume\n"); 
+47 
+48    /* Your device resume code */ 
+49 
+50    return 0; 
+51} 
+52 
+53static const struct dev_pm_ops devicemodel_pm_ops = { 
+54    .suspend = devicemodel_suspend, 
+55    .resume = devicemodel_resume, 
+56    .poweroff = devicemodel_suspend, 
+57    .freeze = devicemodel_suspend, 
+58    .thaw = devicemodel_resume, 
+59    .restore = devicemodel_resume, 
+60}; 
+61 
+62static struct platform_driver devicemodel_driver = { 
+63    .driver = 
+64        { 
+65            .name = "devicemodel_example", 
+66            .pm = &devicemodel_pm_ops, 
+67        }, 
+68    .probe = devicemodel_probe, 
+69    .remove = devicemodel_remove, 
+70}; 
+71 
+72static int __init devicemodel_init(void) 
+73{ 
+74    int ret; 
+75 
+76    pr_info("devicemodel init\n"); 
+77 
+78    ret = platform_driver_register(&devicemodel_driver); 
+79 
+80    if (ret) { 
+81        pr_err("Unable to register driver\n"); 
+82        return ret; 
+83    } 
+84 
+85    return 0; 
+86} 
+87 
+88static void __exit devicemodel_exit(void) 
+89{ 
+90    pr_info("devicemodel exit\n"); 
+91    platform_driver_unregister(&devicemodel_driver); 
+92} 
+93 
+94module_init(devicemodel_init); 
+95module_exit(devicemodel_exit); 
+96 
+97MODULE_LICENSE("GPL"); 
+98MODULE_DESCRIPTION("Linux Device Model example");
    +

    -

    19 Optimizations

    -

    +

    18 Optimizations

    +

    -

    19.1 Likely and Unlikely conditions

    -

    Sometimes you might want your code to run as quickly as possible, +

    18.1 Likely and Unlikely conditions

    +

    Sometimes you might want your code to run as quickly as possible, especially if it is handling an interrupt or doing something which might cause noticeable latency. If your code contains boolean conditions and if you know that the conditions are almost always likely to evaluate as either @@ -6512,40 +6206,40 @@ you know that the conditions are almost always likely to evaluate as either macros. For example, when allocating memory you are almost always expecting this to succeed.

    +

    + 
    1bvl = bvec_alloc(gfp_mask, nr_iovecs, &idx); 
+2if (unlikely(!bvl)) { 
+3    mempool_free(bio, bio_pool); 
+4    bio = NULL; 
+5    goto out; 
+6}
    -

    - 
    1bvl = bvec_alloc(gfp_mask, nr_iovecs, &idx); 
-2if (unlikely(!bvl)) { 
-3    mempool_free(bio, bio_pool); 
-4    bio = NULL; 
-5    goto out; 
-6}
    -

    When the unlikely +

    When the unlikely macro is used, the compiler alters its machine instruction output, so that it continues along the false branch and only jumps if the condition is true. That avoids flushing the processor pipeline. The opposite happens if you use the likely macro. -

    +

    -

    19.2 Static keys

    -

    Static keys allow us to enable or disable kernel code paths based on the runtime state +

    18.2 Static keys

    +

    Static keys allow us to enable or disable kernel code paths based on the runtime state of key. Its APIs have been available since 2010 (most architectures are already supported), use self-modifying code to eliminate the overhead of cache and branch prediction. The most typical use case of static keys is for performance-sensitive kernel code, such as tracepoints, context switching, networking, etc. These hot paths of the kernel often contain branches and can be optimized easily using this technique. Before we can use static keys in the kernel, we need to make sure that gcc supports - asm goto + asm goto inline assembly, and the following kernel configurations are set:

    - 
    1CONFIG_JUMP_LABEL=y 
-2CONFIG_HAVE_ARCH_JUMP_LABEL=y 
-3CONFIG_HAVE_ARCH_JUMP_LABEL_RELATIVE=y
    -

    To declare a static key, we need to define a global variable using the + 

    1CONFIG_JUMP_LABEL=y 
+2CONFIG_HAVE_ARCH_JUMP_LABEL=y 
+3CONFIG_HAVE_ARCH_JUMP_LABEL_RELATIVE=y
    +

    To declare a static key, we need to define a global variable using the DEFINE_STATIC_KEY_FALSE or DEFINE_STATIC_KEY_TRUE macro defined in include/linux/jump_label.h. This macro initializes the key with @@ -6554,268 +6248,268 @@ declare a static key with an initial value of false, we can use the following code:

    - 
    1DEFINE_STATIC_KEY_FALSE(fkey);
    -

    Once the static key has been declared, we need to add branching code to the + 

    1DEFINE_STATIC_KEY_FALSE(fkey);
    +

    Once the static key has been declared, we need to add branching code to the module that uses the static key. For example, the code includes a fastpath, where a no-op instruction will be generated at compile time as the key is initialized to false and the branch is unlikely to be taken.

    - 
    1pr_info("fastpath 1\n"); 
-2if (static_branch_unlikely(&fkey)) 
-3    pr_alert("do unlikely thing\n"); 
-4pr_info("fastpath 2\n");
    + 
    1pr_info("fastpath 1\n"); 
+2if (static_branch_unlikely(&fkey)) 
+3    pr_alert("do unlikely thing\n"); 
+4pr_info("fastpath 2\n");
    +

    If the key is enabled at runtime by calling + static_branch_enable(&fkey) +, the fastpath will be patched with an unconditional jump instruction to the slowpath -

    If the key is enabled at runtime by calling - static_branch_enable(&fkey) -, the fastpath will be patched with an unconditional jump instruction to the slowpath code pr_alert , so the branch will always be taken until the key is disabled again. -

    The following kernel module derived from chardev.c, demonstrates how the +

    The following kernel module derived from chardev.c, demonstrates how the static key works.

    - 
    1/* 
-2 * static_key.c 
-3 */ 
-4 
-5#include <linux/atomic.h> 
-6#include <linux/device.h> 
-7#include <linux/fs.h> 
-8#include <linux/kernel.h> /* for sprintf() */ 
-9#include <linux/module.h> 
-10#include <linux/printk.h> 
-11#include <linux/types.h> 
-12#include <linux/uaccess.h> /* for get_user and put_user */ 
-13#include <linux/jump_label.h> /* for static key macros */ 
-14 
-15#include <asm/errno.h> 
-16 
-17static int device_open(struct inode *inode, struct file *file); 
-18static int device_release(struct inode *inode, struct file *file); 
-19static ssize_t device_read(struct file *file, char __user *buf, size_t count, 
-20                           loff_t *ppos); 
-21static ssize_t device_write(struct file *file, const char __user *buf, 
-22                            size_t count, loff_t *ppos); 
-23 
-24#define SUCCESS 0 
-25#define DEVICE_NAME "key_state" 
-26#define BUF_LEN 10 
-27 
-28static int major; 
-29 
-30enum { 
-31    CDEV_NOT_USED = 0, 
-32    CDEV_EXCLUSIVE_OPEN = 1, 
-33}; 
-34 
-35static atomic_t already_open = ATOMIC_INIT(CDEV_NOT_USED); 
-36 
-37static char msg[BUF_LEN + 1]; 
-38 
-39static struct class *cls; 
-40 
-41static DEFINE_STATIC_KEY_FALSE(fkey); 
-42 
-43static struct file_operations chardev_fops = { 
-44    .owner = THIS_MODULE, 
-45    .open = device_open, 
-46    .release = device_release, 
-47    .read = device_read, 
-48    .write = device_write, 
-49}; 
-50 
-51static int __init chardev_init(void) 
-52{ 
-53    major = register_chrdev(0, DEVICE_NAME, &chardev_fops); 
-54    if (major < 0) { 
-55        pr_alert("Registering char device failed with %d\n", major); 
-56        return major; 
-57    } 
-58 
-59    pr_info("I was assigned major number %d\n", major); 
-60 
-61    cls = class_create(THIS_MODULE, DEVICE_NAME); 
-62 
-63    device_create(cls, NULL, MKDEV(major, 0), NULL, DEVICE_NAME); 
-64 
-65    pr_info("Device created on /dev/%s\n", DEVICE_NAME); 
-66 
-67    return SUCCESS; 
-68} 
-69 
-70static void __exit chardev_exit(void) 
-71{ 
-72    device_destroy(cls, MKDEV(major, 0)); 
-73    class_destroy(cls); 
-74 
-75    /* Unregister the device */ 
-76    unregister_chrdev(major, DEVICE_NAME); 
-77} 
-78 
-79/* Methods */ 
-80 
-81/** 
-82 * Called when a process tried to open the device file, like 
-83 * cat /dev/key_state 
-84 */ 
-85static int device_open(struct inode *inode, struct file *file) 
-86{ 
-87    if (atomic_cmpxchg(&already_open, CDEV_NOT_USED, CDEV_EXCLUSIVE_OPEN)) 
-88        return -EBUSY; 
-89 
-90    sprintf(msg, static_key_enabled(&fkey) ? "enabled\n" : "disabled\n"); 
-91 
-92    pr_info("fastpath 1\n"); 
-93    if (static_branch_unlikely(&fkey)) 
-94        pr_alert("do unlikely thing\n"); 
-95    pr_info("fastpath 2\n"); 
-96 
-97    try_module_get(THIS_MODULE); 
-98 
-99    return SUCCESS; 
-100} 
-101 
-102/** 
-103 * Called when a process closes the device file 
-104 */ 
-105static int device_release(struct inode *inode, struct file *file) 
-106{ 
-107    /* We are now ready for our next caller. */ 
-108    atomic_set(&already_open, CDEV_NOT_USED); 
-109 
-110    /** 
-111     * Decrement the usage count, or else once you opened the file, you will 
-112     * never get rid of the module. 
-113     */ 
-114    module_put(THIS_MODULE); 
-115 
-116    return SUCCESS; 
-117} 
-118 
-119/** 
-120 * Called when a process, which already opened the dev file, attempts to 
-121 * read from it. 
-122 */ 
-123static ssize_t device_read(struct file *filp, /* see include/linux/fs.h */ 
-124                           char __user *buffer, /* buffer to fill with data */ 
-125                           size_t length, /* length of the buffer */ 
-126                           loff_t *offset) 
-127{ 
-128    /* Number of the bytes actually written to the buffer */ 
-129    int bytes_read = 0; 
-130    const char *msg_ptr = msg; 
-131 
-132    if (!*(msg_ptr + *offset)) { /* We are at the end of the message */ 
-133        *offset = 0; /* reset the offset */ 
-134        return 0; /* signify end of file */ 
-135    } 
-136 
-137    msg_ptr += *offset; 
-138 
-139    /* Actually put the data into the buffer */ 
-140    while (length && *msg_ptr) { 
-141        /** 
-142         * The buffer is in the user data segment, not the kernel 
-143         * segment so "*" assignment won't work. We have to use 
-144         * put_user which copies data from the kernel data segment to 
-145         * the user data segment. 
-146         */ 
-147        put_user(*(msg_ptr++), buffer++); 
-148        length--; 
-149        bytes_read++; 
-150    } 
-151 
-152    *offset += bytes_read; 
-153 
-154    /* Most read functions return the number of bytes put into the buffer. */ 
-155    return bytes_read; 
-156} 
-157 
-158/* Called when a process writes to dev file; echo "enable" > /dev/key_state */ 
-159static ssize_t device_write(struct file *filp, const char __user *buffer, 
-160                            size_t length, loff_t *offset) 
-161{ 
-162    char command[10]; 
-163 
-164    if (length > 10) { 
-165        pr_err("command exceeded 10 char\n"); 
-166        return -EINVAL; 
-167    } 
-168 
-169    if (copy_from_user(command, buffer, length)) 
-170        return -EFAULT; 
-171 
-172    if (strncmp(command, "enable", strlen("enable")) == 0) 
-173        static_branch_enable(&fkey); 
-174    else if (strncmp(command, "disable", strlen("disable")) == 0) 
-175        static_branch_disable(&fkey); 
-176    else { 
-177        pr_err("Invalid command: %s\n", command); 
-178        return -EINVAL; 
-179    } 
-180 
-181    /* Again, return the number of input characters used. */ 
-182    return length; 
-183} 
-184 
-185module_init(chardev_init); 
-186module_exit(chardev_exit); 
-187 
-188MODULE_LICENSE("GPL");
    -

