[PATCH] audit: speedup for syscalls when auditing is disabled

[Michael Neuling <mikey@neuling.org>]

We found that when auditing is disabled using "auditctl -D", that
there's still a significant overhead when doing syscalls.  This overhead
is not present when a single never rule is inserted using "auditctl -a
task,never".  

Using Anton's null syscall microbenchmark from
http://ozlabs.org/~anton/junkcode/null_syscall.c we currently have on a
powerpc machine:

  # auditctl -D
  No rules
  # ./null_syscall
	  null_syscall:     739.03 cycles     100.00%
  # auditctl -a task,never
  # ./null_syscall
	  null_syscall:     204.63 cycles     100.00%

This doesn't seem right, as we'd hope that auditing would have the same
minimal impact when disabled via -D as when we have a single never rule.

The patch below creates a fast path when initialising a task.  If the
rules list for tasks is empty (the disabled -D option), we mark auditing
as disabled for this task.  

When this is applied, our null syscall benchmark improves in the
disabled case to match the single never rule case.

  # auditctl -D
  No rules
  # ./null_syscall
	  null_syscall:     204.62 cycles     100.00%
  # auditctl -a task,never
  # ./null_syscall
	  null_syscall:     204.63 cycles     100.00%

Reported-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Michael Neuling <mikey@neuling.org>
---
I'm not familiar with the auditing code/infrastructure so I may have
misunderstood something here

diff --git a/kernel/auditsc.c b/kernel/auditsc.c
index 1b31c13..1cd6ec7 100644
--- a/kernel/auditsc.c
+++ b/kernel/auditsc.c
@@ -666,6 +666,11 @@ static enum audit_state audit_filter_task(struct task_struct *tsk, char **key)
 	enum audit_state   state;
 
 	rcu_read_lock();
+	/* Fast path.  If the list is empty, disable auditing */
+	if (list_empty(&audit_filter_list[AUDIT_FILTER_TASK])) {
+		rcu_read_unlock();
+		return AUDIT_DISABLED;
+	}
 	list_for_each_entry_rcu(e, &audit_filter_list[AUDIT_FILTER_TASK], list) {
 		if (audit_filter_rules(tsk, &e->rule, NULL, NULL, &state)) {
 			if (state == AUDIT_RECORD_CONTEXT)