linux-kernel - Re: execve-under-ptrace API bug (was Re: Ptrace documentation, draft #3)

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Date:	Thu, 2 Jun 2011 11:57:12 +0100
From:	Pedro Alves <pedro@...esourcery.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Denys Vlasenko <vda.linux@...glemail.com>,
	Tejun Heo <tj@...nel.org>, jan.kratochvil@...hat.com,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, indan@....nu
Subject: Re: execve-under-ptrace API bug (was Re: Ptrace documentation, draft #3)

On Tuesday 31 May 2011 14:51:16, Oleg Nesterov wrote:

> The main problem is: it is not clear do we really want EVENT_EXIT
> in this case. I think we do, Roland thought we do not. OTOH I never
> really the purpose of EVENT_EXIT, but this doesn't matter.
> 
> If we decide we do want this notification (in this case), then we
> need fixes. EVENT_EXIT is not reliable. Say, the thread can exit
> before it dequeues SIGKILL and in this case it doesn't stop.
> Also. If we guarantee EVENT_EXIT in this case, then probably the
> implicit SIGKILL should not wakeup the TASK_TRACED tracee (except
> the new PTRACE_LISTEN case).
> 
> In short: currently I do not know what should be documented. I do
> not know the original intent, I can only see what the code actually
> does. 

Daniel Jacobowitz said when he submitted it:

<http://lkml.indiana.edu/hypermail/linux/kernel/0302.0/1051.html>

"PTRACE_EVENT_EXIT, which triggers in do_exit().  This is useful to quickly
 find out where a program is making an exit syscall from, etc. - it
 triggers before the mm is released, so we can still get backtraces et
 cetera."

That said, GDB was never made to use it:

  /* Do not enable PTRACE_O_TRACEEXIT until GDB is more prepared to support
     read-only process state.  */

-- 
Pedro Alves
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