    To check the state of the static key, we can use the /dev/key_state + 

    1/* 
+2 * static_key.c 
+3 */ 
+4 
+5#include <linux/atomic.h> 
+6#include <linux/device.h> 
+7#include <linux/fs.h> 
+8#include <linux/kernel.h> /* for sprintf() */ 
+9#include <linux/module.h> 
+10#include <linux/printk.h> 
+11#include <linux/types.h> 
+12#include <linux/uaccess.h> /* for get_user and put_user */ 
+13#include <linux/jump_label.h> /* for static key macros */ 
+14 
+15#include <asm/errno.h> 
+16 
+17static int device_open(struct inode *inode, struct file *file); 
+18static int device_release(struct inode *inode, struct file *file); 
+19static ssize_t device_read(struct file *file, char __user *buf, size_t count, 
+20                           loff_t *ppos); 
+21static ssize_t device_write(struct file *file, const char __user *buf, 
+22                            size_t count, loff_t *ppos); 
+23 
+24#define SUCCESS 0 
+25#define DEVICE_NAME "key_state" 
+26#define BUF_LEN 10 
+27 
+28static int major; 
+29 
+30enum { 
+31    CDEV_NOT_USED = 0, 
+32    CDEV_EXCLUSIVE_OPEN = 1, 
+33}; 
+34 
+35static atomic_t already_open = ATOMIC_INIT(CDEV_NOT_USED); 
+36 
+37static char msg[BUF_LEN + 1]; 
+38 
+39static struct class *cls; 
+40 
+41static DEFINE_STATIC_KEY_FALSE(fkey); 
+42 
+43static struct file_operations chardev_fops = { 
+44    .owner = THIS_MODULE, 
+45    .open = device_open, 
+46    .release = device_release, 
+47    .read = device_read, 
+48    .write = device_write, 
+49}; 
+50 
+51static int __init chardev_init(void) 
+52{ 
+53    major = register_chrdev(0, DEVICE_NAME, &chardev_fops); 
+54    if (major < 0) { 
+55        pr_alert("Registering char device failed with %d\n", major); 
+56        return major; 
+57    } 
+58 
+59    pr_info("I was assigned major number %d\n", major); 
+60 
+61    cls = class_create(THIS_MODULE, DEVICE_NAME); 
+62 
+63    device_create(cls, NULL, MKDEV(major, 0), NULL, DEVICE_NAME); 
+64 
+65    pr_info("Device created on /dev/%s\n", DEVICE_NAME); 
+66 
+67    return SUCCESS; 
+68} 
+69 
+70static void __exit chardev_exit(void) 
+71{ 
+72    device_destroy(cls, MKDEV(major, 0)); 
+73    class_destroy(cls); 
+74 
+75    /* Unregister the device */ 
+76    unregister_chrdev(major, DEVICE_NAME); 
+77} 
+78 
+79/* Methods */ 
+80 
+81/** 
+82 * Called when a process tried to open the device file, like 
+83 * cat /dev/key_state 
+84 */ 
+85static int device_open(struct inode *inode, struct file *file) 
+86{ 
+87    if (atomic_cmpxchg(&already_open, CDEV_NOT_USED, CDEV_EXCLUSIVE_OPEN)) 
+88        return -EBUSY; 
+89 
+90    sprintf(msg, static_key_enabled(&fkey) ? "enabled\n" : "disabled\n"); 
+91 
+92    pr_info("fastpath 1\n"); 
+93    if (static_branch_unlikely(&fkey)) 
+94        pr_alert("do unlikely thing\n"); 
+95    pr_info("fastpath 2\n"); 
+96 
+97    try_module_get(THIS_MODULE); 
+98 
+99    return SUCCESS; 
+100} 
+101 
+102/** 
+103 * Called when a process closes the device file 
+104 */ 
+105static int device_release(struct inode *inode, struct file *file) 
+106{ 
+107    /* We are now ready for our next caller. */ 
+108    atomic_set(&already_open, CDEV_NOT_USED); 
+109 
+110    /** 
+111     * Decrement the usage count, or else once you opened the file, you will 
+112     * never get rid of the module. 
+113     */ 
+114    module_put(THIS_MODULE); 
+115 
+116    return SUCCESS; 
+117} 
+118 
+119/** 
+120 * Called when a process, which already opened the dev file, attempts to 
+121 * read from it. 
+122 */ 
+123static ssize_t device_read(struct file *filp, /* see include/linux/fs.h */ 
+124                           char __user *buffer, /* buffer to fill with data */ 
+125                           size_t length, /* length of the buffer */ 
+126                           loff_t *offset) 
+127{ 
+128    /* Number of the bytes actually written to the buffer */ 
+129    int bytes_read = 0; 
+130    const char *msg_ptr = msg; 
+131 
+132    if (!*(msg_ptr + *offset)) { /* We are at the end of the message */ 
+133        *offset = 0; /* reset the offset */ 
+134        return 0; /* signify end of file */ 
+135    } 
+136 
+137    msg_ptr += *offset; 
+138 
+139    /* Actually put the data into the buffer */ 
+140    while (length && *msg_ptr) { 
+141        /** 
+142         * The buffer is in the user data segment, not the kernel 
+143         * segment so "*" assignment won't work. We have to use 
+144         * put_user which copies data from the kernel data segment to 
+145         * the user data segment. 
+146         */ 
+147        put_user(*(msg_ptr++), buffer++); 
+148        length--; 
+149        bytes_read++; 
+150    } 
+151 
+152    *offset += bytes_read; 
+153 
+154    /* Most read functions return the number of bytes put into the buffer. */ 
+155    return bytes_read; 
+156} 
+157 
+158/* Called when a process writes to dev file; echo "enable" > /dev/key_state */ 
+159static ssize_t device_write(struct file *filp, const char __user *buffer, 
+160                            size_t length, loff_t *offset) 
+161{ 
+162    char command[10]; 
+163 
+164    if (length > 10) { 
+165        pr_err("command exceeded 10 char\n"); 
+166        return -EINVAL; 
+167    } 
+168 
+169    if (copy_from_user(command, buffer, length)) 
+170        return -EFAULT; 
+171 
+172    if (strncmp(command, "enable", strlen("enable")) == 0) 
+173        static_branch_enable(&fkey); 
+174    else if (strncmp(command, "disable", strlen("disable")) == 0) 
+175        static_branch_disable(&fkey); 
+176    else { 
+177        pr_err("Invalid command: %s\n", command); 
+178        return -EINVAL; 
+179    } 
+180 
+181    /* Again, return the number of input characters used. */ 
+182    return length; 
+183} 
+184 
+185module_init(chardev_init); 
+186module_exit(chardev_exit); 
+187 
+188MODULE_LICENSE("GPL");
    +

    To check the state of the static key, we can use the /dev/key_state interface.

    - 
    1cat /dev/key_state
    -

    This will display the current state of the key, which is disabled by default. -

    To change the state of the static key, we can perform a write operation on the + 

    1cat /dev/key_state
    +

    This will display the current state of the key, which is disabled by default. +

    To change the state of the static key, we can perform a write operation on the file:

    - 
    1echo enable > /dev/key_state
    -

    This will enable the static key, causing the code path to switch from the fastpath + 

    1echo enable > /dev/key_state
    +

    This will enable the static key, causing the code path to switch from the fastpath to the slowpath. -

    In some cases, the key is enabled or disabled at initialization and never changed, +

    In some cases, the key is enabled or disabled at initialization and never changed, we can declare a static key as read-only, which means that it can only be toggled in the module init function. To declare a read-only static key, we can use the DEFINE_STATIC_KEY_FALSE_RO or DEFINE_STATIC_KEY_TRUE_RO macro instead. Attempts to change the key at runtime will result in a page fault. For more information, see Static keys -

    +

    -

    20 Common Pitfalls

    -

    +

    19 Common Pitfalls

    +

    +

    19.1 Using standard libraries

    +

    You can not do that. In a kernel module, you can only use kernel functions which are +the functions you can see in /proc/kallsyms. -

    20.1 Using standard libraries

    -

    You can not do that. In a kernel module, you can only use kernel functions which are -the functions you can see in /proc/kallsyms. -

    +

    -

    20.2 Disabling interrupts

    -

    You might need to do this for a short time and that is OK, but if you do not enable +

    19.2 Disabling interrupts

    +

    You might need to do this for a short time and that is OK, but if you do not enable them afterwards, your system will be stuck and you will have to power it off. -

    +

    -

    21 Where To Go From Here?

    -

    For those deeply interested in kernel programming, kernelnewbies.org and the +

    20 Where To Go From Here?

    +

    For those deeply interested in kernel programming, kernelnewbies.org and the Documentation subdirectory within the kernel source code are highly recommended. Although the latter may not always be straightforward, it serves as a valuable initial step for further exploration. Echoing Linus Torvalds’ perspective, the most effective method to understand the kernel is through personal examination of the source code. -

    Contributions to this guide are welcome, especially if there are any significant +

    Contributions to this guide are welcome, especially if there are any significant inaccuracies identified. To contribute or report an issue, please initiate an issue at https://github.com/sysprog21/lkmpg. Pull requests are greatly appreciated. -

    Happy hacking! +

    Happy hacking!

    1The goal of threaded interrupts is to push more of the work to separate threads, so that the minimum needed for acknowledging an interrupt is reduced, and therefore the time spent handling diff --git a/lkmpg-for-ht.css b/lkmpg-for-ht.css index 6b00ff7..956dc50 100644 --- a/lkmpg-for-ht.css +++ b/lkmpg-for-ht.css @@ -3409,784 +3409,761 @@ pre#fancyvrb82{ border-top: solid 0.4pt; } pre#fancyvrb82{ border-left: solid 0.4pt; } pre#fancyvrb82{ border-bottom: solid 0.4pt; } pre#fancyvrb82{ border-right: solid 0.4pt; } -span#textcolor2789{color:rgb(0,127,0)} -span#textcolor2790{color:rgb(0,127,0)} -span#textcolor2791{color:rgb(0,127,0)} +span#textcolor2789{color:rgb(43,145,175)} +span#textcolor2790{color:rgb(0,0,255)} +span#textcolor2791{color:rgb(0,0,255)} span#textcolor2792{color:rgb(0,0,255)} -span#textcolor2793{color:rgb(0,127,0)} -span#textcolor2794{color:rgb(0,0,255)} -span#textcolor2795{color:rgb(0,127,0)} -span#textcolor2796{color:rgb(0,0,255)} 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      15.1 Interrupt Handlers
      15.2 Detecting button presses
      15.3 Bottom Half -
     16 Crypto -
      16.1 Hash functions -
      16.2 Symmetric key encryption -
     17 Virtual Input Device Driver -
     18 Standardizing the interfaces: The Device Model -
     19 Optimizations -
      19.1 Likely and Unlikely conditions -
      19.2 Static keys -
     20 Common Pitfalls -
      20.1 Using standard libraries -
      20.2 Disabling interrupts -
     21 Where To Go From Here? +
     16 Virtual Input Device Driver +
     17 Standardizing the interfaces: The Device Model +
     18 Optimizations +
      18.1 Likely and Unlikely conditions +
      18.2 Static keys +
     19 Common Pitfalls +
      19.1 Using standard libraries +
      19.2 Disabling interrupts +
     20 Where To Go From Here?

    1 Introduction

    The Linux Kernel Module Programming Guide is a free book; you may reproduce @@ -111,14 +108,14 @@ the provisions of the Open Sof distribute this book free of charge or for a profit. No explicit permission is required from the author for reproduction of this book in any medium, physical or electronic. - - -

    Derivative works and translations of this document must be placed under the Open Software License, and the original copyright notice must remain intact. If you have contributed new material to this book, you must make the material and source code available for your revisions. Please make revisions and updates available directly to the document maintainer, Jim Huang <jserv@ccns.ncku.edu.tw>. This will allow + + + for the merging of updates and provide consistent revisions to the Linux community.

    If you publish or distribute this book commercially, donations, royalties, and/or @@ -153,13 +150,13 @@ Scarpazza, David Porter, demonsome, Dimo Velev, Ekang Monyet, Ethan Chan, fennecJ, Francois Audeon, Gilad Reti, heartofrain, Horst Schirmeier, Hsin-Hsiang Peng, Ignacio Martin, Iûnn Kiàn-îng, Jian-Xing Wu, Johan Calle, keytouch, Kohei Otsuka, Kuan-Wei Chiu, manbing, Marconi Jiang, - - - mengxinayan, Peter Lin, Roman Lakeev, Sam Erickson, Shao-Tse Hung, Shih-Sheng Yang, Stacy Prowell, Steven Lung, Tristan Lelong, Tse-Wei Lin, Tucker Polomik, Tyler Fanelli, VxTeemo, Wei-Lun Tsai, Xatierlike Lee, Yin-Chiuan Chen, Yi-Wei Lin, Ylowy, Yu-Hsiang Tseng.

    + + +

    1.3 What Is A Kernel Module?

    @@ -192,11 +189,11 @@ desired.

