[PATCH] Exclude messages by message type

- Add a new, 5th filter called "exclude".
    - And add a new field AUDIT_MSGTYPE.
    - Define a new function audit_filter_exclude() that takes a message type
      as input and examines all rules in the filter.  It returns '1' if the
      message is to be excluded, and '0' otherwise.
    - Call the audit_filter_exclude() function near the top of
      audit_log_start() just after asserting audit_initialized.  If the
      message type is not to be audited, return NULL very early, before
      doing a lot of work.
[combined with followup fix for bug in original patch, Nov 4, same author]
[combined with later renaming AUDIT_FILTER_EXCLUDE->AUDIT_FILTER_TYPE
and audit_filter_exclude() -> audit_filter_type()]

Signed-off-by: Dustin Kirkland <dustin.kirkland@us.ibm.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

kernel/audit.c

@@ -702,6 +702,9 @@ struct audit_buffer *audit_log_start(struct audit_context *ctx, gfp_t gfp_mask,
 	if (!audit_initialized)
 		return NULL;
 
+	if (unlikely(audit_filter_type(type)))
+		return NULL;
+
 	if (gfp_mask & __GFP_WAIT)
 		reserve = 0;
 	else