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



On Tue, 2010-08-24 at 12:11 +1000, Michael Neuling wrote:
> In message <1282586177 2681 43 camel localhost 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 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)
> > 
> > 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.....

-Eric


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