linux-kernel - Re: [PATCH] audit: speedup for syscalls when auditing is disabled

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Date:	Tue, 24 Aug 2010 12:11:20 +1000
From:	Michael Neuling <mikey@...ling.org>
To:	Eric Paris <eparis@...hat.com>
cc:	linux-audit@...hat.com, linux-kernel@...r.kernel.org,
	Al Viro <viro@...iv.linux.org.uk>, anton@...ba.org,
	sgrubb@...hat.com
Subject: Re: [PATCH] audit: speedup for syscalls when auditing is disabled

In message <1282586177.2681.43.camel@...alhost.localdomain> you wrote:
> On Fri, 2010-08-20 at 12:13 +1000, Michael Neuling wrote:
> > 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@...ba.org>
> > Signed-off-by: Michael Neuling <mikey@...ling.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)
> 
> I don't think this works at all.  I don't see how syscall audit'ing can
> work.  What if I have nothing in the AUDIT_FILTER_TASK list but I want
> to audit all 'open(2)' syscalls?  This patch is going to leave the task
> in the DISABLED state and we won't ever be able to match on the syscall
> rules.

Sorry my bad.  I'm not too familiar with the audit infrastructure.

On reflection, we might have a bug in audit_alloc though.  Currently we
have this:

  int audit_alloc(struct task_struct *tsk)
  {
	  <snip>
	  state = audit_filter_task(tsk, &key);
	  if (likely(state == AUDIT_DISABLED))
		  return 0;

	  <snip>
	  set_tsk_thread_flag(tsk, TIF_SYSCALL_AUDIT);
	  return 0;
  }

This gets called on fork.  If we have "task,never" rule, we hit this
state == AUDIT_DISABLED path, return immediately and the tasks
TIF_SYSCALL_AUDIT flags doesn't get set.  On powerpc, we check
TIF_SYSCALL_AUDIT in asm on syscall entry to fast path not calling the
syscall audit code.

This seems wrong to me as a "never" _task_ audit rule shouldn't effect
_syscall_ auditing?  Is there some interaction between task and syscall
auditing that I'm missing?

> I wonder if you could get much back, in terms of performance, by moving
> the
>          context->dummy = !audit_n_rules;
> line to the top and just returning if context->dummy == 1;

We get 668.09 cycles with this optimisation, so it comes down a bit, but
no where near if the auditing is disabled altogether.

Like I said above, powerpc has a fast path in asm on system call entry
to check the thread_info flags for if syscall auditing is disabled.  If
it's disabled, we don't call the audit code, hence why it's very fast in
this case.

> I'll play a bit, but I thought that was supposed to be a safe thing to
> do....

Thanks!

Mikey

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