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

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Date:	Wed, 16 Jul 2014 13:56:20 -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@...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 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.

>
>> This has one user-visible effect: the audit record written for
>> SECCOMP_RET_TRACE is now a simple indication that SECCOMP_RET_TRACE
>> happened.  It used to depend in a complicated way on what the tracer
>> did.  I couldn't make much sense of it.
>
> I think this change is okay. The only way to get the audit record to
> report SIGSYS before was to have an additional signal come in and kill
> it while the tracer was working on it. Which is confusing too. I like
> this way better.

Thanks :)

--Andy

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



-- 
Andy Lutomirski
AMA Capital Management, LLC
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