gcc -Wall -o hello hello.c
+ with gcc -Wall -o hello hello.c
. Run the executable with strace ./hello
. Are you impressed? Every line you see corresponds to a system call. strace is a
handy program that gives you details about what system calls a program is
@@ -994,15 +997,15 @@ calls (like kill()
with (like cosh()
and random()
).
-
You can even write modules to replace the kernel’s system calls, which we will do
+
You can even write modules to replace the kernel’s system calls, which we will do
shortly. Crackers often make use of this sort of thing for backdoors or trojans, but
you can write your own modules to do more benign things, like have the kernel
write Tee hee, that tickles! every time someone tries to delete a file on your
system.
-
+
0.5.3 User Space vs Kernel Space
-A kernel is all about access to resources, whether the resource in question happens to
+
A kernel is all about access to resources, whether the resource in question happens to
be a video card, a hard drive or even memory. Programs often compete for the same
resource. As I just saved this document, updatedb started updating the locate
database. My vim session and updatedb are both using the hard drive concurrently.
@@ -1013,19 +1016,19 @@ architecture had 4 of these modes, which were called rings. Unix uses only
two rings; the highest ring (ring 0, also known as “supervisor mode” where
everything is allowed to happen) and the lowest ring, which is called “user
mode”.
-
Recall the discussion about library functions vs system calls. Typically, you use a
-library function in user mode. The library function calls one or more system calls,
-and these system calls execute on the library function’s behalf, but do so in
+
Recall the discussion about library functions vs system calls. Typically, you use a
+library function in user mode. The library function calls one or more system calls,
+and these system calls execute on the library function’s behalf, but do so in
supervisor mode since they are part of the kernel itself. Once the system call
completes its task, it returns and execution gets transfered back to user
mode.
-
+
0.5.4 Name Space
-When you write a small C program, you use variables which are convenient and make
+
When you write a small C program, you use variables which are convenient and make
sense to the reader. If, on the other hand, you are writing routines which will be part
of a bigger problem, any global variables you have are part of a community of other
peoples’ global variables; some of the variable names can clash. When a program has
@@ -1033,57 +1036,57 @@ lots of global variables which aren’t meaningful enough to be distinguished, y
namespace pollution. In large projects, effort must be made to remember reserved
names, and to find ways to develop a scheme for naming unique variable names and
symbols.
-
When writing kernel code, even the smallest module will be linked against the
+
When writing kernel code, even the smallest module will be linked against the
entire kernel, so this is definitely an issue. The best way to deal with this is to declare
all your variables as static and to use a well-defined prefix for your symbols. By
convention, all kernel prefixes are lowercase. If you do not want to declare everything
as static, another option is to declare a symbol table and register it with the kernel.
We will get to this later.
-
The file /proc/kallsyms holds all the symbols that the kernel knows about and
+
The file /proc/kallsyms holds all the symbols that the kernel knows about and
which are therefore accessible to your modules since they share the kernel’s
codespace.
-
+
0.5.5 Code space
-Memory management is a very complicated subject and the majority of O’Reilly’s
+
Memory management is a very complicated subject and the majority of O’Reilly’s
Understanding The Linux Kernel exclusively covers memory management!
We are not setting out to be experts on memory managements, but we do
need to know a couple of facts to even begin worrying about writing real
modules.
-
If you have not thought about what a segfault really means, you may be surprised
+
If you have not thought about what a segfault really means, you may be surprised
to hear that pointers do not actually point to memory locations. Not real
ones, anyway. When a process is created, the kernel sets aside a portion of
real physical memory and hands it to the process to use for its executing
code, variables, stack, heap and other things which a computer scientist
would know about. This memory begins with 0x00000000 and extends up to
whatever it needs to be. Since the memory space for any two processes do not
-overlap, every process that can access a memory address, say 0xbffff978, would
-be accessing a different location in real physical memory! The processes
-would be accessing an index named 0xbffff978 which points to some kind of
+overlap, every process that can access a memory address, say 0xbffff978, would
+be accessing a different location in real physical memory! The processes
+would be accessing an index named 0xbffff978 which points to some kind of
offset into the region of memory set aside for that particular process. For
the most part, a process like our Hello, World program can’t access the
space of another process, although there are ways which we will talk about
later.
-
The kernel has its own space of memory as well. Since a module is code which
+
The kernel has its own space of memory as well. Since a module is code which
can be dynamically inserted and removed in the kernel (as opposed to a
semi-autonomous object), it shares the kernel’s codespace rather than having its own.
Therefore, if your module segfaults, the kernel segfaults. And if you start writing
over data because of an off-by-one error, then you’re trampling on kernel
data (or code). This is even worse than it sounds, so try your best to be
careful.
-
By the way, I would like to point out that the above discussion is true for any
+
By the way, I would like to point out that the above discussion is true for any
operating system which uses a monolithic kernel. This is not quite the same thing as
"building all your modules into the kernel", although the idea is the same. There are
things called microkernels which have modules which get their own codespace. The
GNU Hurd and the Zircon kernel of Google Fuchsia are two examples of a
microkernel.
-
+
0.5.6 Device Drivers
-One class of module is the device driver, which provides functionality for hardware
+
One class of module is the device driver, which provides functionality for hardware
like a serial port. On Unix, each piece of hardware is represented by a file located in
/dev named a device file which provides the means to communicate with the
hardware. The device driver provides the communication on behalf of a
@@ -1091,7 +1094,7 @@ user program. So the es1370.ko sound card device driver might connect the
/dev/sound device file to the Ensoniq IS1370 sound card. A userspace program like
mp3blaster can use /dev/sound without ever knowing what kind of sound card is
installed.
-
Let’s look at some device files. Here are device files which represent the first three
+
Let’s look at some device files. Here are device files which represent the first three
partitions on the primary master IDE hard drive:
@@ -1103,18 +1106,18 @@ brw-rw---- 1 root disk 3, 1 Jul 5 2000 /dev/hda1
brw-rw---- 1 root disk 3, 2 Jul 5 2000 /dev/hda2
brw-rw---- 1 root disk 3, 3 Jul 5 2000 /dev/hda3
-
-
Notice the column of numbers separated by a comma. The first number is called
+
+
Notice the column of numbers separated by a comma. The first number is called
the device’s major number. The second number is the minor number. The major
number tells you which driver is used to access the hardware. Each driver is assigned
a unique major number; all device files with the same major number are controlled
by the same driver. All the above major numbers are 3, because they’re all controlled
by the same driver.