    On Arch Linux: 

    1sudo pacman -S gcc kmod
    +

    +

    -

    -

    1.5 What Modules are in my Kernel?

    To discover what modules are already loaded within your current kernel use the command lsmod @@ -5403,314 +5400,8 @@ when an interrupt is triggered. 168MODULE_DESCRIPTION("Interrupt with top and bottom half");

    -

    16 Crypto

    -

    At the dawn of the internet, everybody trusted everybody completely…but that did -not work out so well. When this guide was originally written, it was a more innocent -era in which almost nobody actually gave a damn about crypto - least of all kernel -developers. That is certainly no longer the case now. To handle crypto stuff, the -kernel has its own API enabling common methods of encryption, decryption and your -favourite hash functions. -

    -

    -

    16.1 Hash functions

    -

    Calculating and checking the hashes of things is a common operation. -Here is a demonstration of how to calculate a sha256 hash within a -kernel module. To provide the sha256 algorithm support, make sure - CONFIG_CRYPTO_SHA256 - is enabled in kernel. -

    -

    - 
    1/* 
-2 * cryptosha256.c 
-3 */ 
-4#include <crypto/internal/hash.h> 
-5#include <linux/module.h> 
-6 
-7#define SHA256_LENGTH 32 
-8 
-9static void show_hash_result(char *plaintext, char *hash_sha256) 
-10{ 
-11    int i; 
-12    char str[SHA256_LENGTH * 2 + 1]; 
-13 
-14    pr_info("sha256 test for string: \"%s\"\n", plaintext); 
-15    for (i = 0; i < SHA256_LENGTH; i++) 
-16        sprintf(&str[i * 2], "%02x", (unsigned char)hash_sha256[i]); 
-17    str[i * 2] = 0; 
-18    pr_info("%s\n", str); 
-19} 
-20 
-21static int __init cryptosha256_init(void) 
-22{ 
-23    char *plaintext = "This is a test"; 
-24    char hash_sha256[SHA256_LENGTH]; 
-25    struct crypto_shash *sha256; 
-26    struct shash_desc *shash; 
-27 
-28    sha256 = crypto_alloc_shash("sha256", 0, 0); 
-29    if (IS_ERR(sha256)) { 
-30        pr_err( 
-31            "%s(): Failed to allocate sha256 algorithm, enable CONFIG_CRYPTO_SHA256 and try again.\n", 
-32            __func__); 
-33        return -1; 
-34    } 
-35 
-36    shash = kmalloc(sizeof(struct shash_desc) + crypto_shash_descsize(sha256), 
-37                    GFP_KERNEL); 
-38    if (!shash) 
-39        return -ENOMEM; 
-40 
-41    shash->tfm = sha256; 
-42 
-43    if (crypto_shash_init(shash)) 
-44        return -1; 
-45 
-46    if (crypto_shash_update(shash, plaintext, strlen(plaintext))) 
-47        return -1; 
-48 
-49    if (crypto_shash_final(shash, hash_sha256)) 
-50        return -1; 
-51 
-52    kfree(shash); 
-53    crypto_free_shash(sha256); 
-54 
-55    show_hash_result(plaintext, hash_sha256); 
-56 
-57    return 0; 
-58} 
-59 
-60static void __exit cryptosha256_exit(void) 
-61{ 
-62} 
-63 
-64module_init(cryptosha256_init); 
-65module_exit(cryptosha256_exit); 
-66 
-67MODULE_DESCRIPTION("sha256 hash test"); 
-68MODULE_LICENSE("GPL");
    -

    Install the module: -

    -

    - 
    1sudo insmod cryptosha256.ko 
-2sudo dmesg
    -

    And you should see that the hash was calculated for the test string. -

    Finally, remove the test module: -

    - - - -

    - 
    1sudo rmmod cryptosha256
    -

    -

    -

    16.2 Symmetric key encryption

    -

    Here is an example of symmetrically encrypting a string using the AES algorithm -and a password. -

    -

    - 
    1/* 
-2 * cryptosk.c 
-3 */ 
-4#include <crypto/internal/skcipher.h> 
-5#include <linux/crypto.h> 
-6#include <linux/module.h> 
-7#include <linux/random.h> 
-8#include <linux/scatterlist.h> 
-9 
-10#define SYMMETRIC_KEY_LENGTH 32 
-11#define CIPHER_BLOCK_SIZE 16 
-12 
-13struct tcrypt_result { 
-14    struct completion completion; 
-15    int err; 
-16}; 
-17 
-18struct skcipher_def { 
-19    struct scatterlist sg; 
-20    struct crypto_skcipher *tfm; 
-21    struct skcipher_request *req; 
-22    struct tcrypt_result result; 
-23    char *scratchpad; 
-24    char *ciphertext; 
-25    char *ivdata; 
-26}; 
-27 
-28static struct skcipher_def sk; 
-29 
-30static void test_skcipher_finish(struct skcipher_def *sk) 
-31{ 
-32    if (sk->tfm) 
-33        crypto_free_skcipher(sk->tfm); 
-34    if (sk->req) 
-35        skcipher_request_free(sk->req); 
-36    if (sk->ivdata) 
-37        kfree(sk->ivdata); 
-38    if (sk->scratchpad) 
-39        kfree(sk->scratchpad); 
-40    if (sk->ciphertext) 
-41        kfree(sk->ciphertext); 
-42} 
-43 
-44static int test_skcipher_result(struct skcipher_def *sk, int rc) 
-45{ 
-46    switch (rc) { 
-47    case 0: 
-48        break; 
-49    case -EINPROGRESS: 
-50    case -EBUSY: 
-51        rc = wait_for_completion_interruptible(&sk->result.completion); 
-52        if (!rc && !sk->result.err) { 
-53            reinit_completion(&sk->result.completion); 
-54            break; 
-55        } 
-56        pr_info("skcipher encrypt returned with %d result %d\n", rc, 
-57                sk->result.err); 
-58        break; 
-59    default: 
-60        pr_info("skcipher encrypt returned with %d result %d\n", rc, 
-61                sk->result.err); 
-62        break; 
-63    } 
-64 
-65    init_completion(&sk->result.completion); 
-66 
-67    return rc; 
-68} 
-69 
-70static void test_skcipher_callback(struct crypto_async_request *req, int error) 
-71{ 
-72    struct tcrypt_result *result = req->data; 
-73 
-74    if (error == -EINPROGRESS) 
-75        return; 
-76 
-77    result->err = error; 
-78    complete(&result->completion); 
-79    pr_info("Encryption finished successfully\n"); 
-80 
-81    /* decrypt data */ 
-82#if 0 
-83    memset((void*)sk.scratchpad, '-', CIPHER_BLOCK_SIZE); 
-84    ret = crypto_skcipher_decrypt(sk.req); 
-85    ret = test_skcipher_result(&sk, ret); 
-86    if (ret) 
-87        return; 
-88 
-89    sg_copy_from_buffer(&sk.sg, 1, sk.scratchpad, CIPHER_BLOCK_SIZE); 
-90    sk.scratchpad[CIPHER_BLOCK_SIZE-1] = 0; 
-91 
-92    pr_info("Decryption request successful\n"); 
-93    pr_info("Decrypted: %s\n", sk.scratchpad); 
-94#endif 
-95} 
-96 
-97static int test_skcipher_encrypt(char *plaintext, char *password, 
-98                                 struct skcipher_def *sk) 
-99{ 
-100    int ret = -EFAULT; 
-101    unsigned char key[SYMMETRIC_KEY_LENGTH]; 
-102 
-103    if (!sk->tfm) { 
-104        sk->tfm = crypto_alloc_skcipher("cbc-aes-aesni", 0, 0); 
-105        if (IS_ERR(sk->tfm)) { 
-106            pr_info("could not allocate skcipher handle\n"); 
-107            return PTR_ERR(sk->tfm); 
-108        } 
-109    } 
-110 
-111    if (!sk->req) { 
-112        sk->req = skcipher_request_alloc(sk->tfm, GFP_KERNEL); 
-113        if (!sk->req) { 
-114            pr_info("could not allocate skcipher request\n"); 
-115            ret = -ENOMEM; 
-116            goto out; 
-117        } 
-118    } 
-119 
-120    skcipher_request_set_callback(sk->req, CRYPTO_TFM_REQ_MAY_BACKLOG, 
-121                                  test_skcipher_callback, &sk->result); 
-122 
-123    /* clear the key */ 
-124    memset((void *)key, '\0', SYMMETRIC_KEY_LENGTH); 
-125 
-126    /* Use the world's favourite password */ 
-127    sprintf((char *)key, "%s", password); 
-128 
-129    /* AES 256 with given symmetric key */ 
-130    if (crypto_skcipher_setkey(sk->tfm, key, SYMMETRIC_KEY_LENGTH)) { 
-131        pr_info("key could not be set\n"); 
-132        ret = -EAGAIN; 
-133        goto out; 
-134    } 
-135    pr_info("Symmetric key: %s\n", key); 
-136    pr_info("Plaintext: %s\n", plaintext); 
-137 
-138    if (!sk->ivdata) { 
-139        /* see https://en.wikipedia.org/wiki/Initialization_vector */ 
-140        sk->ivdata = kmalloc(CIPHER_BLOCK_SIZE, GFP_KERNEL); 
-141        if (!sk->ivdata) { 
-142            pr_info("could not allocate ivdata\n"); 
-143            goto out; 
-144        } 
-145        get_random_bytes(sk->ivdata, CIPHER_BLOCK_SIZE); 
-146    } 
-147 
-148    if (!sk->scratchpad) { 
-149        /* The text to be encrypted */ 
-150        sk->scratchpad = kmalloc(CIPHER_BLOCK_SIZE, GFP_KERNEL); 
-151        if (!sk->scratchpad) { 
-152            pr_info("could not allocate scratchpad\n"); 
-153            goto out; 
-154        } 
-155    } 
-156    sprintf((char *)sk->scratchpad, "%s", plaintext); 
-157 
-158    sg_init_one(&sk->sg, sk->scratchpad, CIPHER_BLOCK_SIZE); 
-159    skcipher_request_set_crypt(sk->req, &sk->sg, &sk->sg, CIPHER_BLOCK_SIZE, 
-160                               sk->ivdata); 
-161    init_completion(&sk->result.completion); 
-162 
-163    /* encrypt data */ 
-164    ret = crypto_skcipher_encrypt(sk->req); 
-165    ret = test_skcipher_result(sk, ret); 
-166    if (ret) 
-167        goto out; 
-168 
-169    pr_info("Encryption request successful\n"); 
-170 
-171out: 
-172    return ret; 
-173} 
-174 
-175static int __init cryptoapi_init(void) 
-176{ 
-177    /* The world's favorite password */ 
-178    char *password = "password123"; 
-179 
-180    sk.tfm = NULL; 
-181    sk.req = NULL; 
-182    sk.scratchpad = NULL; 
-183    sk.ciphertext = NULL; 
-184    sk.ivdata = NULL; 
-185 
-186    test_skcipher_encrypt("Testing", password, &sk); 
-187    return 0; 
-188} 
-189 
-190static void __exit cryptoapi_exit(void) 
-191{ 
-192    test_skcipher_finish(&sk); 
-193} 
-194 
-195module_init(cryptoapi_init); 
-196module_exit(cryptoapi_exit); 
-197 
-198MODULE_DESCRIPTION("Symmetric key encryption example"); 
-199MODULE_LICENSE("GPL");
    -

    -

    -

    17 Virtual Input Device Driver

    -

    The input device driver is a module that provides a way to communicate +

    16 Virtual Input Device Driver

    +

    The input device driver is a module that provides a way to communicate with the interaction device via the event. For example, the keyboard can send the press or release event to tell the kernel what we want to do. The input device driver will allocate a new input structure with @@ -5718,7 +5409,7 @@ do. The input device driver will allocate a new input structure with and sets up input bitfields, device id, version, etc. After that, registers it by calling input_register_device() . -

    Here is an example, vinput, It is an API to allow easy +

    Here is an example, vinput, It is an API to allow easy development of virtual input drivers. The drivers needs to export a vinput_device() that contains the virtual device name and @@ -5731,527 +5422,530 @@ development of virtual input drivers. The drivers needs to export a

  • the input event injection function: send() + + +
  • the readback function: read()
    • -

    Then using vinput_register_device() +

    Then using vinput_register_device() and vinput_unregister_device() will add a new device to the list of support virtual input devices.

    - 
    1int init(struct vinput *);
    -

    This function is passed a struct vinput - already initialized with an allocated struct input_dev + 

    1int init(struct vinput *);
    +

    This function is passed a struct vinput + already initialized with an allocated struct input_dev . The init() function is responsible for initializing the capabilities of the input device and register it.

    - 
    1int send(struct vinput *, char *, int);
    -

    This function will receive a user string to interpret and inject the event using the + 

    1int send(struct vinput *, char *, int);
    +

    This function will receive a user string to interpret and inject the event using the input_report_XXXX or input_event call. The string is already copied from user.

    - 
    1int read(struct vinput *, char *, int);
    -

    This function is used for debugging and should fill the buffer parameter with the + 

    1int read(struct vinput *, char *, int);
    +

    This function is used for debugging and should fill the buffer parameter with the last event sent in the virtual input device format. The buffer will then be copied to user. -

    vinput devices are created and destroyed using sysfs. And, event injection is done +

    vinput devices are created and destroyed using sysfs. And, event injection is done through a /dev node. The device name will be used by the userland to export a new virtual input device. -

    The class_attribute +

    The class_attribute structure is similar to other attribute types we talked about in section 8:

    - 
    1struct class_attribute { 
-2    struct attribute attr; 
-3    ssize_t (*show)(struct class *class, struct class_attribute *attr, 
-4                    char *buf); 
-5    ssize_t (*store)(struct class *class, struct class_attribute *attr, 
-6                    const char *buf, size_t count); 
-7};
    - - - -

    In vinput.c, the macro CLASS_ATTR_WO(export/unexport) + 

    1struct class_attribute { 
+2    struct attribute attr; 
+3    ssize_t (*show)(struct class *class, struct class_attribute *attr, 
+4                    char *buf); 
+5    ssize_t (*store)(struct class *class, struct class_attribute *attr, 
+6                    const char *buf, size_t count); 
+7};
    +

    In vinput.c, the macro CLASS_ATTR_WO(export/unexport) defined in include/linux/device.h (in this case, device.h is included in include/linux/input.h) will generate the class_attribute structures which are named class_attr_export/unexport. Then, put them into + + + vinput_class_attrs array and the macro ATTRIBUTE_GROUPS(vinput_class) - will generate the struct attribute_group vinput_class_group + will generate the struct attribute_group vinput_class_group that should be assigned in vinput_class . Finally, call class_register(&vinput_class) to create attributes in sysfs. -

    To create a vinputX sysfs entry and /dev node. +

    To create a vinputX sysfs entry and /dev node.

