Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754216Ab0HXCLY (ORCPT ); Mon, 23 Aug 2010 22:11:24 -0400 Received: from ozlabs.org ([203.10.76.45]:54773 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752227Ab0HXCLV (ORCPT ); Mon, 23 Aug 2010 22:11:21 -0400 From: Michael Neuling To: Eric Paris cc: linux-audit@redhat.com, linux-kernel@vger.kernel.org, Al Viro , anton@samba.org, sgrubb@redhat.com Subject: Re: [PATCH] audit: speedup for syscalls when auditing is disabled In-reply-to: <1282586177.2681.43.camel@localhost.localdomain> References: <29151.1282270393@neuling.org> <1282586177.2681.43.camel@localhost.localdomain> Comments: In-reply-to Eric Paris message dated "Mon, 23 Aug 2010 13:56:17 -0400." X-Mailer: MH-E 8.2; nmh 1.3; GNU Emacs 23.1.1 Date: Tue, 24 Aug 2010 12:11:20 +1000 Message-ID: <20887.1282615880@neuling.org> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4391 Lines: 120 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 > > Signed-off-by: Michael Neuling > > --- > > 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) { state = audit_filter_task(tsk, &key); if (likely(state == AUDIT_DISABLED)) return 0; 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/