linux-kernel - Re: [PATCH v10 07/11] signal, x86: add SIGSYS info and make it synchronous.

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Message-ID: <CABqD9havUbeHDQVqxf7iy1XjNSrUkh=ehSW_h2OfU2QQ_VQaaA@mail.gmail.com>
Date:	Thu, 23 Feb 2012 10:44:29 -0600
From:	Will Drewry <wad@...omium.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Kees Cook <keescook@...omium.org>,
	Andrew Lutomirski <luto@....edu>,
	Indan Zupancic <indan@....nu>, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, linux-doc@...r.kernel.org,
	kernel-hardening@...ts.openwall.com, netdev@...r.kernel.org,
	x86@...nel.org, arnd@...db.de, davem@...emloft.net,
	mingo@...hat.com, oleg@...hat.com, peterz@...radead.org,
	rdunlap@...otime.net, mcgrathr@...omium.org, tglx@...utronix.de,
	eparis@...hat.com, serge.hallyn@...onical.com, djm@...drot.org,
	scarybeasts@...il.com, pmoore@...hat.com,
	akpm@...ux-foundation.org, corbet@....net, eric.dumazet@...il.com,
	markus@...omium.org
Subject: Re: [PATCH v10 07/11] signal, x86: add SIGSYS info and make it synchronous.

On Wed, Feb 22, 2012 at 6:29 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 02/22/2012 04:08 PM, Kees Cook wrote:
>>>
>>> Hrm, it might be possible to do_exit(SIGSYS) which would be both. It
>>> looks like tsk->exit_code would be SIGSYS then, but I'll look a little
>>> more closely to see what that'll actually do.
>>
>> As long as there's no way it can get blocked, I'd be fine with that.
>> It would, actually, be better than SIGKILL because, as Andy said, it's
>> more distinguishable from other situations. I've long wanted a signal
>> to be used for "violated policy" that wasn't just a straight SIGKILL.
>>
>
> Can we really introduce force-kill semantics for a POSIX-defined signal?
>  Other user space programs might use it for other purposes.
>
> I'm wondering if the right thing may be to introduce some variant of
> exit() which can return more information about a signal, including some
> kind of cause code for SIGKILL?

While it'd be harder to send back extra info, passing SIGSYS to
do_exit() should result in the si_status for the emitted SIGCHLD to be
SIGSYS (si_status = (tsk->exit_code & 0x7f)).  I think it'll still
have a si_code of CLD_KILLED, but it'd be enough for a parent to
differentiate the task-death path.  I'll try it out before I post
another patch rev.

A variant that allowed extended exit information would be useful
(especially for this patch series), I'm not sure I'd know where to
start.

cheers!
will
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