    - 
    1echo "vkbd" | sudo tee /sys/class/vinput/export
    -

    To unexport the device, just echo its id in unexport: + 

    1echo "vkbd" | sudo tee /sys/class/vinput/export
    +

    To unexport the device, just echo its id in unexport:

    - 
    1echo "0" | sudo tee /sys/class/vinput/unexport
    + 
    1echo "0" | sudo tee /sys/class/vinput/unexport

    - 
    1/* 
-2 * vinput.h 
-3 */ 
-4 
-5#ifndef VINPUT_H 
-6#define VINPUT_H 
-7 
-8#include <linux/input.h> 
-9#include <linux/spinlock.h> 
-10 
-11#define VINPUT_MAX_LEN 128 
-12#define MAX_VINPUT 32 
-13#define VINPUT_MINORS MAX_VINPUT 
-14 
-15#define dev_to_vinput(dev) container_of(dev, struct vinput, dev) 
-16 
-17struct vinput_device; 
-18 
-19struct vinput { 
-20    long id; 
-21    long devno; 
-22    long last_entry; 
-23    spinlock_t lock; 
-24 
-25    void *priv_data; 
-26 
-27    struct device dev; 
-28    struct list_head list; 
-29    struct input_dev *input; 
-30    struct vinput_device *type; 
-31}; 
-32 
-33struct vinput_ops { 
-34    int (*init)(struct vinput *); 
-35    int (*kill)(struct vinput *); 
-36    int (*send)(struct vinput *, char *, int); 
-37    int (*read)(struct vinput *, char *, int); 
-38}; 
-39 
-40struct vinput_device { 
-41    char name[16]; 
-42    struct list_head list; 
-43    struct vinput_ops *ops; 
-44}; 
-45 
-46int vinput_register(struct vinput_device *dev); 
-47void vinput_unregister(struct vinput_device *dev); 
-48 
-49#endif
    + 
    1/* 
+2 * vinput.h 
+3 */ 
+4 
+5#ifndef VINPUT_H 
+6#define VINPUT_H 
+7 
+8#include <linux/input.h> 
+9#include <linux/spinlock.h> 
+10 
+11#define VINPUT_MAX_LEN 128 
+12#define MAX_VINPUT 32 
+13#define VINPUT_MINORS MAX_VINPUT 
+14 
+15#define dev_to_vinput(dev) container_of(dev, struct vinput, dev) 
+16 
+17struct vinput_device; 
+18 
+19struct vinput { 
+20    long id; 
+21    long devno; 
+22    long last_entry; 
+23    spinlock_t lock; 
+24 
+25    void *priv_data; 
+26 
+27    struct device dev; 
+28    struct list_head list; 
+29    struct input_dev *input; 
+30    struct vinput_device *type; 
+31}; 
+32 
+33struct vinput_ops { 
+34    int (*init)(struct vinput *); 
+35    int (*kill)(struct vinput *); 
+36    int (*send)(struct vinput *, char *, int); 
+37    int (*read)(struct vinput *, char *, int); 
+38}; 
+39 
+40struct vinput_device { 
+41    char name[16]; 
+42    struct list_head list; 
+43    struct vinput_ops *ops; 
+44}; 
+45 
+46int vinput_register(struct vinput_device *dev); 
+47void vinput_unregister(struct vinput_device *dev); 
+48 
+49#endif

    - 
    1/* 
-2 * vinput.c 
-3 */ 
-4 
-5#include <linux/cdev.h> 
-6#include <linux/input.h> 
-7#include <linux/module.h> 
-8#include <linux/slab.h> 
-9#include <linux/spinlock.h> 
-10 
-11#include <asm/uaccess.h> 
-12 
-13#include "vinput.h" 
-14 
-15#define DRIVER_NAME "vinput" 
-16 
-17#define dev_to_vinput(dev) container_of(dev, struct vinput, dev) 
-18 
-19static DECLARE_BITMAP(vinput_ids, VINPUT_MINORS); 
-20 
-21static LIST_HEAD(vinput_devices); 
-22static LIST_HEAD(vinput_vdevices); 
-23 
-24static int vinput_dev; 
-25static struct spinlock vinput_lock; 
-26static struct class vinput_class; 
-27 
-28/* Search the name of vinput device in the vinput_devices linked list, 
-29 * which added at vinput_register(). 
-30 */ 
-31static struct vinput_device *vinput_get_device_by_type(const char *type) 
-32{ 
-33    int found = 0; 
-34    struct vinput_device *vinput; 
-35    struct list_head *curr; 
-36 
-37    spin_lock(&vinput_lock); 
-38    list_for_each (curr, &vinput_devices) { 
-39        vinput = list_entry(curr, struct vinput_device, list); 
-40        if (vinput && strncmp(type, vinput->name, strlen(vinput->name)) == 0) { 
-41            found = 1; 
-42            break; 
-43        } 
-44    } 
-45    spin_unlock(&vinput_lock); 
-46 
-47    if (found) 
-48        return vinput; 
-49    return ERR_PTR(-ENODEV); 
-50} 
-51 
-52/* Search the id of virtual device in the vinput_vdevices linked list, 
-53 * which added at vinput_alloc_vdevice(). 
-54 */ 
-55static struct vinput *vinput_get_vdevice_by_id(long id) 
-56{ 
-57    struct vinput *vinput = NULL; 
-58    struct list_head *curr; 
-59 
-60    spin_lock(&vinput_lock); 
-61    list_for_each (curr, &vinput_vdevices) { 
-62        vinput = list_entry(curr, struct vinput, list); 
-63        if (vinput && vinput->id == id) 
-64            break; 
-65    } 
-66    spin_unlock(&vinput_lock); 
-67 
-68    if (vinput && vinput->id == id) 
-69        return vinput; 
-70    return ERR_PTR(-ENODEV); 
-71} 
-72 
-73static int vinput_open(struct inode *inode, struct file *file) 
-74{ 
-75    int err = 0; 
-76    struct vinput *vinput = NULL; 
-77 
-78    vinput = vinput_get_vdevice_by_id(iminor(inode)); 
-79 
-80    if (IS_ERR(vinput)) 
-81        err = PTR_ERR(vinput); 
-82    else 
-83        file->private_data = vinput; 
-84 
-85    return err; 
-86} 
-87 
-88static int vinput_release(struct inode *inode, struct file *file) 
-89{ 
-90    return 0; 
-91} 
-92 
-93static ssize_t vinput_read(struct file *file, char __user *buffer, size_t count, 
-94                           loff_t *offset) 
-95{ 
-96    int len; 
-97    char buff[VINPUT_MAX_LEN + 1]; 
-98    struct vinput *vinput = file->private_data; 
-99 
-100    len = vinput->type->ops->read(vinput, buff, count); 
-101 
-102    if (*offset > len) 
-103        count = 0; 
-104    else if (count + *offset > VINPUT_MAX_LEN) 
-105        count = len - *offset; 
-106 
-107    if (raw_copy_to_user(buffer, buff + *offset, count)) 
-108        count = -EFAULT; 
-109 
-110    *offset += count; 
-111 
-112    return count; 
-113} 
-114 
-115static ssize_t vinput_write(struct file *file, const char __user *buffer, 
-116                            size_t count, loff_t *offset) 
-117{ 
-118    char buff[VINPUT_MAX_LEN + 1]; 
-119    struct vinput *vinput = file->private_data; 
-120 
-121    memset(buff, 0, sizeof(char) * (VINPUT_MAX_LEN + 1)); 
-122 
-123    if (count > VINPUT_MAX_LEN) { 
-124        dev_warn(&vinput->dev, "Too long. %d bytes allowed\n", VINPUT_MAX_LEN); 
-125        return -EINVAL; 
-126    } 
-127 
-128    if (raw_copy_from_user(buff, buffer, count)) 
-129        return -EFAULT; 
-130 
-131    return vinput->type->ops->send(vinput, buff, count); 
-132} 
-133 
-134static const struct file_operations vinput_fops = { 
-135    .owner = THIS_MODULE, 
-136    .open = vinput_open, 
-137    .release = vinput_release, 
-138    .read = vinput_read, 
-139    .write = vinput_write, 
-140}; 
-141 
-142static void vinput_unregister_vdevice(struct vinput *vinput) 
-143{ 
-144    input_unregister_device(vinput->input); 
-145    if (vinput->type->ops->kill) 
-146        vinput->type->ops->kill(vinput); 
-147} 
-148 
-149static void vinput_destroy_vdevice(struct vinput *vinput) 
-150{ 
-151    /* Remove from the list first */ 
-152    spin_lock(&vinput_lock); 
-153    list_del(&vinput->list); 
-154    clear_bit(vinput->id, vinput_ids); 
-155    spin_unlock(&vinput_lock); 
-156 
-157    module_put(THIS_MODULE); 
-158 
-159    kfree(vinput); 
-160} 
-161 
-162static void vinput_release_dev(struct device *dev) 
-163{ 
-164    struct vinput *vinput = dev_to_vinput(dev); 
-165    int id = vinput->id; 
-166 
-167    vinput_destroy_vdevice(vinput); 
-168 
-169    pr_debug("released vinput%d.\n", id); 
-170} 
-171 
-172static struct vinput *vinput_alloc_vdevice(void) 
-173{ 
-174    int err; 
-175    struct vinput *vinput = kzalloc(sizeof(struct vinput), GFP_KERNEL); 
-176 
-177    try_module_get(THIS_MODULE); 
-178 
-179    memset(vinput, 0, sizeof(struct vinput)); 
-180 
-181    spin_lock_init(&vinput->lock); 
-182 
-183    spin_lock(&vinput_lock); 
-184    vinput->id = find_first_zero_bit(vinput_ids, VINPUT_MINORS); 
-185    if (vinput->id >= VINPUT_MINORS) { 
-186        err = -ENOBUFS; 
-187        goto fail_id; 
-188    } 
-189    set_bit(vinput->id, vinput_ids); 
-190    list_add(&vinput->list, &vinput_vdevices); 
-191    spin_unlock(&vinput_lock); 
-192 
-193    /* allocate the input device */ 
-194    vinput->input = input_allocate_device(); 
-195    if (vinput->input == NULL) { 
-196        pr_err("vinput: Cannot allocate vinput input device\n"); 
-197        err = -ENOMEM; 
-198        goto fail_input_dev; 
-199    } 
-200 
-201    /* initialize device */ 
-202    vinput->dev.class = &vinput_class; 
-203    vinput->dev.release = vinput_release_dev; 
-204    vinput->dev.devt = MKDEV(vinput_dev, vinput->id); 
-205    dev_set_name(&vinput->dev, DRIVER_NAME "%lu", vinput->id); 
-206 
-207    return vinput; 
-208 
-209fail_input_dev: 
-210    spin_lock(&vinput_lock); 
-211    list_del(&vinput->list); 
-212fail_id: 
-213    spin_unlock(&vinput_lock); 
-214    module_put(THIS_MODULE); 
-215    kfree(vinput); 
-216 
-217    return ERR_PTR(err); 
-218} 
-219 
-220static int vinput_register_vdevice(struct vinput *vinput) 
-221{ 
-222    int err = 0; 
-223 
-224    /* register the input device */ 
-225    vinput->input->name = vinput->type->name; 
-226    vinput->input->phys = "vinput"; 
-227    vinput->input->dev.parent = &vinput->dev; 
-228 
-229    vinput->input->id.bustype = BUS_VIRTUAL; 
-230    vinput->input->id.product = 0x0000; 
-231    vinput->input->id.vendor = 0x0000; 
-232    vinput->input->id.version = 0x0000; 
-233 
-234    err = vinput->type->ops->init(vinput); 
-235 
-236    if (err == 0) 
-237        dev_info(&vinput->dev, "Registered virtual input %s %ld\n", 
-238                 vinput->type->name, vinput->id); 
-239 
-240    return err; 
-241} 
-242 
-243static ssize_t export_store(struct class *class, struct class_attribute *attr, 
-244                            const char *buf, size_t len) 
-245{ 
-246    int err; 
-247    struct vinput *vinput; 
-248    struct vinput_device *device; 
-249 
-250    device = vinput_get_device_by_type(buf); 
-251    if (IS_ERR(device)) { 
-252        pr_info("vinput: This virtual device isn't registered\n"); 
-253        err = PTR_ERR(device); 
-254        goto fail; 
-255    } 
-256 
-257    vinput = vinput_alloc_vdevice(); 
-258    if (IS_ERR(vinput)) { 
-259        err = PTR_ERR(vinput); 
-260        goto fail; 
-261    } 
-262 
-263    vinput->type = device; 
-264    err = device_register(&vinput->dev); 
-265    if (err < 0) 
-266        goto fail_register; 
-267 
-268    err = vinput_register_vdevice(vinput); 
-269    if (err < 0) 
-270        goto fail_register_vinput; 
-271 
-272    return len; 
-273 
-274fail_register_vinput: 
-275    device_unregister(&vinput->dev); 
-276fail_register: 
-277    vinput_destroy_vdevice(vinput); 
-278fail: 
-279    return err; 
-280} 
-281/* This macro generates class_attr_export structure and export_store() */ 
-282static CLASS_ATTR_WO(export); 
-283 
-284static ssize_t unexport_store(struct class *class, struct class_attribute *attr, 
-285                              const char *buf, size_t len) 
-286{ 
-287    int err; 
-288    unsigned long id; 
-289    struct vinput *vinput; 
-290 
-291    err = kstrtol(buf, 10, &id); 
-292    if (err) { 
-293        err = -EINVAL; 
-294        goto failed; 
-295    } 
-296 
-297    vinput = vinput_get_vdevice_by_id(id); 
-298    if (IS_ERR(vinput)) { 
-299        pr_err("vinput: No such vinput device %ld\n", id); 
-300        err = PTR_ERR(vinput); 
-301        goto failed; 
-302    } 
-303 
-304    vinput_unregister_vdevice(vinput); 
-305    device_unregister(&vinput->dev); 
-306 
-307    return len; 
-308failed: 
-309    return err; 
-310} 
-311/* This macro generates class_attr_unexport structure and unexport_store() */ 
-312static CLASS_ATTR_WO(unexport); 
-313 
-314static struct attribute *vinput_class_attrs[] = { 
-315    &class_attr_export.attr, 
-316    &class_attr_unexport.attr, 
-317    NULL, 
-318}; 
-319 
-320/* This macro generates vinput_class_groups structure */ 
-321ATTRIBUTE_GROUPS(vinput_class); 
-322 
-323static struct class vinput_class = { 
-324    .name = "vinput", 
-325    .owner = THIS_MODULE, 
-326    .class_groups = vinput_class_groups, 
-327}; 
-328 
-329int vinput_register(struct vinput_device *dev) 
-330{ 
-331    spin_lock(&vinput_lock); 
-332    list_add(&dev->list, &vinput_devices); 
-333    spin_unlock(&vinput_lock); 
-334 
-335    pr_info("vinput: registered new virtual input device '%s'\n", dev->name); 
-336 
-337    return 0; 
-338} 
-339EXPORT_SYMBOL(vinput_register); 
-340 
-341void vinput_unregister(struct vinput_device *dev) 
-342{ 
-343    struct list_head *curr, *next; 
-344 
-345    /* Remove from the list first */ 
-346    spin_lock(&vinput_lock); 
-347    list_del(&dev->list); 
-348    spin_unlock(&vinput_lock); 
-349 
-350    /* unregister all devices of this type */ 
-351    list_for_each_safe (curr, next, &vinput_vdevices) { 
-352        struct vinput *vinput = list_entry(curr, struct vinput, list); 
-353        if (vinput && vinput->type == dev) { 
-354            vinput_unregister_vdevice(vinput); 
-355            device_unregister(&vinput->dev); 
-356        } 
-357    } 
-358 
-359    pr_info("vinput: unregistered virtual input device '%s'\n", dev->name); 
-360} 
-361EXPORT_SYMBOL(vinput_unregister); 
-362 
-363static int __init vinput_init(void) 
-364{ 
-365    int err = 0; 
-366 
-367    pr_info("vinput: Loading virtual input driver\n"); 
-368 
-369    vinput_dev = register_chrdev(0, DRIVER_NAME, &vinput_fops); 
-370    if (vinput_dev < 0) { 
-371        pr_err("vinput: Unable to allocate char dev region\n"); 
-372        err = vinput_dev; 
-373        goto failed_alloc; 
-374    } 
-375 
-376    spin_lock_init(&vinput_lock); 
-377 
-378    err = class_register(&vinput_class); 
-379    if (err < 0) { 
-380        pr_err("vinput: Unable to register vinput class\n"); 
-381        goto failed_class; 
-382    } 
-383 
-384    return 0; 
-385failed_class: 
-386    class_unregister(&vinput_class); 
-387failed_alloc: 
-388    return err; 
-389} 
-390 
-391static void __exit vinput_end(void) 
-392{ 
-393    pr_info("vinput: Unloading virtual input driver\n"); 
-394 
-395    unregister_chrdev(vinput_dev, DRIVER_NAME); 
-396    class_unregister(&vinput_class); 
-397} 
-398 
-399module_init(vinput_init); 
-400module_exit(vinput_end); 
-401 
-402MODULE_LICENSE("GPL"); 
-403MODULE_DESCRIPTION("Emulate input events");
    -

