linux-kernel - Re: [PATCH 5/7] seccomp_filter: Document what seccomp

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Date:	Wed, 04 May 2011 11:54:22 -0400
From:	Eric Paris <eparis@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Will Drewry <wad@...omium.org>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org, kees.cook@...onical.com,
	agl@...omium.org, jmorris@...ei.org,
	Randy Dunlap <rdunlap@...otime.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tom Zanussi <tzanussi@...il.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 5/7] seccomp_filter: Document what seccomp_filter is and
 how it works.

On Wed, 2011-05-04 at 08:16 -0400, Steven Rostedt wrote:
> On Tue, 2011-05-03 at 03:47 +0200, Frederic Weisbecker wrote:
> > 2011/5/3 Frederic Weisbecker <fweisbec@...il.com>:
> 
> > Even better: applying a filter would always automatically be an
> > intersection of the previous one.
> > 
> > If you do:
> > 
> > SECCOMP_FILTER_SET, __NR_foo, "a == 1 || a == 2"
> > SECCOMP_FILTER_APPLY
> > SECCOMP_FILTER_SET, __NR_foo, "b == 2"
> > SECCOMP_FILTER_APPLY
> > SECCOMP_FILTER_SET, __NR_foo, "c == 3"
> > SECCOMP_FILTER_APPLY
> > 
> > The end result is:
> > 
> > "(a == 1 || a == 2) && b == 2  && c == 3"
> > 
> 
> I'm a little confused. Why do we have both a FILTER_SET and a
> FILTER_APPLY? Maybe this was discussed earlier in the thread and I
> missed it or simply forgot.
> 
> Why not just apply on the set call?

As this is a deny by default interface which only allows you to further
restrict you couldn't add more than 1 syscall if you didn't have an
explict 'apply' action.

SECCOMP_FILTER_SET, __NR_fo, "a=0"
SECCOMP_FILTER_SET, __NR_read, "1" == EPERM

Maybe apply on set is fine after the first apply, but we definitely need
some way to do more than 1 set before the rules are applied....

-Eric

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