24.2.4.8 Writing Audit Plugins

This section describes how to write an audit server plugin, using the example plugin found in the plugin/audit_null directory of MySQL source distributions. The audit_null.c source file in that directory implements a simple example audit plugin named NULL_AUDIT.

Within the server, the pluggable audit interface is implemented in the sql_audit.h and sql_audit.cc files in the sql directory of MySQL source distributions. Additionally, several places in the server are modified to call the audit interface when an auditable event occurs, so that registered audit plugins can be notified about the event if necessary. To see where such calls occur, look for invocations of functions with names of the form mysql_audit_xxx(). Audit notification occurs for server operations such as these:

To write an audit plugin, include the following header file in the plugin source file. Other MySQL or general header files might also be needed, depending on the plugin capabilities and requirements.

#include <mysql/plugin_audit.h>

plugin_audit.h includes plugin.h, so you need not include the latter file explicitly. plugin.h defines the MYSQL_AUDIT_PLUGIN server plugin type and the data structures needed to declare the plugin. plugin_audit.h defines data structures specific to audit plugins.

An audit plugin, like any MySQL server plugin, has a general plugin descriptor (see Section 24.2.4.2.1, “Server Plugin Library and Plugin Descriptors”). In audit_null.c, the general descriptor looks like this:

mysql_declare_plugin(audit_null)
{
  MYSQL_AUDIT_PLUGIN,         /* type                            */
  &audit_null_descriptor,     /* descriptor                      */
  "NULL_AUDIT",               /* name                            */
  "Oracle Corp",              /* author                          */
  "Simple NULL Audit",        /* description                     */
  PLUGIN_LICENSE_GPL,
  audit_null_plugin_init,     /* init function (when loaded)     */
  audit_null_plugin_deinit,   /* deinit function (when unloaded) */
  0x0002,                     /* version                         */
  simple_status,              /* status variables                */
  NULL,                       /* system variables                */
  NULL,
  0,
}
mysql_declare_plugin_end;

The name member (NULL_AUDIT) indicates the name to use for references to the plugin in statements such as INSTALL PLUGIN or UNINSTALL PLUGIN. This is also the name displayed by INFORMATION_SCHEMA.PLUGINS or SHOW PLUGINS.

The general descriptor also refers to simple_status, a structure that exposes several status variables to the SHOW STATUS statement:

static struct st_mysql_show_var simple_status[]=
{
  { "Audit_null_called", (char *) &number_of_calls, SHOW_INT },
  { "Audit_null_general_log", (char *) &number_of_calls_general_log,
    SHOW_INT },
  { "Audit_null_general_error", (char *) &number_of_calls_general_error,
    SHOW_INT },
  { "Audit_null_general_result", (char *) &number_of_calls_general_result,
    SHOW_INT },
  { 0, 0, 0}
};

The audit_null_plugin_init initialization function sets the status variables to zero when the plugin is loaded. The audit_null_plugin_deinit function performs cleanup with the plugin is unloaded. During operation, the plugin increments the first status variable for each notification it receives. It also increments the others according to the event class and subclass. In effect, the first variable is the aggregate of the counts for the event subclasses.

The audit_null_descriptor value in the general descriptor points to the type-specific descriptor. For audit plugins, this descriptor has the following structure:

struct st_mysql_audit
{
  int interface_version;
  void (*release_thd)(MYSQL_THD);
  void (*event_notify)(MYSQL_THD, unsigned int, const void *);
  unsigned long class_mask[MYSQL_AUDIT_CLASS_MASK_SIZE];
};

The type-specific descriptor has these members:

The server uses the event_notify and release_thd functions together. They are called within the context of a specific thread, and a thread might perform an activity that produces several event notifications. The first time the server calls event_notify for a thread, it creates a binding of the plugin to the thread. The plugin cannot be uninstalled while this binding exists. When no more events for the thread will occur, the server informs the plugin of this by calling the release_thd function, and then destroys the binding. For example, when a client issues a statement, the thread processing the statement might notify audit plugins about the result set produced by the statement and about the statement being logged. After these notifications occur, the server releases the plugin before putting the thread to sleep until the client issues another statement.

This design enables the plugin to allocate resources needed for a given thread in the first call to the event_notify function and release them in the release_thd function:

event_notify function:
  if memory is needed to service the thread
    allocate memory
  ... rest of notification processing ...

release_thd function:
  if memory was allocated
    release memory
  ... rest of release processing ...

That is more efficient than allocating and releasing memory repeatedly in the notification function.

For the NULL_AUDIT example audit plugin, the type-specific descriptor looks like this:

static struct st_mysql_audit audit_null_descriptor=
{
  MYSQL_AUDIT_INTERFACE_VERSION,                    /* interface version    */
  NULL,                                             /* release_thd function */
  audit_null_notify,                                /* notify function      */
  { (unsigned long) MYSQL_AUDIT_GENERAL_CLASSMASK } /* class mask           */
};

The server calls audit_null_notify to pass audit event information to the plugin. There is no release_thd function.