    Here the virtual keyboard is one of example to use vinput. It supports all + 

    1/* 
+2 * vinput.c 
+3 */ 
+4 
+5#include <linux/cdev.h> 
+6#include <linux/input.h> 
+7#include <linux/module.h> 
+8#include <linux/slab.h> 
+9#include <linux/spinlock.h> 
+10 
+11#include <asm/uaccess.h> 
+12 
+13#include "vinput.h" 
+14 
+15#define DRIVER_NAME "vinput" 
+16 
+17#define dev_to_vinput(dev) container_of(dev, struct vinput, dev) 
+18 
+19static DECLARE_BITMAP(vinput_ids, VINPUT_MINORS); 
+20 
+21static LIST_HEAD(vinput_devices); 
+22static LIST_HEAD(vinput_vdevices); 
+23 
+24static int vinput_dev; 
+25static struct spinlock vinput_lock; 
+26static struct class vinput_class; 
+27 
+28/* Search the name of vinput device in the vinput_devices linked list, 
+29 * which added at vinput_register(). 
+30 */ 
+31static struct vinput_device *vinput_get_device_by_type(const char *type) 
+32{ 
+33    int found = 0; 
+34    struct vinput_device *vinput; 
+35    struct list_head *curr; 
+36 
+37    spin_lock(&vinput_lock); 
+38    list_for_each (curr, &vinput_devices) { 
+39        vinput = list_entry(curr, struct vinput_device, list); 
+40        if (vinput && strncmp(type, vinput->name, strlen(vinput->name)) == 0) { 
+41            found = 1; 
+42            break; 
+43        } 
+44    } 
+45    spin_unlock(&vinput_lock); 
+46 
+47    if (found) 
+48        return vinput; 
+49    return ERR_PTR(-ENODEV); 
+50} 
+51 
+52/* Search the id of virtual device in the vinput_vdevices linked list, 
+53 * which added at vinput_alloc_vdevice(). 
+54 */ 
+55static struct vinput *vinput_get_vdevice_by_id(long id) 
+56{ 
+57    struct vinput *vinput = NULL; 
+58    struct list_head *curr; 
+59 
+60    spin_lock(&vinput_lock); 
+61    list_for_each (curr, &vinput_vdevices) { 
+62        vinput = list_entry(curr, struct vinput, list); 
+63        if (vinput && vinput->id == id) 
+64            break; 
+65    } 
+66    spin_unlock(&vinput_lock); 
+67 
+68    if (vinput && vinput->id == id) 
+69        return vinput; 
+70    return ERR_PTR(-ENODEV); 
+71} 
+72 
+73static int vinput_open(struct inode *inode, struct file *file) 
+74{ 
+75    int err = 0; 
+76    struct vinput *vinput = NULL; 
+77 
+78    vinput = vinput_get_vdevice_by_id(iminor(inode)); 
+79 
+80    if (IS_ERR(vinput)) 
+81        err = PTR_ERR(vinput); 
+82    else 
+83        file->private_data = vinput; 
+84 
+85    return err; 
+86} 
+87 
+88static int vinput_release(struct inode *inode, struct file *file) 
+89{ 
+90    return 0; 
+91} 
+92 
+93static ssize_t vinput_read(struct file *file, char __user *buffer, size_t count, 
+94                           loff_t *offset) 
+95{ 
+96    int len; 
+97    char buff[VINPUT_MAX_LEN + 1]; 
+98    struct vinput *vinput = file->private_data; 
+99 
+100    len = vinput->type->ops->read(vinput, buff, count); 
+101 
+102    if (*offset > len) 
+103        count = 0; 
+104    else if (count + *offset > VINPUT_MAX_LEN) 
+105        count = len - *offset; 
+106 
+107    if (raw_copy_to_user(buffer, buff + *offset, count)) 
+108        count = -EFAULT; 
+109 
+110    *offset += count; 
+111 
+112    return count; 
+113} 
+114 
+115static ssize_t vinput_write(struct file *file, const char __user *buffer, 
+116                            size_t count, loff_t *offset) 
+117{ 
+118    char buff[VINPUT_MAX_LEN + 1]; 
+119    struct vinput *vinput = file->private_data; 
+120 
+121    memset(buff, 0, sizeof(char) * (VINPUT_MAX_LEN + 1)); 
+122 
+123    if (count > VINPUT_MAX_LEN) { 
+124        dev_warn(&vinput->dev, "Too long. %d bytes allowed\n", VINPUT_MAX_LEN); 
+125        return -EINVAL; 
+126    } 
+127 
+128    if (raw_copy_from_user(buff, buffer, count)) 
+129        return -EFAULT; 
+130 
+131    return vinput->type->ops->send(vinput, buff, count); 
+132} 
+133 
+134static const struct file_operations vinput_fops = { 
+135    .owner = THIS_MODULE, 
+136    .open = vinput_open, 
+137    .release = vinput_release, 
+138    .read = vinput_read, 
+139    .write = vinput_write, 
+140}; 
+141 
+142static void vinput_unregister_vdevice(struct vinput *vinput) 
+143{ 
+144    input_unregister_device(vinput->input); 
+145    if (vinput->type->ops->kill) 
+146        vinput->type->ops->kill(vinput); 
+147} 
+148 
+149static void vinput_destroy_vdevice(struct vinput *vinput) 
+150{ 
+151    /* Remove from the list first */ 
+152    spin_lock(&vinput_lock); 
+153    list_del(&vinput->list); 
+154    clear_bit(vinput->id, vinput_ids); 
+155    spin_unlock(&vinput_lock); 
+156 
+157    module_put(THIS_MODULE); 
+158 
+159    kfree(vinput); 
+160} 
+161 
+162static void vinput_release_dev(struct device *dev) 
+163{ 
+164    struct vinput *vinput = dev_to_vinput(dev); 
+165    int id = vinput->id; 
+166 
+167    vinput_destroy_vdevice(vinput); 
+168 
+169    pr_debug("released vinput%d.\n", id); 
+170} 
+171 
+172static struct vinput *vinput_alloc_vdevice(void) 
+173{ 
+174    int err; 
+175    struct vinput *vinput = kzalloc(sizeof(struct vinput), GFP_KERNEL); 
+176 
+177    try_module_get(THIS_MODULE); 
+178 
+179    memset(vinput, 0, sizeof(struct vinput)); 
+180 
+181    spin_lock_init(&vinput->lock); 
+182 
+183    spin_lock(&vinput_lock); 
+184    vinput->id = find_first_zero_bit(vinput_ids, VINPUT_MINORS); 
+185    if (vinput->id >= VINPUT_MINORS) { 
+186        err = -ENOBUFS; 
+187        goto fail_id; 
+188    } 
+189    set_bit(vinput->id, vinput_ids); 
+190    list_add(&vinput->list, &vinput_vdevices); 
+191    spin_unlock(&vinput_lock); 
+192 
+193    /* allocate the input device */ 
+194    vinput->input = input_allocate_device(); 
+195    if (vinput->input == NULL) { 
+196        pr_err("vinput: Cannot allocate vinput input device\n"); 
+197        err = -ENOMEM; 
+198        goto fail_input_dev; 
+199    } 
+200 
+201    /* initialize device */ 
+202    vinput->dev.class = &vinput_class; 
+203    vinput->dev.release = vinput_release_dev; 
+204    vinput->dev.devt = MKDEV(vinput_dev, vinput->id); 
+205    dev_set_name(&vinput->dev, DRIVER_NAME "%lu", vinput->id); 
+206 
+207    return vinput; 
+208 
+209fail_input_dev: 
+210    spin_lock(&vinput_lock); 
+211    list_del(&vinput->list); 
+212fail_id: 
+213    spin_unlock(&vinput_lock); 
+214    module_put(THIS_MODULE); 
+215    kfree(vinput); 
+216 
+217    return ERR_PTR(err); 
+218} 
+219 
+220static int vinput_register_vdevice(struct vinput *vinput) 
+221{ 
+222    int err = 0; 
+223 
+224    /* register the input device */ 
+225    vinput->input->name = vinput->type->name; 
+226    vinput->input->phys = "vinput"; 
+227    vinput->input->dev.parent = &vinput->dev; 
+228 
+229    vinput->input->id.bustype = BUS_VIRTUAL; 
+230    vinput->input->id.product = 0x0000; 
+231    vinput->input->id.vendor = 0x0000; 
+232    vinput->input->id.version = 0x0000; 
+233 
+234    err = vinput->type->ops->init(vinput); 
+235 
+236    if (err == 0) 
+237        dev_info(&vinput->dev, "Registered virtual input %s %ld\n", 
+238                 vinput->type->name, vinput->id); 
+239 
+240    return err; 
+241} 
+242 
+243static ssize_t export_store(struct class *class, struct class_attribute *attr, 
+244                            const char *buf, size_t len) 
+245{ 
+246    int err; 
+247    struct vinput *vinput; 
+248    struct vinput_device *device; 
+249 
+250    device = vinput_get_device_by_type(buf); 
+251    if (IS_ERR(device)) { 
+252        pr_info("vinput: This virtual device isn't registered\n"); 
+253        err = PTR_ERR(device); 
+254        goto fail; 
+255    } 
+256 
+257    vinput = vinput_alloc_vdevice(); 
+258    if (IS_ERR(vinput)) { 
+259        err = PTR_ERR(vinput); 
+260        goto fail; 
+261    } 
+262 
+263    vinput->type = device; 
+264    err = device_register(&vinput->dev); 
+265    if (err < 0) 
+266        goto fail_register; 
+267 
+268    err = vinput_register_vdevice(vinput); 
+269    if (err < 0) 
+270        goto fail_register_vinput; 
+271 
+272    return len; 
+273 
+274fail_register_vinput: 
+275    device_unregister(&vinput->dev); 
+276fail_register: 
+277    vinput_destroy_vdevice(vinput); 
+278fail: 
+279    return err; 
+280} 
+281/* This macro generates class_attr_export structure and export_store() */ 
+282static CLASS_ATTR_WO(export); 
+283 
+284static ssize_t unexport_store(struct class *class, struct class_attribute *attr, 
+285                              const char *buf, size_t len) 
+286{ 
+287    int err; 
+288    unsigned long id; 
+289    struct vinput *vinput; 
+290 
+291    err = kstrtol(buf, 10, &id); 
+292    if (err) { 
+293        err = -EINVAL; 
+294        goto failed; 
+295    } 
+296 
+297    vinput = vinput_get_vdevice_by_id(id); 
+298    if (IS_ERR(vinput)) { 
+299        pr_err("vinput: No such vinput device %ld\n", id); 
+300        err = PTR_ERR(vinput); 
+301        goto failed; 
+302    } 
+303 
+304    vinput_unregister_vdevice(vinput); 
+305    device_unregister(&vinput->dev); 
+306 
+307    return len; 
+308failed: 
+309    return err; 
+310} 
+311/* This macro generates class_attr_unexport structure and unexport_store() */ 
+312static CLASS_ATTR_WO(unexport); 
+313 
+314static struct attribute *vinput_class_attrs[] = { 
+315    &class_attr_export.attr, 
+316    &class_attr_unexport.attr, 
+317    NULL, 
+318}; 
+319 
+320/* This macro generates vinput_class_groups structure */ 
+321ATTRIBUTE_GROUPS(vinput_class); 
+322 
+323static struct class vinput_class = { 
+324    .name = "vinput", 
+325    .owner = THIS_MODULE, 
+326    .class_groups = vinput_class_groups, 
+327}; 
+328 
+329int vinput_register(struct vinput_device *dev) 
+330{ 
+331    spin_lock(&vinput_lock); 
+332    list_add(&dev->list, &vinput_devices); 
+333    spin_unlock(&vinput_lock); 
+334 
+335    pr_info("vinput: registered new virtual input device '%s'\n", dev->name); 
+336 
+337    return 0; 
+338} 
+339EXPORT_SYMBOL(vinput_register); 
+340 
+341void vinput_unregister(struct vinput_device *dev) 
+342{ 
+343    struct list_head *curr, *next; 
+344 
+345    /* Remove from the list first */ 
+346    spin_lock(&vinput_lock); 
+347    list_del(&dev->list); 
+348    spin_unlock(&vinput_lock); 
+349 
+350    /* unregister all devices of this type */ 
+351    list_for_each_safe (curr, next, &vinput_vdevices) { 
+352        struct vinput *vinput = list_entry(curr, struct vinput, list); 
+353        if (vinput && vinput->type == dev) { 
+354            vinput_unregister_vdevice(vinput); 
+355            device_unregister(&vinput->dev); 
+356        } 
+357    } 
+358 
+359    pr_info("vinput: unregistered virtual input device '%s'\n", dev->name); 
+360} 
+361EXPORT_SYMBOL(vinput_unregister); 
+362 
+363static int __init vinput_init(void) 
+364{ 
+365    int err = 0; 
+366 
+367    pr_info("vinput: Loading virtual input driver\n"); 
+368 
+369    vinput_dev = register_chrdev(0, DRIVER_NAME, &vinput_fops); 
+370    if (vinput_dev < 0) { 
+371        pr_err("vinput: Unable to allocate char dev region\n"); 
+372        err = vinput_dev; 
+373        goto failed_alloc; 
+374    } 
+375 
+376    spin_lock_init(&vinput_lock); 
+377 
+378    err = class_register(&vinput_class); 
+379    if (err < 0) { 
+380        pr_err("vinput: Unable to register vinput class\n"); 
+381        goto failed_class; 
+382    } 
+383 
+384    return 0; 
+385failed_class: 
+386    class_unregister(&vinput_class); 
+387failed_alloc: 
+388    return err; 
+389} 
+390 
+391static void __exit vinput_end(void) 
+392{ 
+393    pr_info("vinput: Unloading virtual input driver\n"); 
+394 
+395    unregister_chrdev(vinput_dev, DRIVER_NAME); 
+396    class_unregister(&vinput_class); 
+397} 
+398 
+399module_init(vinput_init); 
+400module_exit(vinput_end); 
+401 
+402MODULE_LICENSE("GPL"); 
+403MODULE_DESCRIPTION("Emulate input events");
    +

