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

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