The event class mask indicates an interest in all events of the general class. plugin_audit.h defines its symbol, MYSQL_AUDIT_GENERAL_CLASS, and a mask with a bit for this class:

#define MYSQL_AUDIT_GENERAL_CLASS 0
#define MYSQL_AUDIT_GENERAL_CLASSMASK (1 << MYSQL_AUDIT_GENERAL_CLASS)

plugin_audit.h also has defines for a connection event class, although the NULL_AUDIT plugin does nothing with such events:

#define MYSQL_AUDIT_CONNECTION_CLASS 1
#define MYSQL_AUDIT_CONNECTION_CLASSMASK (1 << MYSQL_AUDIT_CONNECTION_CLASS)

A plugin could be written to receive both general and connection events by setting its type-specific descriptor class mask like this:

  { (unsigned long) MYSQL_AUDIT_GENERAL_CLASSMASK |
                 MYSQL_AUDIT_CONNECTION_CLASSMASK } /* class mask        */

In the type-specific descriptor, the second and third parameters of the event_notify function prototype represent the event class and a generic pointer to an event structure:

void (*event_notify)(MYSQL_THD, unsigned int, const void *);

Events in different classes may have different structures, so the notification function should use the event class value to determine how to interpret the pointer to the event structure.

If the server calls the notification function with an event class of MYSQL_AUDIT_GENERAL_CLASS, it passes the event structure as a pointer to a mysql_event_general structure:

struct mysql_event_general
{
  unsigned int event_subclass;
  int general_error_code;
  unsigned long general_thread_id;
  const char *general_user;
  unsigned int general_user_length;
  const char *general_command;
  unsigned int general_command_length;
  const char *general_query;
  unsigned int general_query_length;
  struct charset_info_st *general_charset;
  unsigned long long general_time;
  unsigned long long general_rows;
};

Audit plugins can interpret mysql_event_general members as follows:

The general_host, general_sql_command, general_external_user, and general_ip members are new in MySQL 5.5.34. These are MYSQL_LEX_STRING structures that pair a string and its length. For example, if event_general is a pointer to a general event, you can access the members of the general_host value as follows:

event_general->general_host.length
event_general->general_host.str

The NULL_AUDIT plugin notification function is quite simple. It increments a global event counter, verifies that the event is of the general class, then looks at the event subclass to determine which subclass counter to increment:

static void audit_null_notify(MYSQL_THD thd __attribute__((unused)),
                              unsigned int event_class,
                              const void *event)
{
  number_of_calls++;
  if (event_class == MYSQL_AUDIT_GENERAL_CLASS)
  {
    const struct mysql_event_general *event_general=
      (const struct mysql_event_general *) event;
    switch (event_general->event_subclass)
    {
    case MYSQL_AUDIT_GENERAL_LOG:
      number_of_calls_general_log++;
      break;
    case MYSQL_AUDIT_GENERAL_ERROR:
      number_of_calls_general_error++;
      break;
    case MYSQL_AUDIT_GENERAL_RESULT:
      number_of_calls_general_result++;
      break;
    default:
      break;
    }
  }
}

To compile and install a plugin library object file, use the instructions in Section 24.2.4.3, “Compiling and Installing Plugin Libraries”. To use the library file, it must be installed in the plugin directory (the directory named by the plugin_dir system variable). For the AUDIT_NULL plugin, it is compiled and installed when you build MySQL from source. It is also included in binary distributions. The build process produces a shared object library with a name of adt_null.so (the suffix might differ depending on your platform).

To register the plugin at runtime, use this statement (change the suffix as necessary):

mysql> INSTALL PLUGIN NULL_AUDIT SONAME 'adt_null.so';

For additional information about plugin loading, see Section 5.1.8.1, “Installing and Uninstalling Plugins”.

To verify plugin installation, examine the INFORMATION_SCHEMA.PLUGINS table or use the SHOW PLUGINS statement.

While the audit plugin is installed, it exposes status variables that indicate the events for which the plugin has been called:

mysql> SHOW STATUS LIKE 'Audit_null%';
+---------------------------+-------+
| Variable_name             | Value |
+---------------------------+-------+
| Audit_null_called         | 1385  |
| Audit_null_general_error  | 1     |
| Audit_null_general_log    | 741   |
| Audit_null_general_result | 643   |
+---------------------------+-------+

Audit_null_called counts all events, and the other variables count instances of event subclasses. For example, the preceding SHOW STATUS statement causes the server to send a result to the client and to write a message to the general query log if that log is enabled. Thus, a client that issues the statement repeatedly causes Audit_null_called and Audit_null_general_result to be incremented each time, and Audit_null_general_log to be incremented if the log is enabled.

To disable the plugin after testing it, use this statement to unload it:

mysql> UNINSTALL PLUGIN NULL_AUDIT;