-
The minor number is used by the driver to distinguish between the various
+
The minor number is used by the driver to distinguish between the various
hardware it controls. Returning to the example above, although all three devices are
handled by the same driver they have unique minor numbers because the driver sees
them as being different pieces of hardware.
-
Devices are divided into two types: character devices and block devices. The
+
Devices are divided into two types: character devices and block devices. The
difference is that block devices have a buffer for requests, so they can choose the best
order in which to respond to the requests. This is important in the case of storage
devices, where it is faster to read or write sectors which are close to each
@@ -1139,10 +1142,10 @@ crw-r----- 1 root dial 4, 65 Nov 17 10:26 /dev/ttyS1
crw-rw---- 1 root dial 4, 66 Jul 5 2000 /dev/ttyS2
crw-rw---- 1 root dial 4, 67 Jul 5 2000 /dev/ttyS3
-
-
If you want to see which major numbers have been assigned, you can look at
+
+
If you want to see which major numbers have been assigned, you can look at
Documentation/admin-guide/devices.txt.
-
When the system was installed, all of those device files were created by the
+
When the system was installed, all of those device files were created by the
mknod
command. To create a new char device named coffee with major/minor number 12 and 2,
simply do mknod /dev/coffee c 12 2
@@ -1151,14 +1154,14 @@ Linus put his device files in I would like to make a few last points which are implicit from the above
+
I would like to make a few last points which are implicit from the above
discussion, but I would like to make them explicit just in case. When a device file is
accessed, the kernel uses the major number of the file to determine which driver
should be used to handle the access. This means that the kernel doesn’t really need
to use or even know about the minor number. The driver itself is the only thing that
cares about the minor number. It uses the minor number to distinguish between
different pieces of hardware.
-
By the way, when I say "hardware", I mean something a bit more abstract
+
By the way, when I say "hardware", I mean something a bit more abstract
than a PCI card that you can hold in your hand. Look at these two device
files:
@@ -1170,24 +1173,24 @@ $ ls -l /dev/sda /dev/sdb
brw-rw---- 1 root disk 8, 0 Jan 3 09:02 /dev/sda
brw-rw---- 1 root disk 8, 16 Jan 3 09:02 /dev/sdb
-
-
By now you can look at these two device files and know instantly that they are
+
+
By now you can look at these two device files and know instantly that they are
block devices and are handled by same driver (block major 8). Sometimes two device
files with the same major but different minor number can actually represent the same
piece of physical hardware. So just be aware that the word “hardware” in our
discussion can mean something very abstract.
-
+
0.6 Character Device drivers
-
+
0.6.1 The file_operations Structure
-The file_operations
+The file_operations
structure is defined in include/linux/fs.h, and holds pointers to functions defined by
the driver that perform various operations on the device. Each field of the structure
corresponds to the address of some function defined by the driver to handle a
requested operation.
-
For example, every character driver needs to define a function that reads from the
+
For example, every character driver needs to define a function that reads from the
device. The file_operations
structure holds the address of the module’s function that performs that operation.
Here is what the definition looks like for kernel 5.4:
@@ -1231,12 +1234,12 @@ Here is what the definition looks like for kernel 5.4:
36 loff_t len, unsigned int remap_flags);
37 int (*fadvise)(struct file *, loff_t, loff_t, int);
38} __randomize_layout;
-
Some operations are not implemented by a driver. For example, a driver that handles
+
Some operations are not implemented by a driver. For example, a driver that handles
a video card will not need to read from a directory structure. The corresponding entries
in the file_operations
structure should be set to NULL
.
-
There is a gcc extension that makes assigning to this structure more convenient.
+
There is a gcc extension that makes assigning to this structure more convenient.
You will see it in modern drivers, and may catch you by surprise. This is what the
new way of assigning to the structure looks like:
@@ -1250,7 +1253,7 @@ new way of assigning to the structure looks like:
4 open: device_open,
5 release: device_release
6};
-
However, there is also a C99 way of assigning to elements of a structure,
+
However, there is also a C99 way of assigning to elements of a structure,
designated initializers, and this is definitely preferred over using the GNU extension.
You should use this syntax in case someone wants to port your driver. It will help
with compatibility:
@@ -1262,25 +1265,25 @@ with compatibility:
4 .open = device_open,
5 .release = device_release
6};
-
The meaning is clear, and you should be aware that any member of
+
The meaning is clear, and you should be aware that any member of
the structure which you do not explicitly assign will be initialized to
NULL
by gcc.
-
An instance of struct file_operations
+
An instance of struct file_operations
containing pointers to functions that are used to implement
read
, write
, open
, … system calls is commonly named fops
.
-
Since Linux v5.6, the proc_ops
+
Since Linux v5.6, the proc_ops
structure was introduced to replace the use of the
file_operations
structure when registering proc handlers.
-
+
0.6.2 The file structure
-Each device is represented in the kernel by a file structure, which is defined
+
Each device is represented in the kernel by a file structure, which is defined
in include/linux/fs.h. Be aware that a file is a kernel level structure and
never appears in a user space program. It is not the same thing as a
FILE
@@ -1289,34 +1292,34 @@ function. Also, its name is a bit misleading; it represents an abstract open
‘file’, not a file on a disk, which is represented by a structure named
inode
.
-
An instance of struct file is commonly named
+
An instance of struct file is commonly named
filp
. You’ll also see it referred to as a struct file object. Resist the temptation.
-
Go ahead and look at the definition of file. Most of the entries you see, like struct
+
Go ahead and look at the definition of file. Most of the entries you see, like struct
dentry are not used by device drivers, and you can ignore them. This is because
drivers do not fill file directly; they only use structures contained in file which are
created elsewhere.
-
+
0.6.3 Registering A Device
-As discussed earlier, char devices are accessed through device files, usually located in
+
As discussed earlier, char devices are accessed through device files, usually located in
/dev. This is by convention. When writing a driver, it is OK to put the
device file in your current directory. Just make sure you place it in /dev for a
production driver. The major number tells you which driver handles which
device file. The minor number is used only by the driver itself to differentiate
which device it is operating on, just in case the driver handles more than one
device.
-
Adding a driver to your system means registering it with the kernel. This is synonymous
+
Adding a driver to your system means registering it with the kernel. This is synonymous
with assigning it a major number during the module’s initialization. You do this by
using the register_chrdev
function, defined by include/linux/fs.h.
