linux-kernel - Re: [PATCH 1/1] ptrace: make sure do_wait() won't hang after PTRACE

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Date:	Fri, 4 Feb 2011 18:06:02 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Roland McGrath <roland@...hat.com>, jan.kratochvil@...hat.com,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org
Subject: Re: [PATCH 1/1] ptrace: make sure do_wait() won't hang after
	PTRACE_ATTACH

On 02/04, Tejun Heo wrote:
>
> