linux-kernel - Re: [RFC] Proposal for ptrace improvements

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Date:	Wed, 2 Mar 2011 12:52:36 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Indan Zupancic <indan@....nu>
Cc:	Tejun Heo <tj@...nel.org>, Oleg Nesterov <oleg@...hat.com>,
	Roland McGrath <roland@...hat.com>, jan.kratochvil@...hat.com,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org
Subject: Re: [RFC] Proposal for ptrace improvements

On Wed, Mar 2, 2011 at 12:32 PM, Indan Zupancic <indan@....nu> wrote:
> On Wed, March 2, 2011 08:44, Tejun Heo wrote:
>> On Wed, Mar 02, 2011 at 06:07:35AM +0100, Indan Zupancic wrote:
>>> I'm not sure what Denys is talking about: Currently it's impossible to
>>> pass along SIGSTOP to traced processes. Quoting the ptrace manpage:
>>>
>>>    PTRACE_CONT
>>>           Restarts  the stopped child process.  If data is nonzero and not
>>>           SIGSTOP, it is interpreted as a signal to be  delivered  to  the
>>>           child;  otherwise,  no  signal is delivered.
>>
>> AFAICS, that's not true.  SIGSTOP isn't treated differently from other
>> signals in the ptrace signal delivery path.  Maybe it was true in the
>> past.
>
> Well, I can't find it in the code either, but it's probably a side-effect
> of how ptrace is currently implemented. Test program code below, see for
> yourself. I hope it's a program bug, but perhaps it's a kernel bug, as I
> seem to get two SIGSTOP events when I allow the SIGSTOP, but only one when
> denying it.

This was discussed recently (again). Second SIGSTOP you see
is a job control stop notification (as opposed to signal delivery notification).
It's not a bug.

-- 
vda
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