1int register_chrdev(unsigned int major, const char *name, struct file_operations *fops);
- where unsigned int major is the major number you want to request,
+
where unsigned int major is the major number you want to request,
const char *name
is the name of the device as it will appear in /proc/devices and
struct file_operations *fops
@@ -1326,13 +1329,13 @@ registration failed. Note that we didn’t pass the minor number to
register_chrdev
. That is because the kernel doesn’t care about the minor number; only our driver
uses it.
-
Now the question is, how do you get a major number without hijacking
+
Now the question is, how do you get a major number without hijacking
one that’s already in use? The easiest way would be to look through
Documentation/admin-guide/devices.txt and pick an unused one. That is a bad way
of doing things because you will never be sure if the number you picked will be
assigned later. The answer is that you can ask the kernel to assign you a dynamic
major number.
-
If you pass a major number of 0 to register_chrdev
+
If you pass a major number of 0 to register_chrdev
, the return value will be the dynamically allocated major number. The
downside is that you can not make a device file in advance, since you do not
know what the major number will be. There are a couple of ways to do
@@ -1349,10 +1352,10 @@ third method is that we can have our driver make the device file using the
device_destroy
during the call to cleanup_module
.
-
+
0.6.4 Unregistering A Device
-We can not allow the kernel module to be
+
We can not allow the kernel module to be
rmmod
’ed whenever root feels like it. If the device file is opened by a process and then we
remove the kernel module, using the file would cause a call to the memory location
@@ -1362,7 +1365,7 @@ unlucky, another kernel module was loaded into the same location, which
means a jump into the middle of another function within the kernel. The
results of this would be impossible to predict, but they can not be very
positive.
-
Normally, when you do not want to allow something, you return an error code
+
Normally, when you do not want to allow something, you return an error code
(a negative number) from the function which is supposed to do it. With
cleanup_module
that’s impossible because it is a void function. However, there is a counter
@@ -1388,19 +1391,19 @@ decrease and display this counter:
module_refcount(THIS_MODULE)
: Return the value of reference count of current module. It is important to keep the counter accurate; if you ever do lose track of the
+
It is important to keep the counter accurate; if you ever do lose track of the
correct usage count, you will never be able to unload the module; it’s now reboot
time, boys and girls. This is bound to happen to you sooner or later during a
module’s development.
-
+
0.6.5 chardev.c
-The next code sample creates a char driver named chardev. You can cat its device
+
The next code sample creates a char driver named chardev. You can cat its device
file.
1cat /proc/devices
- (or open the file with a program) and the driver will put the number of times the
+
(or open the file with a program) and the driver will put the number of times the
device file has been read from into the file. We do not support writing to the file (like
echo "hi" > /dev/hello
), but catch these attempts and tell the user that the operation is not supported.
@@ -1558,10 +1561,10 @@ acknowledging that we received it.
147module_exit(chardev_exit);
148
149MODULE_LICENSE("GPL");
-
+
0.6.6 Writing Modules for Multiple Kernel Versions
-The system calls, which are the major interface the kernel shows to the processes,
+
The system calls, which are the major interface the kernel shows to the processes,
generally stay the same across versions. A new system call may be added, but
usually the old ones will behave exactly like they used to. This is necessary for
backward compatibility – a new kernel version is not supposed to break regular
@@ -1571,22 +1574,22 @@ backward compatibility – a new kernel version is not supposed to break regular
processes. In most cases, the device files will also remain the same. On the other
hand, the internal interfaces within the kernel can and do change between
versions.
-
There are differences between different kernel versions, and if you want
+
There are differences between different kernel versions, and if you want
to support multiple kernel versions, you will find yourself having to code
conditional compilation directives. The way to do this to compare the macro
LINUX_VERSION_CODE
to the macro KERNEL_VERSION
. In version a.b.c of the kernel, the value of this macro would be 
.
-
+
0.7 The /proc File System
-In Linux, there is an additional mechanism for the kernel and kernel modules to send
+
In Linux, there is an additional mechanism for the kernel and kernel modules to send
information to processes — the /proc file system. Originally designed to allow easy
access to information about processes (hence the name), it is now used by every bit
of the kernel which has something interesting to report, such as /proc/modules
which provides the list of modules and /proc/meminfo which gathers memory usage
statistics.
-
The method to use the proc file system is very similar to the one used with device
+
The method to use the proc file system is very similar to the one used with device
drivers — a structure is created with all the information needed for the /proc file,
including pointers to any handler functions (in our case there is only one, the
one called when somebody attempts to read from the /proc file). Then,
@@ -1594,18 +1597,18 @@ one called when somebody attempts to read from the registers the structure with the kernel and
cleanup_module
unregisters it.
-
Normal file systems are located on a disk, rather than just in memory (which is
+
Normal file systems are located on a disk, rather than just in memory (which is
where /proc is), and in that case the inode number is a pointer to a disk
location where the file’s index-node (inode for short) is located. The inode
contains information about the file, for example the file’s permissions, together
with a pointer to the disk location or locations where the file’s data can be
found.
-
Because we don’t get called when the file is opened or closed, there’s nowhere for
+
Because we don’t get called when the file is opened or closed, there’s nowhere for
us to put try_module_get
and module_put
in this module, and if the file is opened and then the module is removed, there’s no
way to avoid the consequences.
-
Here a simple example showing how to use a /proc file. This is the HelloWorld for
+
Here a simple example showing how to use a /proc file. This is the HelloWorld for
the /proc filesystem. There are three parts: create the file /proc/helloworld in the
function init_module
, return a value (and a buffer) when the file /proc/helloworld is read in the callback
@@ -1616,12 +1619,12 @@ function procfile_read
cleanup_module
.
-
The /proc/helloworld is created when the module is loaded with the function
+
The /proc/helloworld is created when the module is loaded with the function
proc_create
. The return value is a struct proc_dir_entry
, and it will be used to configure the file /proc/helloworld (for example, the owner
of this file). A null return value means that the creation has failed.
-
Every time the file /proc/helloworld is read, the function
+
Every time the file /proc/helloworld is read, the function
procfile_read
is called. Two parameters of this function are very important: the buffer
(the second parameter) and the offset (the fourth one). The content of the
@@ -1638,7 +1641,7 @@ function, if it never returns zero, the read function is called endlessly.
$ cat /proc/helloworld
HelloWorld!
-
+
1/*
@@ -1713,10 +1716,10 @@ HelloWorld!
70module_exit(procfs1_exit);
71
72MODULE_LICENSE("GPL");
-
+
0.7.1 The proc_ops Structure
-The proc_ops
+The proc_ops
structure is defined in include/linux/proc_fs.h in Linux v5.6+. In older kernels, it
used file_operations
for custom hooks in /proc file system, but it contains some
@@ -1728,10 +1731,10 @@ performance. For example, the file which never disappears in proc_flag
as PROC_ENTRY_PERMANENT
to save 2 atomic ops, 1 allocation, 1 free in per open/read/close sequence.
