linux-kernel - Re: [PATCH -v2] Audit: push audit success and retcode into arch ptrace.h

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Date:	Wed, 08 Jun 2011 14:13:36 -0400
From:	Eric Paris <eparis@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	linux-kernel@...r.kernel.org, tony.luck@...el.com,
	fenghua.yu@...el.com, monstr@...str.eu, ralf@...ux-mips.org,
	benh@...nel.crashing.org, paulus@...ba.org, schwidefsky@...ibm.com,
	heiko.carstens@...ibm.com, linux390@...ibm.com,
	lethal@...ux-sh.org, davem@...emloft.net, jdike@...toit.com,
	richard@....at, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, x86@...nel.org, viro@...iv.linux.org.uk,
	akpm@...ux-foundation.org, linux-ia64@...r.kernel.org,
	microblaze-uclinux@...e.uq.edu.au, linux-mips@...ux-mips.org,
	linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
	linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
	user-mode-linux-devel@...ts.sourceforge.net
Subject: Re: [PATCH -v2] Audit: push audit success and retcode into arch
 ptrace.h

On Wed, 2011-06-08 at 18:36 +0200, Oleg Nesterov wrote:
> On 06/07, Eric Paris wrote:
> >
> > On Tue, 2011-06-07 at 19:19 +0200, Oleg Nesterov wrote:
> > >
> > > With or without this patch, can't we call audit_syscall_exit() twice
> > > if there is something else in _TIF_WORK_SYSCALL_EXIT mask apart from
> > > SYSCALL_AUDIT ? First time it is called from asm, then from
> > > syscall_trace_leave(), no?
> > >
> > > For example. The task has TIF_SYSCALL_AUDIT and nothing else, it does
> > > system_call->auditsys->system_call_fastpath. What if it gets, say,
> > > TIF_SYSCALL_TRACE before ret_from_sys_call?
> >
> > No harm is done calling twice.  The first call will do the real work and
> > cleanup.  It will set a flag in the audit data that the work has been
> > done (in_syscall == 0) thus the second call will then not do any real
> > work and won't have anything to clean up.
> 
> Hmm... and I assume context->previous != NULL is not possible on x86_64.
> OK, thanks.
> 
> And I guess, all CONFIG_AUDITSYSCALL code in entry.S is only needed to
> microoptimize the case when TIF_SYSCALL_AUDIT is the only reason for the
> slow path. I wonder if it really makes the measureble difference...

All I know is what Roland put in the changelog:

Avoiding the iret return path when syscall audit is enabled helps
performance a lot.

I believe this was a result of Fedora starting auditd by default and
then Linus bitching about how slow a null syscall in a tight loop was.
It was an optimization for a microbenchmark.  How much it affects things
on a real syscall that does real work is probably going to be determined
by how much work is done in the syscall.  (or just disable auditd in
userspace)

-Eric

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