linux-kernel - Re: [RFC,PATCH 1/2] seccomp_filters: system call filtering using BPF

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Date:	Mon, 16 Jan 2012 14:15:10 -0600
From:	Will Drewry <wad@...omium.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	linux-kernel@...r.kernel.org, keescook@...omium.org,
	john.johansen@...onical.com, serge.hallyn@...onical.com,
	coreyb@...ux.vnet.ibm.com, pmoore@...hat.com, eparis@...hat.com,
	djm@...drot.org, torvalds@...ux-foundation.org,
	segoon@...nwall.com, rostedt@...dmis.org, jmorris@...ei.org,
	scarybeasts@...il.com, avi@...hat.com, penberg@...helsinki.fi,
	viro@...iv.linux.org.uk, luto@....edu, mingo@...e.hu,
	akpm@...ux-foundation.org, khilman@...com, borislav.petkov@....com,
	amwang@...hat.com, ak@...ux.intel.com, eric.dumazet@...il.com,
	gregkh@...e.de, dhowells@...hat.com, daniel.lezcano@...e.fr,
	linux-fsdevel@...r.kernel.org,
	linux-security-module@...r.kernel.org, olofj@...omium.org,
	mhalcrow@...gle.com, dlaor@...hat.com,
	Roland McGrath <mcgrathr@...omium.org>,
	Andi Kleen <andi@...stfloor.org>, indan@....nu
Subject: Re: [RFC,PATCH 1/2] seccomp_filters: system call filtering using BPF

On Mon, Jan 16, 2012 at 12:37 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> On 01/13, Will Drewry wrote:
>>
>> On Fri, Jan 13, 2012 at 1:01 PM, Will Drewry <wad@...omium.org> wrote:
>> > On Fri, Jan 13, 2012 at 11:31 AM, Oleg Nesterov <oleg@...hat.com> wrote:
>> >>
>> >> Me too. I see no point in using user_regs_struct.
>> >
>> > I'll rev the change to use pt_regs and drop all the helper code.  If
>> > no one says otherwise, that certainly seems ideal from a performance
>> > perspective, and I see pt_regs exported to userland along with ptrace
>> > abi register offset macros.
>>
>> On second thought, pt_regs is scary :)
>>
>> From looking at
>>   http://lxr.linux.no/linux+v3.2.1/arch/x86/include/asm/syscall.h#L97
>> and ia32syscall enty code, it appears that for x86, at least, the
>> pt_regs for compat processes will be 8 bytes wide per register on the
>> stack.  This means if a self-filtering 32-bit program runs on a 64-bit host in
>> IA32_EMU, its filters will always index into pt_regs incorrectly.
>
> Yes, thanks, I forgot about compat tasks again. But this is easy, just
> we need regs_64_to_32().

Yup - we could make the assumption that is_compat_task is always
32-bit and the pt_regs is always 64-bit, then copy_and_truncate with
regs_64_to_32.  Seems kinda wonky though :/

> Doesn't matter. I think Indan has a better suggestion.

I disagree, but perhaps I'm not fully understanding!

Thanks!
will
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