linux-kernel - Re: PT_EXITKILL (Was: pdeath

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Message-ID: <20121108123716.GA20917@redhat.com>
Date:	Thu, 8 Nov 2012 13:37:16 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Amnon Shiloh <u3557@...o.sublimeip.com>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Chris Evans <scarybeasts@...il.com>,
	David Howells <dhowells@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andrew Morton <akpm@...ux-foundation.org>, u3557@...lix.com.au,
	security@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: PT_EXITKILL (Was: pdeath_signal)

Hi Amnon,

On 11/08, Amnon Shiloh wrote:
>
> Thanks for the patch, I tried it and it works nicely!

OK, thanks. I'll wait for other comments a bit and then send
the patch to Andrew.

> Also, I just noticed that this new option (PTRACE_O_EXITKILL) is not
> safe with ptrace(PTRACE_TRACEME)+execve, because the parent may die
> before even having a chance to set PTRACE_O_EXITKILL!

Not safe? I guess you meant that PTRACE_O_EXITKILL can't prevent
this race? Yes sure. Or the parent can die before it does
PTRACE_O_EXITKILL.

> However, that's easy to fix by using PTRACE_ATTACH instead.

Or the child can check getppid() == saved_parent_pid after PTRACE_TRACEME.

But if you switch to PTRACE_ATTACH, I'd suggest to use PTRACE_SEIZE.
Note that, in particular, this allows you to specify the options at
attach.

As for PF_NO_SIGSTOP, I'll write another email.

Oleg.

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