    Here the virtual keyboard is one of example to use vinput. It supports all KEY_MAX keycodes. The injection format is the KEY_CODE such as defined in include/linux/input.h. A positive value means @@ -6259,136 +5953,136 @@ will generate the class_attribute while a negative value is a KEY_RELEASE . The keyboard supports repetition when the key stays pressed for too long. The following demonstrates how simulation work. -

    Simulate a key press on "g" ( KEY_G +

    Simulate a key press on "g" ( KEY_G = 34):

    - 
    1echo "+34" | sudo tee /dev/vinput0
    -

    Simulate a key release on "g" ( KEY_G + 

    1echo "+34" | sudo tee /dev/vinput0
    +

    Simulate a key release on "g" ( KEY_G = 34):

    +

    + 
    1echo "-34" | sudo tee /dev/vinput0
    -

    - 
    1echo "-34" | sudo tee /dev/vinput0

    - 
    1/* 
-2 * vkbd.c 
-3 */ 
-4 
-5#include <linux/init.h> 
-6#include <linux/input.h> 
-7#include <linux/module.h> 
-8#include <linux/spinlock.h> 
-9 
-10#include "vinput.h" 
-11 
-12#define VINPUT_KBD "vkbd" 
-13#define VINPUT_RELEASE 0 
-14#define VINPUT_PRESS 1 
-15 
-16static unsigned short vkeymap[KEY_MAX]; 
-17 
-18static int vinput_vkbd_init(struct vinput *vinput) 
-19{ 
-20    int i; 
-21 
-22    /* Set up the input bitfield */ 
-23    vinput->input->evbit[0] = BIT_MASK(EV_KEY) | BIT_MASK(EV_REP); 
-24    vinput->input->keycodesize = sizeof(unsigned short); 
-25    vinput->input->keycodemax = KEY_MAX; 
-26    vinput->input->keycode = vkeymap; 
-27 
-28    for (i = 0; i < KEY_MAX; i++) 
-29        set_bit(vkeymap[i], vinput->input->keybit); 
-30 
-31    /* vinput will help us allocate new input device structure via 
-32     * input_allocate_device(). So, we can register it straightforwardly. 
-33     */ 
-34    return input_register_device(vinput->input); 
-35} 
-36 
-37static int vinput_vkbd_read(struct vinput *vinput, char *buff, int len) 
-38{ 
-39    spin_lock(&vinput->lock); 
-40    len = snprintf(buff, len, "%+ld\n", vinput->last_entry); 
-41    spin_unlock(&vinput->lock); 
-42 
-43    return len; 
-44} 
-45 
-46static int vinput_vkbd_send(struct vinput *vinput, char *buff, int len) 
-47{ 
-48    int ret; 
-49    long key = 0; 
-50    short type = VINPUT_PRESS; 
-51 
-52    /* Determine which event was received (press or release) 
-53     * and store the state. 
-54     */ 
-55    if (buff[0] == '+') 
-56        ret = kstrtol(buff + 1, 10, &key); 
-57    else 
-58        ret = kstrtol(buff, 10, &key); 
-59    if (ret) 
-60        dev_err(&vinput->dev, "error during kstrtol: -%d\n", ret); 
-61    spin_lock(&vinput->lock); 
-62    vinput->last_entry = key; 
-63    spin_unlock(&vinput->lock); 
-64 
-65    if (key < 0) { 
-66        type = VINPUT_RELEASE; 
-67        key = -key; 
-68    } 
-69 
-70    dev_info(&vinput->dev, "Event %s code %ld\n", 
-71             (type == VINPUT_RELEASE) ? "VINPUT_RELEASE" : "VINPUT_PRESS", key); 
-72 
-73    /* Report the state received to input subsystem. */ 
-74    input_report_key(vinput->input, key, type); 
-75    /* Tell input subsystem that it finished the report. */ 
-76    input_sync(vinput->input); 
-77 
-78    return len; 
-79} 
-80 
-81static struct vinput_ops vkbd_ops = { 
-82    .init = vinput_vkbd_init, 
-83    .send = vinput_vkbd_send, 
-84    .read = vinput_vkbd_read, 
-85}; 
-86 
-87static struct vinput_device vkbd_dev = { 
-88    .name = VINPUT_KBD, 
-89    .ops = &vkbd_ops, 
-90}; 
-91 
-92static int __init vkbd_init(void) 
-93{ 
-94    int i; 
-95 
-96    for (i = 0; i < KEY_MAX; i++) 
-97        vkeymap[i] = i; 
-98    return vinput_register(&vkbd_dev); 
-99} 
-100 
-101static void __exit vkbd_end(void) 
-102{ 
-103    vinput_unregister(&vkbd_dev); 
-104} 
-105 
-106module_init(vkbd_init); 
-107module_exit(vkbd_end); 
-108 
-109MODULE_LICENSE("GPL"); 
-110MODULE_DESCRIPTION("Emulate keyboard input events through /dev/vinput");
    -

    + 

    1/* 
+2 * vkbd.c 
+3 */ 
+4 
+5#include <linux/init.h> 
+6#include <linux/input.h> 
+7#include <linux/module.h> 
+8#include <linux/spinlock.h> 
+9 
+10#include "vinput.h" 
+11 
+12#define VINPUT_KBD "vkbd" 
+13#define VINPUT_RELEASE 0 
+14#define VINPUT_PRESS 1 
+15 
+16static unsigned short vkeymap[KEY_MAX]; 
+17 
+18static int vinput_vkbd_init(struct vinput *vinput) 
+19{ 
+20    int i; 
+21 
+22    /* Set up the input bitfield */ 
+23    vinput->input->evbit[0] = BIT_MASK(EV_KEY) | BIT_MASK(EV_REP); 
+24    vinput->input->keycodesize = sizeof(unsigned short); 
+25    vinput->input->keycodemax = KEY_MAX; 
+26    vinput->input->keycode = vkeymap; 
+27 
+28    for (i = 0; i < KEY_MAX; i++) 
+29        set_bit(vkeymap[i], vinput->input->keybit); 
+30 
+31    /* vinput will help us allocate new input device structure via 
+32     * input_allocate_device(). So, we can register it straightforwardly. 
+33     */ 
+34    return input_register_device(vinput->input); 
+35} 
+36 
+37static int vinput_vkbd_read(struct vinput *vinput, char *buff, int len) 
+38{ 
+39    spin_lock(&vinput->lock); 
+40    len = snprintf(buff, len, "%+ld\n", vinput->last_entry); 
+41    spin_unlock(&vinput->lock); 
+42 
+43    return len; 
+44} 
+45 
+46static int vinput_vkbd_send(struct vinput *vinput, char *buff, int len) 
+47{ 
+48    int ret; 
+49    long key = 0; 
+50    short type = VINPUT_PRESS; 
+51 
+52    /* Determine which event was received (press or release) 
+53     * and store the state. 
+54     */ 
+55    if (buff[0] == '+') 
+56        ret = kstrtol(buff + 1, 10, &key); 
+57    else 
+58        ret = kstrtol(buff, 10, &key); 
+59    if (ret) 
+60        dev_err(&vinput->dev, "error during kstrtol: -%d\n", ret); 
+61    spin_lock(&vinput->lock); 
+62    vinput->last_entry = key; 
+63    spin_unlock(&vinput->lock); 
+64 
+65    if (key < 0) { 
+66        type = VINPUT_RELEASE; 
+67        key = -key; 
+68    } 
+69 
+70    dev_info(&vinput->dev, "Event %s code %ld\n", 
+71             (type == VINPUT_RELEASE) ? "VINPUT_RELEASE" : "VINPUT_PRESS", key); 
+72 
+73    /* Report the state received to input subsystem. */ 
+74    input_report_key(vinput->input, key, type); 
+75    /* Tell input subsystem that it finished the report. */ 
+76    input_sync(vinput->input); 
+77 
+78    return len; 
+79} 
+80 
+81static struct vinput_ops vkbd_ops = { 
+82    .init = vinput_vkbd_init, 
+83    .send = vinput_vkbd_send, 
+84    .read = vinput_vkbd_read, 
+85}; 
+86 
+87static struct vinput_device vkbd_dev = { 
+88    .name = VINPUT_KBD, 
+89    .ops = &vkbd_ops, 
+90}; 
+91 
+92static int __init vkbd_init(void) 
+93{ 
+94    int i; 
+95 
+96    for (i = 0; i < KEY_MAX; i++) 
+97        vkeymap[i] = i; 
+98    return vinput_register(&vkbd_dev); 
+99} 
+100 
+101static void __exit vkbd_end(void) 
+102{ 
+103    vinput_unregister(&vkbd_dev); 
+104} 
+105 
+106module_init(vkbd_init); 
+107module_exit(vkbd_end); 
+108 
+109MODULE_LICENSE("GPL"); 
+110MODULE_DESCRIPTION("Emulate keyboard input events through /dev/vinput");
    +

    -

    18 Standardizing the interfaces: The Device Model

    -

    Up to this point we have seen all kinds of modules doing all kinds of things, but there +

    17 Standardizing the interfaces: The Device Model

    +

    Up to this point we have seen all kinds of modules doing all kinds of things, but there was no consistency in their interfaces with the rest of the kernel. To impose some consistency such that there is at minimum a standardized way to start, suspend and resume a device model was added. An example is shown below, and you can @@ -6396,111 +6090,111 @@ use this as a template to add your own suspend, resume or other interface functions.