-
+
0.7.2 Read and Write a /proc File
-We have seen a very simple example for a /proc file where we only read
+
We have seen a very simple example for a /proc file where we only read
the file /proc/helloworld. It is also possible to write in a /proc file. It
works the same way as read, a function is called when the /proc file
is written. But there is a little difference with read, data comes from
@@ -1739,7 +1742,7 @@ user, so you have to import data from user space to kernel space (with
copy_from_user
or get_user
)
-
The reason for copy_from_user
+
The reason for copy_from_user
or get_user
is that Linux memory (on Intel architecture, it may be different under some
@@ -1750,7 +1753,7 @@ not reference a unique location in memory, only a location in a memory
segment, and you need to know which memory segment it is to be able to use
it. There is one memory segment for the kernel, and one for each of the
processes.
-
The only memory segment accessible to a process is its own, so when
+
The only memory segment accessible to a process is its own, so when
writing regular programs to run as processes, there is no need to worry about
segments. When you write a kernel module, normally you want to access
the kernel memory segment, which is handled automatically by the system.
@@ -1867,22 +1870,22 @@ because data is already in kernel space.
97module_exit(procfs2_exit);
98
99MODULE_LICENSE("GPL");
-
+
0.7.3 Manage /proc file with standard filesystem
-We have seen how to read and write a /proc file with the /proc interface. But it is
+
We have seen how to read and write a /proc file with the /proc interface. But it is
also possible to manage /proc file with inodes. The main concern is to use advanced
functions, like permissions.
-
In Linux, there is a standard mechanism for file system registration.
+
In Linux, there is a standard mechanism for file system registration.
Since every file system has to have its own functions to handle inode and file
operations, there is a special structure to hold pointers to all those functions,
struct inode_operations
, which includes a pointer to struct proc_ops
.
-
The difference between file and inode operations is that file operations deal with
+
The difference between file and inode operations is that file operations deal with
the file itself whereas inode operations deal with ways of referencing the file, such as
creating links to it.
-
In /proc, whenever we register a new file, we’re allowed to specify which
+
In /proc, whenever we register a new file, we’re allowed to specify which
struct inode_operations
will be used to access to it. This is the mechanism we use, a
struct inode_operations
@@ -1893,7 +1896,7 @@ creating links to it.
which includes pointers to our procf_read
and procfs_write
functions.
-
Another interesting point here is the
+
Another interesting point here is the
module_permission
function. This function is called whenever a process tries to do something with the
/proc file, and it can decide whether to allow access or not. Right now it is only
@@ -1902,7 +1905,7 @@ pointer to a structure which includes information on the currently running
process), but it could be based on anything we like, such as what other
processes are doing with the same file, the time of day, or the last input we
received.
-
It is important to note that the standard roles of read and write are reversed in
+
It is important to note that the standard roles of read and write are reversed in
the kernel. Read functions are used for output, whereas write functions are used for
input. The reason for that is that read and write refer to the user’s point of view — if
a process reads something from the kernel, then the kernel needs to output it, and
@@ -2017,14 +2020,14 @@ input.
105module_exit(procfs3_exit);
106
107MODULE_LICENSE("GPL");
-
Still hungry for procfs examples? Well, first of all keep in mind, there are rumors
+
Still hungry for procfs examples? Well, first of all keep in mind, there are rumors
around, claiming that procfs is on its way out, consider using sysfs instead. Consider
using this mechanism, in case you want to document something kernel related
yourself.
-
+
0.7.4 Manage /proc file with seq_file
-As we have seen, writing a /proc file may be quite “complex”.
+
As we have seen, writing a /proc file may be quite “complex”.
So to help people writting /proc file, there is an API named
seq_file
that helps formating a /proc file for output. It is based on sequence, which is composed of
@@ -2033,7 +2036,7 @@ So to help people writting , and stop()
. The seq_file
API starts a sequence when a user read the /proc file.
-
A sequence begins with the call of the function
+
A sequence begins with the call of the function
start()
. If the return is a non NULL
value, the function next()
@@ -2050,7 +2053,7 @@ time next()
returns NULL
, then the function stop()
is called.
-
BE CAREFUL: when a sequence is finished, another one starts. That means that at the end
+
BE CAREFUL: when a sequence is finished, another one starts. That means that at the end
of function stop()
, the function start()
is called again. This loop finishes when the function
@@ -2067,14 +2070,14 @@ of function stop()
-
+
Figure 1:How seq_file works
- The seq_file
+ The seq_file
provides basic functions for proc_ops
, such as seq_read
, seq_lseek
@@ -2198,23 +2201,23 @@ the same way as in the previous example.
115module_exit(procfs4_exit);
116
117MODULE_LICENSE("GPL");
- If you want more information, you can read this web page:
+
If you want more information, you can read this web page:
- You can also read the code of fs/seq_file.c in the linux kernel.
+
You can also read the code of fs/seq_file.c in the linux kernel.
0.8 sysfs: Interacting with your module
-sysfs allows you to interact with the running kernel from userspace by reading or
+
sysfs allows you to interact with the running kernel from userspace by reading or
setting variables inside of modules. This can be useful for debugging purposes, or just
as an interface for applications or scripts. You can find sysfs directories and files
under the /sys directory on your system.
1ls -l /sys
- An example of a hello world module which includes the creation of a variable
+
An example of a hello world module which includes the creation of a variable
accessible via sysfs is given below.
@@ -2283,7 +2286,7 @@ accessible via sysfs is given below.
63module_exit(mymodule_exit);
64
65MODULE_LICENSE("GPL");
- Make and install the module:
+
Make and install the module:
1make
@@ -2291,36 +2294,36 @@ accessible via sysfs is given below.
- Check that it exists:
+
Check that it exists:
1sudo lsmod | grep hello_sysfs
- What is the current value of myvariable
+ What is the current value of myvariable
?
1cat /sys/kernel/mymodule/myvariable
- Set the value of myvariable
+ Set the value of myvariable
and check that it changed.
