4.4.8 Writing Audit Plugins

This section describes how to write a server-side audit 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 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, search the server source files for invocations of functions with names of the form mysql_audit_xxx(). Audit notification occurs for server operations such as these:

• Client connect and disconnect events

• Writing a message to the general query log (if the log is enabled)

• Writing a message to the error log

• Sending a query result to a client

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 4.4.2.1, “Server Plugin Library and Plugin Descriptors”) and a type-specific plugin descriptor. In audit_null.c, the general descriptor for audit_null 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) */
  0x0003,                     /* version                         */
  simple_status,              /* status variables                */
  NULL,                       /* system variables                */
  NULL,
  0,
}
mysql_declare_plugin_end;

The first member, MYSQL_AUDIT_PLUGIN, identifies this plugin as an audit plugin.

audit_null_descriptor points to the type-specific plugin descriptor, described later.

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 plugin 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 },
  { "Audit_null_general_status",
    (char *) &number_of_calls_general_status,
    SHOW_INT },
  { "Audit_null_connection_connect",
    (char *) &number_of_calls_connection_connect,
    SHOW_INT },
  { "Audit_null_connection_disconnect",
    (char *) &number_of_calls_connection_disconnect,
    SHOW_INT },
  { "Audit_null_connection_change_user",
    (char *) &number_of_calls_connection_change_user,
    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 when 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 plugin descriptor points to the type-specific plugin 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 plugin descriptor for audit plugins has these members:

• interface_version: By convention, type-specific plugin descriptors begin with the interface version for the given plugin type. The server checks interface_version when it loads the plugin to see whether the plugin is compatible with it. For audit plugins, the value of the interface_version member is MYSQL_AUDIT_INTERFACE_VERSION (defined in plugin_audit.h).

• release_thd: A function that the server calls to inform the plugin that it is being dissociated from its thread context. This should be NULL if there is no such function.

• event_notify: A function that the server calls to notify the plugin that an auditable event has occurred. This function should not be NULL; that would not make sense because no auditing would occur.

• class_mask: A bitmask that indicates the event classes for which the plugin wants to receive notification. If this value is 0, the server passes no events to the plugin.

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 audit plugin, the type-specific plugin 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 |
                    MYSQL_AUDIT_CONNECTION_CLASSMASK } /* class mask        */
};

The server calls audit_null_notify() to pass audit event information to the plugin. The plugin has no release_thd function.

The event class mask indicates an interest in all events of the “general” and “connection” classes. plugin_audit.h defines symbols for these classes and their corresponding class masks:

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

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

In the type-specific plugin 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:

• event_subclass: The event subclass, one of the following values:

  #define MYSQL_AUDIT_GENERAL_LOG 0
  #define MYSQL_AUDIT_GENERAL_ERROR 1
  #define MYSQL_AUDIT_GENERAL_RESULT 2
  #define MYSQL_AUDIT_GENERAL_STATUS 3

• general_error_code: The error code. This is a value like that returned by the mysql_errno() C API function; 0 means “no error.”

• general_thread_id: The ID of the thread for which the event occurred.

• general_user: The current user for the event.

• general_user_length: The length of general_user, in bytes.

• general_command: For general query log events, the type of operation. Examples: Connect, Query, Shutdown. For error log events, the error message. This is a value like that returned by the mysql_error() C API function; an empty string means “no error.” For result events, this is empty.

• general_command_length: The length of general_command, in bytes.

• general_query: The SQL statement that was logged or produced a result.

• general_query_length: The length of general_query, in bytes.

• general_charset: Character set information for the event.

• general_time: A TIMESTAMP value indicating the time just before the notification function was called.

• general_rows: For general query log events, zero. For error log events, the row number at which an error occurred. For result events, the number of rows in the result plus one. For statements that produce no result set, the value is 0. This encoding enables statements that produce no result set to be distinguished from those that produce an empty result set. For example, for a DELETE statement, this value is 0. For a SELECT, the result is always 1 or more, where 1 represents an empty result set.

• general_host: For general query log events, a string representing the client host name.

• general_sql_command: For general query log events, a string that indicates the type of action performed, such as connect or drop_table.

• general_external_user: For general query log events, a string representing the external user (empty if none).

• general_ip: For general query log events, a string representing the client IP address.

The general_host, general_sql_command, general_external_user, and general_ip members are new in MySQL 5.6.14. 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

If the server calls the notification function with an event class of MYSQL_AUDIT_CONNECTION_CLASS, it passes the event structure as a pointer to a mysql_event_connection structure, which is similar to and interpreted much the same way as the mysql_event_general structure.

The NULL_AUDIT plugin notification function is quite simple. It increments a global event counter, determines the event 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)
{
  /* prone to races, oh well */
  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;
    case MYSQL_AUDIT_GENERAL_STATUS:
      number_of_calls_general_status++;
      break;
    default:
      break;
    }
  }
  else if (event_class == MYSQL_AUDIT_CONNECTION_CLASS)
  {
    const struct mysql_event_connection *event_connection=
      (const struct mysql_event_connection *) event;
    switch (event_connection->event_subclass)
    {
    case MYSQL_AUDIT_CONNECTION_CONNECT:
      number_of_calls_connection_connect++;
      break;
    case MYSQL_AUDIT_CONNECTION_DISCONNECT:
      number_of_calls_connection_disconnect++;
      break;
    case MYSQL_AUDIT_CONNECTION_CHANGE_USER:
      number_of_calls_connection_change_user++;
      break;
    default:
      break;
    }
  }
}

To compile and install a plugin library file, use the instructions in Section 4.4.3, “Compiling and Installing Plugin Libraries”. To make the library file available for use, install it in the plugin directory (the directory named by the plugin_dir system variable). For the NULL_AUDIT 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 .so suffix might differ depending on your platform).

To register the plugin at runtime, use this statement, adjusting the .so suffix for your platform as necessary:

INSTALL PLUGIN NULL_AUDIT SONAME 'adt_null.so';

For additional information about plugin loading, see Installing and Uninstalling Plugins.

To verify plugin installation, examine the INFORMATION_SCHEMA.PLUGINS table or use the SHOW PLUGINS statement. See Obtaining Server Plugin Information.

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                 | 1388  |
| Audit_null_connection_change_user | 0     |
| Audit_null_connection_connect     | 22    |
| Audit_null_connection_disconnect  | 21    |
| Audit_null_general_error          | 1     |
| Audit_null_general_log            | 513   |
| Audit_null_general_result         | 415   |
| Audit_null_general_status         | 416   |
+-----------------------------------+-------+

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. (Before MySQL 5.6.24, notification of events for the general query log are received only if the general query log is enabled. As of 5.6.24, notifications are received regardless of whether that log is enabled.)

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

UNINSTALL PLUGIN NULL_AUDIT;