    - 
    1/* 
-2 * devicemodel.c 
-3 */ 
-4#include <linux/kernel.h> 
-5#include <linux/module.h> 
-6#include <linux/platform_device.h> 
-7 
-8struct devicemodel_data { 
-9    char *greeting; 
-10    int number; 
-11}; 
-12 
-13static int devicemodel_probe(struct platform_device *dev) 
-14{ 
-15    struct devicemodel_data *pd = 
-16        (struct devicemodel_data *)(dev->dev.platform_data); 
-17 
-18    pr_info("devicemodel probe\n"); 
-19    pr_info("devicemodel greeting: %s; %d\n", pd->greeting, pd->number); 
-20 
-21    /* Your device initialization code */ 
-22 
-23    return 0; 
-24} 
-25 
-26static int devicemodel_remove(struct platform_device *dev) 
-27{ 
-28    pr_info("devicemodel example removed\n"); 
-29 
-30    /* Your device removal code */ 
-31 
-32    return 0; 
-33} 
-34 
-35static int devicemodel_suspend(struct device *dev) 
-36{ 
-37    pr_info("devicemodel example suspend\n"); 
-38 
-39    /* Your device suspend code */ 
-40 
-41    return 0; 
-42} 
-43 
-44static int devicemodel_resume(struct device *dev) 
-45{ 
-46    pr_info("devicemodel example resume\n"); 
-47 
-48    /* Your device resume code */ 
-49 
-50    return 0; 
-51} 
-52 
-53static const struct dev_pm_ops devicemodel_pm_ops = { 
-54    .suspend = devicemodel_suspend, 
-55    .resume = devicemodel_resume, 
-56    .poweroff = devicemodel_suspend, 
-57    .freeze = devicemodel_suspend, 
-58    .thaw = devicemodel_resume, 
-59    .restore = devicemodel_resume, 
-60}; 
-61 
-62static struct platform_driver devicemodel_driver = { 
-63    .driver = 
-64        { 
-65            .name = "devicemodel_example", 
-66            .pm = &devicemodel_pm_ops, 
-67        }, 
-68    .probe = devicemodel_probe, 
-69    .remove = devicemodel_remove, 
-70}; 
-71 
-72static int __init devicemodel_init(void) 
-73{ 
-74    int ret; 
-75 
-76    pr_info("devicemodel init\n"); 
-77 
-78    ret = platform_driver_register(&devicemodel_driver); 
-79 
-80    if (ret) { 
-81        pr_err("Unable to register driver\n"); 
-82        return ret; 
-83    } 
-84 
-85    return 0; 
-86} 
-87 
-88static void __exit devicemodel_exit(void) 
-89{ 
-90    pr_info("devicemodel exit\n"); 
-91    platform_driver_unregister(&devicemodel_driver); 
-92} 
-93 
-94module_init(devicemodel_init); 
-95module_exit(devicemodel_exit); 
-96 
-97MODULE_LICENSE("GPL"); 
-98MODULE_DESCRIPTION("Linux Device Model example");
    -

    + 

    1/* 
+2 * devicemodel.c 
+3 */ 
+4#include <linux/kernel.h> 
+5#include <linux/module.h> 
+6#include <linux/platform_device.h> 
+7 
+8struct devicemodel_data { 
+9    char *greeting; 
+10    int number; 
+11}; 
+12 
+13static int devicemodel_probe(struct platform_device *dev) 
+14{ 
+15    struct devicemodel_data *pd = 
+16        (struct devicemodel_data *)(dev->dev.platform_data); 
+17 
+18    pr_info("devicemodel probe\n"); 
+19    pr_info("devicemodel greeting: %s; %d\n", pd->greeting, pd->number); 
+20 
+21    /* Your device initialization code */ 
+22 
+23    return 0; 
+24} 
+25 
+26static int devicemodel_remove(struct platform_device *dev) 
+27{ 
+28    pr_info("devicemodel example removed\n"); 
+29 
+30    /* Your device removal code */ 
+31 
+32    return 0; 
+33} 
+34 
+35static int devicemodel_suspend(struct device *dev) 
+36{ 
+37    pr_info("devicemodel example suspend\n"); 
+38 
+39    /* Your device suspend code */ 
+40 
+41    return 0; 
+42} 
+43 
+44static int devicemodel_resume(struct device *dev) 
+45{ 
+46    pr_info("devicemodel example resume\n"); 
+47 
+48    /* Your device resume code */ 
+49 
+50    return 0; 
+51} 
+52 
+53static const struct dev_pm_ops devicemodel_pm_ops = { 
+54    .suspend = devicemodel_suspend, 
+55    .resume = devicemodel_resume, 
+56    .poweroff = devicemodel_suspend, 
+57    .freeze = devicemodel_suspend, 
+58    .thaw = devicemodel_resume, 
+59    .restore = devicemodel_resume, 
+60}; 
+61 
+62static struct platform_driver devicemodel_driver = { 
+63    .driver = 
+64        { 
+65            .name = "devicemodel_example", 
+66            .pm = &devicemodel_pm_ops, 
+67        }, 
+68    .probe = devicemodel_probe, 
+69    .remove = devicemodel_remove, 
+70}; 
+71 
+72static int __init devicemodel_init(void) 
+73{ 
+74    int ret; 
+75 
+76    pr_info("devicemodel init\n"); 
+77 
+78    ret = platform_driver_register(&devicemodel_driver); 
+79 
+80    if (ret) { 
+81        pr_err("Unable to register driver\n"); 
+82        return ret; 
+83    } 
+84 
+85    return 0; 
+86} 
+87 
+88static void __exit devicemodel_exit(void) 
+89{ 
+90    pr_info("devicemodel exit\n"); 
+91    platform_driver_unregister(&devicemodel_driver); 
+92} 
+93 
+94module_init(devicemodel_init); 
+95module_exit(devicemodel_exit); 
+96 
+97MODULE_LICENSE("GPL"); 
+98MODULE_DESCRIPTION("Linux Device Model example");
    +

    -

    19 Optimizations

    -

    +

    18 Optimizations

    +

    -

    19.1 Likely and Unlikely conditions

    -

    Sometimes you might want your code to run as quickly as possible, +

    18.1 Likely and Unlikely conditions

    +

    Sometimes you might want your code to run as quickly as possible, especially if it is handling an interrupt or doing something which might cause noticeable latency. If your code contains boolean conditions and if you know that the conditions are almost always likely to evaluate as either @@ -6512,40 +6206,40 @@ you know that the conditions are almost always likely to evaluate as either macros. For example, when allocating memory you are almost always expecting this to succeed.

    +

    + 
    1bvl = bvec_alloc(gfp_mask, nr_iovecs, &idx); 
+2if (unlikely(!bvl)) { 
+3    mempool_free(bio, bio_pool); 
+4    bio = NULL; 
+5    goto out; 
+6}
    -

    - 
    1bvl = bvec_alloc(gfp_mask, nr_iovecs, &idx); 
-2if (unlikely(!bvl)) { 
-3    mempool_free(bio, bio_pool); 
-4    bio = NULL; 
-5    goto out; 
-6}
    -

    When the unlikely +

    When the unlikely macro is used, the compiler alters its machine instruction output, so that it continues along the false branch and only jumps if the condition is true. That avoids flushing the processor pipeline. The opposite happens if you use the likely macro. -

    +

    -

    19.2 Static keys

    -

    Static keys allow us to enable or disable kernel code paths based on the runtime state +

    18.2 Static keys

    +

    Static keys allow us to enable or disable kernel code paths based on the runtime state of key. Its APIs have been available since 2010 (most architectures are already supported), use self-modifying code to eliminate the overhead of cache and branch prediction. The most typical use case of static keys is for performance-sensitive kernel code, such as tracepoints, context switching, networking, etc. These hot paths of the kernel often contain branches and can be optimized easily using this technique. Before we can use static keys in the kernel, we need to make sure that gcc supports - asm goto + asm goto inline assembly, and the following kernel configurations are set:

    - 
    1CONFIG_JUMP_LABEL=y 
-2CONFIG_HAVE_ARCH_JUMP_LABEL=y 
-3CONFIG_HAVE_ARCH_JUMP_LABEL_RELATIVE=y
    -

    To declare a static key, we need to define a global variable using the + 

    1CONFIG_JUMP_LABEL=y 
+2CONFIG_HAVE_ARCH_JUMP_LABEL=y 
+3CONFIG_HAVE_ARCH_JUMP_LABEL_RELATIVE=y
    +

    To declare a static key, we need to define a global variable using the DEFINE_STATIC_KEY_FALSE or DEFINE_STATIC_KEY_TRUE macro defined in include/linux/jump_label.h. This macro initializes the key with @@ -6554,268 +6248,268 @@ declare a static key with an initial value of false, we can use the following code:

    - 
    1DEFINE_STATIC_KEY_FALSE(fkey);
    -

    Once the static key has been declared, we need to add branching code to the + 

    1DEFINE_STATIC_KEY_FALSE(fkey);
    +

    Once the static key has been declared, we need to add branching code to the module that uses the static key. For example, the code includes a fastpath, where a no-op instruction will be generated at compile time as the key is initialized to false and the branch is unlikely to be taken.

    - 
    1pr_info("fastpath 1\n"); 
-2if (static_branch_unlikely(&fkey)) 
-3    pr_alert("do unlikely thing\n"); 
-4pr_info("fastpath 2\n");
    + 
    1pr_info("fastpath 1\n"); 
+2if (static_branch_unlikely(&fkey)) 
+3    pr_alert("do unlikely thing\n"); 
+4pr_info("fastpath 2\n");
    +

    If the key is enabled at runtime by calling + static_branch_enable(&fkey) +, the fastpath will be patched with an unconditional jump instruction to the slowpath -

    If the key is enabled at runtime by calling - static_branch_enable(&fkey) -, the fastpath will be patched with an unconditional jump instruction to the slowpath code pr_alert , so the branch will always be taken until the key is disabled again. -

    The following kernel module derived from chardev.c, demonstrates how the +

    The following kernel module derived from chardev.c, demonstrates how the static key works.