1echo "32" > /sys/kernel/mymodule/myvariable
2cat /sys/kernel/mymodule/myvariable
- Finally, remove the test module:
+
Finally, remove the test module:
1sudo rmmod hello_sysfs
-
+
0.9 Talking To Device Files
-Device files are supposed to represent physical devices. Most physical devices are
+
Device files are supposed to represent physical devices. Most physical devices are
used for output as well as input, so there has to be some mechanism for
device drivers in the kernel to get the output to send to the device from
processes. This is done by opening the device file for output and writing to it,
just like writing to a file. In the following example, this is implemented by
device_write
.
-
This is not always enough. Imagine you had a serial port connected to a modem
+
This is not always enough. Imagine you had a serial port connected to a modem
(even if you have an internal modem, it is still implemented from the CPU’s
perspective as a serial port connected to a modem, so you don’t have to tax
your imagination too hard). The natural thing to do would be to use the
@@ -2333,7 +2336,7 @@ received.
-
The answer in Unix is to use a special function called
+
The answer in Unix is to use a special function called
ioctl
(short for Input Output ConTroL). Every device can have its own
ioctl
@@ -2342,12 +2345,12 @@ kernel), write ioctl’s (to return information to a process), both or neither.
here the roles of read and write are reversed again, so in ioctl’s read is to
send information to the kernel and write is to receive information from the
kernel.
-
The ioctl function is called with three parameters: the file descriptor of the
+
The ioctl function is called with three parameters: the file descriptor of the
appropriate device file, the ioctl number, and a parameter, which is of type
long so you can use a cast to use it to pass anything. You will not be able
to pass a structure this way, but you will be able to pass a pointer to the
structure.
-
The ioctl number encodes the major device number, the type of the ioctl, the
+
The ioctl number encodes the major device number, the type of the ioctl, the
command, and the type of the parameter. This ioctl number is usually created by a macro
call ( _IO
, _IOR
@@ -2358,7 +2361,7 @@ included both by the programs which will use ioctl (so they can generate the
appropriate ioctl’s) and by the kernel module (so it can understand it). In the
example below, the header file is chardev.h and the program which uses it is
ioctl.c.
-
If you want to use ioctls in your own kernel modules, it is best to receive an
+
If you want to use ioctls in your own kernel modules, it is best to receive an
official ioctl assignment, so if you accidentally get somebody else’s ioctls, or if they
get yours, you’ll know something is wrong. For more information, consult the kernel
source tree at Documentation/userspace-api/ioctl/ioctl-number.rst.
@@ -2844,18 +2847,18 @@ source tree at 196
197MODULE_LICENSE("GPL");
198MODULE_DESCRIPTION("This is test_ioctl module");
-
+
0.10 System Calls
-So far, the only thing we’ve done was to use well defined kernel mechanisms to
+
So far, the only thing we’ve done was to use well defined kernel mechanisms to
register /proc files and device handlers. This is fine if you want to do something the
kernel programmers thought you’d want, such as write a device driver. But what if
you want to do something unusual, to change the behavior of the system in some
way? Then, you are mostly on your own.
-
If you are not being sensible and using a virtual machine then this is where kernel
+
If you are not being sensible and using a virtual machine then this is where kernel
programming can become hazardous. While writing the example below, I killed the
open()
system call. This meant I could not open any files, I could not run any
@@ -2867,7 +2870,7 @@ ensure you do not lose any files, even within a test environment, please run
right before you do the insmod
and the rmmod
.
-
Forget about /proc files, forget about device files. They are just minor details.
+
Forget about /proc files, forget about device files. They are just minor details.
Minutiae in the vast expanse of the universe. The real process to kernel
communication mechanism, the one used by all processes, is system calls. When a
process requests a service from the kernel (such as opening a file, forking to a new
@@ -2876,11 +2879,11 @@ change the behaviour of the kernel in interesting ways, this is the place to do
it. By the way, if you want to see which system calls a program uses, run
strace <arguments>
.
-
In general, a process is not supposed to be able to access the kernel. It can not
+
In general, a process is not supposed to be able to access the kernel. It can not
access kernel memory and it can’t call kernel functions. The hardware of the CPU
enforces this (that is the reason why it is called “protected mode” or “page
protection”).
-
System calls are an exception to this general rule. What happens is that the
+
System calls are an exception to this general rule. What happens is that the
process fills the registers with the appropriate values and then calls a special
instruction which jumps to a previously defined location in the kernel (of course, that
location is readable by user processes, it is not writable by them). Under Intel CPUs,
@@ -2888,7 +2891,7 @@ this is done by means of interrupt 0x80. The hardware knows that once you jump t
this location, you are no longer running in restricted user mode, but as the
operating system kernel — and therefore you’re allowed to do whatever you
want.
-
The location in the kernel a process can jump to is called system_call. The
+
The location in the kernel a process can jump to is called system_call. The
procedure at that location checks the system call number, which tells the kernel what
service the process requested. Then, it looks at the table of system calls
( sys_call_table
@@ -2901,7 +2904,7 @@ different process, if the process time ran out). If you want to read this code,
at the source file arch/$(architecture)/kernel/entry.S, after the line
ENTRY(system_call)
.
-
So, if we want to change the way a certain system call works, what we need to do
+
So, if we want to change the way a certain system call works, what we need to do
is to write our own function to implement it (usually by adding a bit of our own
code, and then calling the original function) and then change the pointer at
sys_call_table
@@ -2909,7 +2912,7 @@ code, and then calling the original function) and then change the pointer at
don’t want to leave the system in an unstable state, it’s important for
cleanup_module
to restore the table to its original state.
-
The source code here is an example of such a kernel module. We want to “spy” on a certain
+
The source code here is an example of such a kernel module. We want to “spy” on a certain
user, and to pr_info()
a message whenever that user opens a file. Towards this end, we
replace the system call to open a file with our own function, called
@@ -2919,7 +2922,7 @@ spy on, it calls pr_info()
to display the name of the file to be opened. Then, either way, it calls the original
open()
function with the same parameters, to actually open the file.
-
The init_module
+
The init_module
function replaces the appropriate location in
sys_call_table
and keeps the original pointer in a variable. The
@@ -2937,7 +2940,7 @@ with B_open
, which will call what it thinks is the original system call,
A_open
, when it’s done.
-
Now, if B is removed first, everything will be well — it will simply restore the system
+
Now, if B is removed first, everything will be well — it will simply restore the system
call to A_open
, which calls the original. However, if A is removed and then B is removed, the
system will crash. A’s removal will restore the system call to the original,
@@ -2960,7 +2963,7 @@ problem. When A is removed, it sees that the system call was changed to
will still try to call A_open
which is no longer there, so that even without removing B the system would
crash.
