MXS-2403 Reject if masked filed is used in subquery

There's some overlapping work being done now. To be cleanup up
in a subsequent commit.

server/modules/filter/masking/maskingfilter.cc

@@ -133,6 +133,12 @@ extern "C" MXS_MODULE* MXS_CREATE_MODULE()
                 Config::check_unions_default,
                 MXS_MODULE_OPT_NONE,
             },
+            {
+                Config::check_subqueries_name,
+                MXS_MODULE_PARAM_BOOL,
+                Config::check_subqueries_default,
+                MXS_MODULE_OPT_NONE,
+            },
             {MXS_END_MODULE_PARAMS}
         }
     };

server/modules/filter/masking/maskingfiltersession.cc

@@ -110,6 +110,14 @@ bool MaskingFilterSession::check_query(GWBUF* pPacket)
         }
     }
 
+    if (rv && config.check_subqueries())
+    {
+        if (is_subquery_used(pPacket, zUser, zHost))
+        {
+            rv = false;
+        }
+    }
+
     return rv;
 }
 
@@ -683,3 +691,67 @@ bool MaskingFilterSession::is_union_used(GWBUF* pPacket, const char* zUser, cons
 
     return is_used;
 }
+
+bool MaskingFilterSession::is_subquery_used(GWBUF* pPacket, const char* zUser, const char* zHost)
+{
+    if (qc_get_operation(pPacket) != QUERY_OP_SELECT)
+    {
+        return false;
+    }
+
+    bool is_used = false;
+
+    SMaskingRules sRules = m_filter.rules();
+
+    auto pred = [&sRules, zUser, zHost](const QC_FIELD_INFO& field_info) {
+        bool rv = false;
+
+        if (field_info.context & QC_FIELD_SUBQUERY)
+        {
+            if (strcmp(field_info.column, "*") == 0)
+            {
+                // If "*" is used, then we must block if there is any rule for the current user.
+                rv = sRules->has_rule_for(zUser, zHost);
+            }
+            else
+            {
+                rv = sRules->get_rule_for(field_info, zUser, zHost) ? true : false;
+            }
+        }
+
+        return rv;
+    };
+
+    const QC_FIELD_INFO* pInfos;
+    size_t nInfos;
+
+    qc_get_field_info(pPacket, &pInfos, &nInfos);
+
+    const QC_FIELD_INFO* begin = pInfos;
+    const QC_FIELD_INFO* end = begin + nInfos;
+
+    auto i = std::find_if(begin, end, pred);
+
+    if (i != end)
+    {
+        const char* zColumn = i->column;
+
+        std::stringstream ss;
+
+        if (strcmp(zColumn, "*") == 0)
+        {
+            ss << "'*' is used in a subquery and there are masking rules for '"
+               << zUser << "'@'" << zHost << "', access is denied.";
+        }
+        else
+        {
+            ss << "The field " << zColumn << " that should be masked for '"
+               << zUser << "'@'" << zHost << "' is used in a subquery, access is denied.";
+        }
+
+        set_response(create_error_response(ss.str().c_str()));
+        is_used = true;
+    }
+
+    return is_used;
+}

server/modules/filter/masking/maskingfiltersession.hh

@@ -68,6 +68,7 @@ private:
     bool is_function_used(GWBUF* pPacket, const char* zUser, const char* zHost);
     bool is_variable_defined(GWBUF* pPacket, const char* zUser, const char* zHost);
     bool is_union_used(GWBUF* pPacket, const char* zUser, const char* zHost);
+    bool is_subquery_used(GWBUF* pPacket, const char* zUser, const char* zHost);
 
 private:
     typedef std::shared_ptr<MaskingRules> SMaskingRules;