    - 
    1/* 
-2 * static_key.c 
-3 */ 
-4 
-5#include <linux/atomic.h> 
-6#include <linux/device.h> 
-7#include <linux/fs.h> 
-8#include <linux/kernel.h> /* for sprintf() */ 
-9#include <linux/module.h> 
-10#include <linux/printk.h> 
-11#include <linux/types.h> 
-12#include <linux/uaccess.h> /* for get_user and put_user */ 
-13#include <linux/jump_label.h> /* for static key macros */ 
-14 
-15#include <asm/errno.h> 
-16 
-17static int device_open(struct inode *inode, struct file *file); 
-18static int device_release(struct inode *inode, struct file *file); 
-19static ssize_t device_read(struct file *file, char __user *buf, size_t count, 
-20                           loff_t *ppos); 
-21static ssize_t device_write(struct file *file, const char __user *buf, 
-22                            size_t count, loff_t *ppos); 
-23 
-24#define SUCCESS 0 
-25#define DEVICE_NAME "key_state" 
-26#define BUF_LEN 10 
-27 
-28static int major; 
-29 
-30enum { 
-31    CDEV_NOT_USED = 0, 
-32    CDEV_EXCLUSIVE_OPEN = 1, 
-33}; 
-34 
-35static atomic_t already_open = ATOMIC_INIT(CDEV_NOT_USED); 
-36 
-37static char msg[BUF_LEN + 1]; 
-38 
-39static struct class *cls; 
-40 
-41static DEFINE_STATIC_KEY_FALSE(fkey); 
-42 
-43static struct file_operations chardev_fops = { 
-44    .owner = THIS_MODULE, 
-45    .open = device_open, 
-46    .release = device_release, 
-47    .read = device_read, 
-48    .write = device_write, 
-49}; 
-50 
-51static int __init chardev_init(void) 
-52{ 
-53    major = register_chrdev(0, DEVICE_NAME, &chardev_fops); 
-54    if (major < 0) { 
-55        pr_alert("Registering char device failed with %d\n", major); 
-56        return major; 
-57    } 
-58 
-59    pr_info("I was assigned major number %d\n", major); 
-60 
-61    cls = class_create(THIS_MODULE, DEVICE_NAME); 
-62 
-63    device_create(cls, NULL, MKDEV(major, 0), NULL, DEVICE_NAME); 
-64 
-65    pr_info("Device created on /dev/%s\n", DEVICE_NAME); 
-66 
-67    return SUCCESS; 
-68} 
-69 
-70static void __exit chardev_exit(void) 
-71{ 
-72    device_destroy(cls, MKDEV(major, 0)); 
-73    class_destroy(cls); 
-74 
-75    /* Unregister the device */ 
-76    unregister_chrdev(major, DEVICE_NAME); 
-77} 
-78 
-79/* Methods */ 
-80 
-81/** 
-82 * Called when a process tried to open the device file, like 
-83 * cat /dev/key_state 
-84 */ 
-85static int device_open(struct inode *inode, struct file *file) 
-86{ 
-87    if (atomic_cmpxchg(&already_open, CDEV_NOT_USED, CDEV_EXCLUSIVE_OPEN)) 
-88        return -EBUSY; 
-89 
-90    sprintf(msg, static_key_enabled(&fkey) ? "enabled\n" : "disabled\n"); 
-91 
-92    pr_info("fastpath 1\n"); 
-93    if (static_branch_unlikely(&fkey)) 
-94        pr_alert("do unlikely thing\n"); 
-95    pr_info("fastpath 2\n"); 
-96 
-97    try_module_get(THIS_MODULE); 
-98 
-99    return SUCCESS; 
-100} 
-101 
-102/** 
-103 * Called when a process closes the device file 
-104 */ 
-105static int device_release(struct inode *inode, struct file *file) 
-106{ 
-107    /* We are now ready for our next caller. */ 
-108    atomic_set(&already_open, CDEV_NOT_USED); 
-109 
-110    /** 
-111     * Decrement the usage count, or else once you opened the file, you will 
-112     * never get rid of the module. 
-113     */ 
-114    module_put(THIS_MODULE); 
-115 
-116    return SUCCESS; 
-117} 
-118 
-119/** 
-120 * Called when a process, which already opened the dev file, attempts to 
-121 * read from it. 
-122 */ 
-123static ssize_t device_read(struct file *filp, /* see include/linux/fs.h */ 
-124                           char __user *buffer, /* buffer to fill with data */ 
-125                           size_t length, /* length of the buffer */ 
-126                           loff_t *offset) 
-127{ 
-128    /* Number of the bytes actually written to the buffer */ 
-129    int bytes_read = 0; 
-130    const char *msg_ptr = msg; 
-131 
-132    if (!*(msg_ptr + *offset)) { /* We are at the end of the message */ 
-133        *offset = 0; /* reset the offset */ 
-134        return 0; /* signify end of file */ 
-135    } 
-136 
-137    msg_ptr += *offset; 
-138 
-139    /* Actually put the data into the buffer */ 
-140    while (length && *msg_ptr) { 
-141        /** 
-142         * The buffer is in the user data segment, not the kernel 
-143         * segment so "*" assignment won't work. We have to use 
-144         * put_user which copies data from the kernel data segment to 
-145         * the user data segment. 
-146         */ 
-147        put_user(*(msg_ptr++), buffer++); 
-148        length--; 
-149        bytes_read++; 
-150    } 
-151 
-152    *offset += bytes_read; 
-153 
-154    /* Most read functions return the number of bytes put into the buffer. */ 
-155    return bytes_read; 
-156} 
-157 
-158/* Called when a process writes to dev file; echo "enable" > /dev/key_state */ 
-159static ssize_t device_write(struct file *filp, const char __user *buffer, 
-160                            size_t length, loff_t *offset) 
-161{ 
-162    char command[10]; 
-163 
-164    if (length > 10) { 
-165        pr_err("command exceeded 10 char\n"); 
-166        return -EINVAL; 
-167    } 
-168 
-169    if (copy_from_user(command, buffer, length)) 
-170        return -EFAULT; 
-171 
-172    if (strncmp(command, "enable", strlen("enable")) == 0) 
-173        static_branch_enable(&fkey); 
-174    else if (strncmp(command, "disable", strlen("disable")) == 0) 
-175        static_branch_disable(&fkey); 
-176    else { 
-177        pr_err("Invalid command: %s\n", command); 
-178        return -EINVAL; 
-179    } 
-180 
-181    /* Again, return the number of input characters used. */ 
-182    return length; 
-183} 
-184 
-185module_init(chardev_init); 
-186module_exit(chardev_exit); 
-187 
-188MODULE_LICENSE("GPL");
    -

    To check the state of the static key, we can use the /dev/key_state + 

    1/* 
+2 * static_key.c 
+3 */ 
+4 
+5#include <linux/atomic.h> 
+6#include <linux/device.h> 
+7#include <linux/fs.h> 
+8#include <linux/kernel.h> /* for sprintf() */ 
+9#include <linux/module.h> 
+10#include <linux/printk.h> 
+11#include <linux/types.h> 
+12#include <linux/uaccess.h> /* for get_user and put_user */ 
+13#include <linux/jump_label.h> /* for static key macros */ 
+14 
+15#include <asm/errno.h> 
+16 
+17static int device_open(struct inode *inode, struct file *file); 
+18static int device_release(struct inode *inode, struct file *file); 
+19static ssize_t device_read(struct file *file, char __user *buf, size_t count, 
+20                           loff_t *ppos); 
+21static ssize_t device_write(struct file *file, const char __user *buf, 
+22                            size_t count, loff_t *ppos); 
+23 
+24#define SUCCESS 0 
+25#define DEVICE_NAME "key_state" 
+26#define BUF_LEN 10 
+27 
+28static int major; 
+29 
+30enum { 
+31    CDEV_NOT_USED = 0, 
+32    CDEV_EXCLUSIVE_OPEN = 1, 
+33}; 
+34 
+35static atomic_t already_open = ATOMIC_INIT(CDEV_NOT_USED); 
+36 
+37static char msg[BUF_LEN + 1]; 
+38 
+39static struct class *cls; 
+40 
+41static DEFINE_STATIC_KEY_FALSE(fkey); 
+42 
+43static struct file_operations chardev_fops = { 
+44    .owner = THIS_MODULE, 
+45    .open = device_open, 
+46    .release = device_release, 
+47    .read = device_read, 
+48    .write = device_write, 
+49}; 
+50 
+51static int __init chardev_init(void) 
+52{ 
+53    major = register_chrdev(0, DEVICE_NAME, &chardev_fops); 
+54    if (major < 0) { 
+55        pr_alert("Registering char device failed with %d\n", major); 
+56        return major; 
+57    } 
+58 
+59    pr_info("I was assigned major number %d\n", major); 
+60 
+61    cls = class_create(THIS_MODULE, DEVICE_NAME); 
+62 
+63    device_create(cls, NULL, MKDEV(major, 0), NULL, DEVICE_NAME); 
+64 
+65    pr_info("Device created on /dev/%s\n", DEVICE_NAME); 
+66 
+67    return SUCCESS; 
+68} 
+69 
+70static void __exit chardev_exit(void) 
+71{ 
+72    device_destroy(cls, MKDEV(major, 0)); 
+73    class_destroy(cls); 
+74 
+75    /* Unregister the device */ 
+76    unregister_chrdev(major, DEVICE_NAME); 
+77} 
+78 
+79/* Methods */ 
+80 
+81/** 
+82 * Called when a process tried to open the device file, like 
+83 * cat /dev/key_state 
+84 */ 
+85static int device_open(struct inode *inode, struct file *file) 
+86{ 
+87    if (atomic_cmpxchg(&already_open, CDEV_NOT_USED, CDEV_EXCLUSIVE_OPEN)) 
+88        return -EBUSY; 
+89 
+90    sprintf(msg, static_key_enabled(&fkey) ? "enabled\n" : "disabled\n"); 
+91 
+92    pr_info("fastpath 1\n"); 
+93    if (static_branch_unlikely(&fkey)) 
+94        pr_alert("do unlikely thing\n"); 
+95    pr_info("fastpath 2\n"); 
+96 
+97    try_module_get(THIS_MODULE); 
+98 
+99    return SUCCESS; 
+100} 
+101 
+102/** 
+103 * Called when a process closes the device file 
+104 */ 
+105static int device_release(struct inode *inode, struct file *file) 
+106{ 
+107    /* We are now ready for our next caller. */ 
+108    atomic_set(&already_open, CDEV_NOT_USED); 
+109 
+110    /** 
+111     * Decrement the usage count, or else once you opened the file, you will 
+112     * never get rid of the module. 
+113     */ 
+114    module_put(THIS_MODULE); 
+115 
+116    return SUCCESS; 
+117} 
+118 
+119/** 
+120 * Called when a process, which already opened the dev file, attempts to 
+121 * read from it. 
+122 */ 
+123static ssize_t device_read(struct file *filp, /* see include/linux/fs.h */ 
+124                           char __user *buffer, /* buffer to fill with data */ 
+125                           size_t length, /* length of the buffer */ 
+126                           loff_t *offset) 
+127{ 
+128    /* Number of the bytes actually written to the buffer */ 
+129    int bytes_read = 0; 
+130    const char *msg_ptr = msg; 
+131 
+132    if (!*(msg_ptr + *offset)) { /* We are at the end of the message */ 
+133        *offset = 0; /* reset the offset */ 
+134        return 0; /* signify end of file */ 
+135    } 
+136 
+137    msg_ptr += *offset; 
+138 
+139    /* Actually put the data into the buffer */ 
+140    while (length && *msg_ptr) { 
+141        /** 
+142         * The buffer is in the user data segment, not the kernel 
+143         * segment so "*" assignment won't work. We have to use 
+144         * put_user which copies data from the kernel data segment to 
+145         * the user data segment. 
+146         */ 
+147        put_user(*(msg_ptr++), buffer++); 
+148        length--; 
+149        bytes_read++; 
+150    } 
+151 
+152    *offset += bytes_read; 
+153 
+154    /* Most read functions return the number of bytes put into the buffer. */ 
+155    return bytes_read; 
+156} 
+157 
+158/* Called when a process writes to dev file; echo "enable" > /dev/key_state */ 
+159static ssize_t device_write(struct file *filp, const char __user *buffer, 
+160                            size_t length, loff_t *offset) 
+161{ 
+162    char command[10]; 
+163 
+164    if (length > 10) { 
+165        pr_err("command exceeded 10 char\n"); 
+166        return -EINVAL; 
+167    } 
+168 
+169    if (copy_from_user(command, buffer, length)) 
+170        return -EFAULT; 
+171 
+172    if (strncmp(command, "enable", strlen("enable")) == 0) 
+173        static_branch_enable(&fkey); 
+174    else if (strncmp(command, "disable", strlen("disable")) == 0) 
+175        static_branch_disable(&fkey); 
+176    else { 
+177        pr_err("Invalid command: %s\n", command); 
+178        return -EINVAL; 
+179    } 
+180 
+181    /* Again, return the number of input characters used. */ 
+182    return length; 
+183} 
+184 
+185module_init(chardev_init); 
+186module_exit(chardev_exit); 
+187 
+188MODULE_LICENSE("GPL");
    +

    To check the state of the static key, we can use the /dev/key_state interface.

    - 
    1cat /dev/key_state
    -

    This will display the current state of the key, which is disabled by default. -

    To change the state of the static key, we can perform a write operation on the + 

    1cat /dev/key_state
    +

    This will display the current state of the key, which is disabled by default. +

    To change the state of the static key, we can perform a write operation on the file:

    - 
    1echo enable > /dev/key_state
    -

    This will enable the static key, causing the code path to switch from the fastpath + 

    1echo enable > /dev/key_state
    +

    This will enable the static key, causing the code path to switch from the fastpath to the slowpath. -

    In some cases, the key is enabled or disabled at initialization and never changed, +

    In some cases, the key is enabled or disabled at initialization and never changed, we can declare a static key as read-only, which means that it can only be toggled in the module init function. To declare a read-only static key, we can use the DEFINE_STATIC_KEY_FALSE_RO or DEFINE_STATIC_KEY_TRUE_RO macro instead. Attempts to change the key at runtime will result in a page fault. For more information, see Static keys -

    +

    -

    20 Common Pitfalls

    -

    +

    19 Common Pitfalls

    +

    +

    19.1 Using standard libraries

    +

    You can not do that. In a kernel module, you can only use kernel functions which are +the functions you can see in /proc/kallsyms. -

    20.1 Using standard libraries

    -

    You can not do that. In a kernel module, you can only use kernel functions which are -the functions you can see in /proc/kallsyms. -

    +

    -

    20.2 Disabling interrupts

    -

    You might need to do this for a short time and that is OK, but if you do not enable +

    19.2 Disabling interrupts

    +

    You might need to do this for a short time and that is OK, but if you do not enable them afterwards, your system will be stuck and you will have to power it off. -

    +

    -

    21 Where To Go From Here?

    -

    For those deeply interested in kernel programming, kernelnewbies.org and the +

    20 Where To Go From Here?

    +

    For those deeply interested in kernel programming, kernelnewbies.org and the Documentation subdirectory within the kernel source code are highly recommended. Although the latter may not always be straightforward, it serves as a valuable initial step for further exploration. Echoing Linus Torvalds’ perspective, the most effective method to understand the kernel is through personal examination of the source code. -

    Contributions to this guide are welcome, especially if there are any significant +

    Contributions to this guide are welcome, especially if there are any significant inaccuracies identified. To contribute or report an issue, please initiate an issue at https://github.com/sysprog21/lkmpg. Pull requests are greatly appreciated. -

    Happy hacking! +

    Happy hacking!

    1The goal of threaded interrupts is to push more of the work to separate threads, so that the minimum needed for acknowledging an interrupt is reduced, and therefore the time spent handling