-
Note that all the related problems make syscall stealing unfeasible for
+
Note that all the related problems make syscall stealing unfeasible for
production use. In order to keep people from doing potential harmful things
sys_call_table
is no longer exported. This means, if you want to do something more than a mere
@@ -3111,13 +3114,13 @@ hand apply the patch.
135module_exit(syscall_end);
136
137MODULE_LICENSE("GPL");
-
+
0.11 Blocking Processes and threads
-
+
0.11.1 Sleep
-What do you do when somebody asks you for something you can not do right
+
What do you do when somebody asks you for something you can not do right
away? If you are a human being and you are bothered by a human being, the
only thing you can say is: "Not right now, I’m busy. Go away!". But if you
are a kernel module and you are bothered by a process, you have another
@@ -3128,21 +3131,21 @@ processes are being put to sleep by the kernel and woken up all the time (that
is the way multiple processes appear to run on the same time on a single
CPU).
-
This kernel module is an example of this. The file (called /proc/sleep) can only
+
This kernel module is an example of this. The file (called /proc/sleep) can only
be opened by a single process at a time. If the file is already open, the kernel module
calls wait_event_interruptible
. The easiest way to keep a file open is to open it with:
1tail -f
- This function changes the status of the task (a task is the kernel data structure
+
This function changes the status of the task (a task is the kernel data structure
which holds information about a process and the system call it is in, if any) to
TASK_INTERRUPTIBLE
, which means that the task will not run until it is woken up somehow, and adds it to
WaitQ, the queue of tasks waiting to access the file. Then, the function calls the
scheduler to context switch to a different process, one which has some use for the
CPU.
-
When a process is done with the file, it closes it, and
+
When a process is done with the file, it closes it, and
module_close
is called. That function wakes up all the processes in the queue (there’s no
mechanism to only wake up one of them). It then returns and the process which just
@@ -3152,20 +3155,20 @@ Eventually, one of the processes which was in the queue will be given control
of the CPU by the scheduler. It starts at the point right after the call to
module_interruptible_sleep_on
.
-
This means that the process is still in kernel mode - as far as the process
+
This means that the process is still in kernel mode - as far as the process
is concerned, it issued the open system call and the system call has not
returned yet. The process does not know somebody else used the CPU for
most of the time between the moment it issued the call and the moment it
returned.
-
It can then proceed to set a global variable to tell all the other processes that the
+
It can then proceed to set a global variable to tell all the other processes that the
file is still open and go on with its life. When the other processes get a piece of the
CPU, they’ll see that global variable and go back to sleep.
-
So we will use tail -f
+
So we will use tail -f
to keep the file open in the background, while trying to access it with another
process (again in the background, so that we need not switch to a different vt). As
soon as the first background process is killed with kill %1 , the second is woken up, is
able to access the file and finally terminates.
-
To make our life more interesting, module_close
+
To make our life more interesting, module_close
does not have a monopoly on waking up the processes which wait to access the file.
A signal, such as Ctrl +c (SIGINT) can also wake up a process. This is because we
used module_interruptible_sleep_on
@@ -3175,11 +3178,11 @@ used module_interruptible_sleep_on
instead, but that would have resulted in extremely angry users whose Ctrl+c’s are
ignored.
-
In that case, we want to return with
+
In that case, we want to return with
-EINTR
immediately. This is important so users can, for example, kill the process before it
receives the file.
-
There is one more point to remember. Some times processes don’t want to sleep, they want
+
There is one more point to remember. Some times processes don’t want to sleep, they want
either to get what they want immediately, or to be told it cannot be done. Such processes
use the O_NONBLOCK
flag when opening the file. The kernel is supposed to respond by returning with the error
@@ -3215,7 +3218,7 @@ $ cat_nonblock /proc/sleep
Last input:
$
-
+
1/*
@@ -3499,14 +3502,14 @@ $
57
58 return 0;
59}
-
+
0.11.2 Completions
-Sometimes one thing should happen before another within a module having multiple threads.
+
Sometimes one thing should happen before another within a module having multiple threads.
Rather than using /bin/sleep
commands, the kernel has another way to do this which allows timeouts or
interrupts to also happen.
-
In the following example two threads are started, but one needs to start before
+
In the following example two threads are started, but one needs to start before
another.
@@ -3589,31 +3592,31 @@ another.
74
75MODULE_DESCRIPTION("Completions example");
76MODULE_LICENSE("GPL");
-
The machine
+ The machine
structure stores the completion states for the two threads. At the exit
point of each thread the respective completion state is updated, and
wait_for_completion
is used by the flywheel thread to ensure that it does not begin prematurely.
-
So even though flywheel_thread
+
So even though flywheel_thread
is started first you should notice if you load this module and run
dmesg
that turning the crank always happens first because the flywheel thread waits for it
to complete.
-
There are other variations upon the
+
There are other variations upon the
wait_for_completion
function, which include timeouts or being interrupted, but this basic mechanism is
enough for many common situations without adding a lot of complexity.
-
+
0.12 Avoiding Collisions and Deadlocks
-If processes running on different CPUs or in different threads try to access the same
+
If processes running on different CPUs or in different threads try to access the same
memory, then it is possible that strange things can happen or your system can lock
up. To avoid this, various types of mutual exclusion kernel functions are available.
These indicate if a section of code is "locked" or "unlocked" so that simultaneous
attempts to run it can not happen.
0.12.1 Mutex
-You can use kernel mutexes (mutual exclusions) in much the same manner that you
+
You can use kernel mutexes (mutual exclusions) in much the same manner that you
might deploy them in userland. This may be all that is needed to avoid collisions in
most cases.
@@ -3659,10 +3662,10 @@ most cases.
39
40MODULE_DESCRIPTION("Mutex example");
41MODULE_LICENSE("GPL");
-
+
0.12.2 Spinlocks
-As the name suggests, spinlocks lock up the CPU that the code is running on,
+
As the name suggests, spinlocks lock up the CPU that the code is running on,
taking 100% of its resources. Because of this you should only use the spinlock
@@ -3670,7 +3673,7 @@ taking 100% of its resources. Because of this you should only use the spinlock
mechanism around code which is likely to take no more than a few milliseconds to
run and so will not noticeably slow anything down from the user’s point of
view.
-
The example here is "irq safe" in that if interrupts happen during the lock then
+
The example here is "irq safe" in that if interrupts happen during the lock then
they will not be forgotten and will activate when the unlock happens, using the
flags
variable to retain their state.
