linux-kernel - Re: [PATCH 2/7] seccomp: Refactor the filter callback and the API

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Date:	Wed, 16 Jul 2014 15:06:07 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Kees Cook <keescook@...omium.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Will Drewry <wad@...omium.org>,
	James Morris <james.l.morris@...cle.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Linux MIPS Mailing List <linux-mips@...ux-mips.org>,
	linux-arch <linux-arch@...r.kernel.org>,
	linux-security-module <linux-security-module@...r.kernel.org>,
	Alexei Starovoitov <ast@...mgrid.com>
Subject: Re: [PATCH 2/7] seccomp: Refactor the filter callback and the API

On Wed, Jul 16, 2014 at 2:56 PM, Kees Cook <keescook@...omium.org> wrote:
> On Wed, Jul 16, 2014 at 1:56 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> On Wed, Jul 16, 2014 at 1:12 PM, Kees Cook <keescook@...omium.org> wrote:
>>> On Tue, Jul 15, 2014 at 12:32 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>>> The reason I did this is to add a seccomp API that will be usable
>>>> for an x86 fast path.  The x86 entry code needs to use a rather
>>>> expensive slow path for a syscall that might be visible to things
>>>> like ptrace.  By splitting seccomp into two phases, we can check
>>>> whether we need the slow path and then use the fast path in if the
>>>> filter allows the syscall or just returns some errno.
>>>>
>>>> As a side effect, I think the new code is much easier to understand
>>>> than the old code.
>>>
>>> I'd agree. The #idefs got a little weirder, but the actual code flow
>>> was much easier to read. I wonder if "phase1" and "phase2" should be
>>> renamed "pretrace" and "tracing" or something more meaningful? Or
>>> "fast" and "slow"?
>>
>> Queue the bikeshedding :)
>>
>> I like "phase1" and "phase2" because it makes it clear that phase1 has
>> to come first.  But I'd be amenable to counterarguments.
>
> That works. I didn't have a strong feeling about it. I was just
> wondering if there was a good way to self-document that phase1 is on
> the fast path, and phase2 was on the slow path for tracing. The
> existing comments really should be sufficient, though.
>
> You mentioned architectures providing "sd" directly. I wonder if that
> new optional ability should be mentioned in the Kconfig help text that
> defines what's needed for an arch to support SECCOMP_FILTER?

Good call.  Queued for v2.

>
> -Kees
>
> --
> Kees Cook
> Chrome OS Security



-- 
Andy Lutomirski
AMA Capital Management, LLC
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