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

[Michael Neuling]

In message <1282621410.26616.406.camel@xxxxxxxxxxxxxxxxxxxxx> you wrote:
> On Tue, 2010-08-24 at 12:11 +1000, Michael Neuling wrote:
> > In message <1282586177.2681.43.camel@xxxxxxxxxxxxxxxxxxxxx> 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 overhea
d
> > > > 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 auditin
g
> > > > 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@xxxxxxxxx>
> > > > Signed-off-by: Michael Neuling <mikey@xxxxxxxxxxx>
> > > > ---
> > > > 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 t
ask_
> > 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, &stat
e)) {
> > > >  			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.
> 
> I'm guessing it actually bypasses audit if the flag is not set?  So we
> might have a bug, but i'd be surprised since I think we tested audit on
> powerpc....
> 
> > 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?
> 
> There are 3 states for a given task, I don't remember the names off the
> top of my head, so I'll guess with: on, off, build.  'Build' is the
> state most processes usually live in.  In this state we collect audit
> information about the task during the whole syscall and then we might
> (likely) throw that information away at syscall exit.
> 
> Some types of audit rule, which alter this state, can be checked at
> either 'entry' or 'exit' (first rule wins) At syscall entry we only have
> enough information (questionable if we even have enough information at
> all but that's a different question) to filter based on the task.  You
> can create rules that will audit all tasks, or in your case will
> explicitly disable auditing for all tasks.
> 
> Normally a process would be in the default 'build' state after syscall
> entry, we will collect information about the syscall, and then we will
> check syscall rules at exit.  Once you explicitly say 'I do not want any
> audit messages for this task' you are in 'off' instead of 'build.'
> 
> > > 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.
> 
> Clean that patch up and send it.  Sounds like a win no matter what else
> we do.
> 
> > 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.
> 
> Here's a new idea to think about with obvious tradeoffs.  What do you
> think about doing a little bit of assembly rejiggering?
> 
> Add a new spot in the assembly which will call a function which will
> check if audit_n_rules > 0 and if so will set TIF_SYSCALL_AUDIT and if
> not will clear TIF_SYSCALL_AUDIT?  It might make things slightly worse
> on systems which explictly disable audit and the flag would always be
> clear on every task (like you did with the explicit rule) but I'm
> guessing might be a win on systems with no rules which are wasting time
> on the audit slow path.....

BTW, do you think we can do this in audit_syscall_exit() too?  

If I do, I get down to 387 cycles (739.03 vanilla, 668.09 with
audit_syscall_entry() optimisation, 204 best case) so about
another 50% perf improvement.

Patch was simply:

--- linux-next.orig/kernel/auditsc.c
+++ linux-next/kernel/auditsc.c
@@ -1681,7 +1683,7 @@ void audit_syscall_exit(int valid, long 
 
 	context = audit_get_context(tsk, valid, return_code);
 
-	if (likely(!context))
+	if (likely((!context) || (audit_n_rules == 0)))
 		return;
 
 	if (context->in_syscall && context->current_state == AUDIT_RECORD_CONTEXT)

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