@@ -3739,10 +3742,10 @@ they will not be forgotten and will activate when the unlock happens, using the
61
62MODULE_DESCRIPTION("Spinlock example");
63MODULE_LICENSE("GPL");
-
+
0.12.3 Read and write locks
-Read and write locks are specialised kinds of spinlocks so that you can exclusively
+
Read and write locks are specialised kinds of spinlocks so that you can exclusively
read from something or write to something. Like the earlier spinlocks example, the
one below shows an "irq safe" situation in which if other functions were triggered
from irqs which might also read and write to whatever you are concerned with
@@ -3807,14 +3810,14 @@ module.
53
54MODULE_DESCRIPTION("Read/Write locks example");
55MODULE_LICENSE("GPL");
-
Of course, if you know for sure that there are no functions triggered by irqs
+
Of course, if you know for sure that there are no functions triggered by irqs
which could possibly interfere with your logic then you can use the simpler
read_lock(&myrwlock)
and read_unlock(&myrwlock)
or the corresponding write functions.
0.12.4 Atomic operations
-If you are doing simple arithmetic: adding, subtracting or bitwise operations, then
+
If you are doing simple arithmetic: adding, subtracting or bitwise operations, then
there is another way in the multi-CPU and multi-hyperthreaded world to stop other
parts of the system from messing with your mojo. By using atomic operations you
can be confident that your addition, subtraction or bit flip did actually happen
@@ -3899,21 +3902,21 @@ below.
-
+
0.13 Replacing Print Macros
-
+
0.13.1 Replacement
-In Section 2, I said that X Window System and kernel module programming do not
+
In Section 2, I said that X Window System and kernel module programming do not
mix. That is true for developing kernel modules. But in actual use, you want to be
able to send messages to whichever tty the command to load the module came
from.
-
"tty" is an abbreviation of teletype: originally a combination keyboard-printer
+
"tty" is an abbreviation of teletype: originally a combination keyboard-printer
used to communicate with a Unix system, and today an abstraction for the text
stream used for a Unix program, whether it is a physical terminal, an xterm on an X
display, a network connection used with ssh, etc.
-
The way this is done is by using current, a pointer to the currently running task,
+
The way this is done is by using current, a pointer to the currently running task,
to get the current task’s tty structure. Then, we look inside that tty structure to find
a pointer to a string write function, which we use to write a string to the
tty.
@@ -3996,16 +3999,16 @@ tty.
75module_exit(print_string_exit);
76
77MODULE_LICENSE("GPL");
-
+
0.13.2 Flashing keyboard LEDs
-In certain conditions, you may desire a simpler and more direct way to communicate
+
In certain conditions, you may desire a simpler and more direct way to communicate
to the external world. Flashing keyboard LEDs can be such a solution: It is an
immediate way to attract attention or to display a status condition. Keyboard LEDs
are present on every hardware, they are always visible, they do not need any setup,
and their use is rather simple and non-intrusive, compared to writing to a tty or a
file.
-
The following source code illustrates a minimal kernel module which, when
+
The following source code illustrates a minimal kernel module which, when
loaded, starts blinking the keyboard LEDs until it is unloaded.
@@ -4101,7 +4104,7 @@ loaded, starts blinking the keyboard LEDs until it is unloaded.
- If none of the examples in this chapter fit your debugging needs,
+
If none of the examples in this chapter fit your debugging needs,
there might yet be some other tricks to try. Ever wondered what
CONFIG_LL_DEBUG
in make menuconfig
@@ -4112,22 +4115,22 @@ everything what your code does over a serial line. If you find yourself porting
kernel to some new and former unsupported architecture, this is usually amongst the
first things that should be implemented. Logging over a netconsole might also be
worth a try.
-
While you have seen lots of stuff that can be used to aid debugging here, there are
+
While you have seen lots of stuff that can be used to aid debugging here, there are
some things to be aware of. Debugging is almost always intrusive. Adding debug code
can change the situation enough to make the bug seem to dissappear. Thus you
should try to keep debug code to a minimum and make sure it does not show up in
production code.
-
+
0.14 Scheduling Tasks
-There are two main ways of running tasks: tasklets and work queues. Tasklets are a
+
There are two main ways of running tasks: tasklets and work queues. Tasklets are a
quick and easy way of scheduling a single function to be run. For example, when
triggered from an interrupt, whereas work queues are more complicated but also
better suited to running multiple things in a sequence.
-
+
0.14.1 Tasklets
-Here is an example tasklet module. The
+
Here is an example tasklet module. The
tasklet_fn
function runs for a few seconds and in the mean time execution of the
example_tasklet_init
@@ -4171,7 +4174,7 @@ better suited to running multiple things in a sequence.
35
36MODULE_DESCRIPTION("Tasklet example");
37MODULE_LICENSE("GPL");
- So with this example loaded dmesg
+ So with this example loaded dmesg
should show:
@@ -4183,11 +4186,11 @@ Example tasklet starts
Example tasklet init continues...
Example tasklet ends
-
-
+
+
0.14.2 Work queues
-To add a task to the scheduler we can use a workqueue. The kernel then uses the
+
To add a task to the scheduler we can use a workqueue. The kernel then uses the
Completely Fair Scheduler (CFS) to execute work within the queue.
@@ -4225,19 +4228,19 @@ Completely Fair Scheduler (CFS) to execute work within the queue.
32
33MODULE_LICENSE("GPL");
34MODULE_DESCRIPTION("Workqueue example");
-
+
0.15 Interrupt Handlers
-
+
0.15.1 Interrupt Handlers
-Except for the last chapter, everything we did in the kernel so far we have done as a
+
Except for the last chapter, everything we did in the kernel so far we have done as a
response to a process asking for it, either by dealing with a special file, sending an
ioctl()
, or issuing a system call. But the job of the kernel is not just to respond to process
requests. Another job, which is every bit as important, is to speak to the hardware
connected to the machine.
-
There are two types of interaction between the CPU and the rest of the
+
There are two types of interaction between the CPU and the rest of the
computer’s hardware. The first type is when the CPU gives orders to the hardware,
the order is when the hardware needs to tell the CPU something. The second, called
interrupts, is much harder to implement because it has to be dealt with when
@@ -4247,14 +4250,14 @@ lost.
-
Under Linux, hardware interrupts are called IRQ’s (Interrupt ReQuests). There
+
Under Linux, hardware interrupts are called IRQ’s (Interrupt ReQuests). There
are two types of IRQ’s, short and long. A short IRQ is one which is expected to take
a very short period of time, during which the rest of the machine will be blocked and
no other interrupts will be handled. A long IRQ is one which can take longer, and
during which other interrupts may occur (but not interrupts from the same
device). If at all possible, it is better to declare an interrupt handler to be
long.
