linux-kernel - Re: [PATCH] audit: Only use the syscall slowpath when syscall audit rules exist

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Date:	Mon, 3 Feb 2014 14:08:43 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Steve Grubb <sgrubb@...hat.com>
Cc:	linux-audit@...hat.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>,
	Oleg Nesterov <oleg@...hat.com>, Eric Paris <eparis@...hat.com>
Subject: Re: [PATCH] audit: Only use the syscall slowpath when syscall audit
 rules exist

On Mon, Feb 3, 2014 at 12:23 PM, Steve Grubb <sgrubb@...hat.com> wrote:
> On Monday, February 03, 2014 09:53:23 AM Andy Lutomirski wrote:
>> This toggles TIF_SYSCALL_AUDIT as needed when rules change instead of
>> leaving it set whenever rules might be set in the future.  This reduces
>> syscall latency from >60ns to closer to 40ns on my laptop.
>
> Does this mean that we have processes that don't have the  TIF_SYSCALL_AUDIT
> flag set? When rules get loaded, how do we get the flag put back into all
> processes?

By looping over all processes and setting the flag, which is what my patch does.

>
> The theory of ops is supposed to be that for anyone not needing audit, there
> is only the cost of  "if (tif & TIF_SYSCALL_AUDIT)".

On current kernels *all* processes have TIF_SYSCALL_AUDIT, even if
they don't need auditing because there's nothing to audit.  So
everything pays the full cost.

> That should be it. If you
> have audit enabled or had it enabled (which means it might be loaded with new
> rules), we want to inspect the syscall.
>

My point is that there's nothing to inspect -- there are no rules.
Unless the audit code needs to do something just in case a non-syscall
audit event gets written, in which case the audit code should IMO be
fixed.  (This is what Eric is talking about, I think.)

--Andy
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