-
When the CPU receives an interrupt, it stops whatever it is doing (unless it is
+
When the CPU receives an interrupt, it stops whatever it is doing (unless it is
processing a more important interrupt, in which case it will deal with this one
only when the more important one is done), saves certain parameters on
the stack and calls the interrupt handler. This means that certain things
@@ -4266,10 +4269,10 @@ the new information at a later time (this is called the "bottom half") and
return. The kernel is then guaranteed to call the bottom half as soon as
possible – and when it does, everything allowed in kernel modules will be
allowed.
-
The way to implement this is to call
+
The way to implement this is to call
request_irq()
to get your interrupt handler called when the relevant IRQ is received.
-
In practice IRQ handling can be a bit more complex. Hardware is often
+
In practice IRQ handling can be a bit more complex. Hardware is often
designed in a way that chains two interrupt controllers, so that all the IRQs
from interrupt controller B are cascaded to a certain IRQ from interrupt
controller A. Of course, that requires that the kernel finds out which IRQ it
@@ -4283,7 +4286,7 @@ need to solve another truckload of problems. It is not enough to know if a
certain IRQs has happened, it’s also important to know what CPU(s) it was
for. People still interested in more details, might want to refer to "APIC"
now.
-
This function receives the IRQ number, the name of the function,
+
This function receives the IRQ number, the name of the function,
flags, a name for /proc/interrupts and a parameter to be passed to the
interrupt handler. Usually there is a certain number of IRQs available.
How many IRQs there are is hardware-dependent. The flags can include
@@ -4296,16 +4299,16 @@ already a handler on this IRQ, or if you are both willing to share.
-
+
0.15.2 Detecting button presses
-Many popular single board computers, such as Raspberry Pi or Beagleboards, have a
+
Many popular single board computers, such as Raspberry Pi or Beagleboards, have a
bunch of GPIO pins. Attaching buttons to those and then having a button press do
something is a classic case in which you might need to use interrupts, so that instead
of having the CPU waste time and battery power polling for a change in input state,
it is better for the input to trigger the CPU to then run a particular handling
function.
-
Here is an example where buttons are connected to GPIO numbers 17 and 18 and
+
Here is an example where buttons are connected to GPIO numbers 17 and 18 and
an LED is connected to GPIO 4. You can change those numbers to whatever is
appropriate for your board.
@@ -4455,14 +4458,14 @@ appropriate for your board.
143
144MODULE_LICENSE("GPL");
145MODULE_DESCRIPTION("Handle some GPIO interrupts");
-
+
0.15.3 Bottom Half
-Suppose you want to do a bunch of stuff inside of an interrupt routine. A common
+
Suppose you want to do a bunch of stuff inside of an interrupt routine. A common
way to do that without rendering the interrupt unavailable for a significant duration
is to combine it with a tasklet. This pushes the bulk of the work off into the
scheduler.
-
The example below modifies the previous example to also run an additional task
+
The example below modifies the previous example to also run an additional task
when an interrupt is triggered.
@@ -4627,10 +4630,10 @@ when an interrupt is triggered.
159
160MODULE_LICENSE("GPL");
161MODULE_DESCRIPTION("Interrupt with top and bottom half");
-
+
0.16 Crypto
-At the dawn of the internet, everybody trusted everybody completely…but that did
+
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
@@ -4639,10 +4642,10 @@ favourite hash functions.
-
+
0.16.1 Hash functions
-Calculating and checking the hashes of things is a common operation. Here is a
+
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.
@@ -4708,21 +4711,21 @@ demonstration of how to calculate a sha256 hash within a kernel module.
60
61MODULE_DESCRIPTION("sha256 hash test");
62MODULE_LICENSE("GPL");
- Make and install the module:
+
Make and install the module:
1make
2sudo insmod cryptosha256.ko
3dmesg
- And you should see that the hash was calculated for the test string.
-
Finally, remove the test module:
+
And you should see that the hash was calculated for the test string.
+
Finally, remove the test module:
1sudo rmmod cryptosha256
-
+
0.16.2 Symmetric key encryption
-Here is an example of symmetrically encrypting a string using the AES algorithm
+
Here is an example of symmetrically encrypting a string using the AES algorithm
and a password.
@@ -4923,10 +4926,10 @@ and a password.
195
196MODULE_DESCRIPTION("Symmetric key encryption example");
197MODULE_LICENSE("GPL");
-
+
0.17 Standardizing the interfaces: The Device Model
-Up to this point we have seen all kinds of modules doing all kinds of things, but there
+
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 a device model was added. An example is shown below, and you can
@@ -5036,13 +5039,13 @@ functions.
-
+
0.18 Optimizations
-
+
0.18.1 Likely and Unlikely conditions
-Sometimes you might want your code to run as quickly as possible,
+
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
@@ -5061,50 +5064,50 @@ to succeed.
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.
-
+
0.19 Common Pitfalls
-
+
0.19.1 Using standard libraries
-You can not do that. In a kernel module, you can only use kernel functions which are
+
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.
-
+
0.19.2 Disabling interrupts
-You might need to do this for a short time and that is OK, but if you do not enable
+
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.
-
+
0.20 Where To Go From Here?
-For people seriously interested in kernel programming, I recommend kernelnewbies.org
+
For people seriously interested in kernel programming, I recommend kernelnewbies.org
and the Documentation subdirectory within the kernel source code which is not
always easy to understand but can be a starting point for further investigation. Also,
as Linus Torvalds said, the best way to learn the kernel is to read the source code
yourself.
-
If you are interested in more examples of short kernel modules then searching on
+
If you are interested in more examples of short kernel modules then searching on
sites such as Github and Gitlab is a good way to start, although there is a lot of
duplication of older LKMPG examples which may not compile with newer kernel
versions. You will also be able to find examples of the use of kernel modules to attack
or compromise systems or exfiltrate data and those can be useful for thinking about
how to defend systems and learning about existing security mechanisms within the
kernel.
-
I hope I have helped you in your quest to become a better programmer, or at
+
I hope I have helped you in your quest to become a better programmer, or at
least to have fun through technology. And, if you do write useful kernel modules, I
hope you publish them under the GPL, so I can use them too.
-
If you would like to contribute to this guide or notice anything glaringly wrong,
+
If you would like to contribute to this guide or notice anything glaringly wrong,
please create an issue at https://github.com/sysprog21/lkmpg.
-
Happy hacking!
+
Happy hacking!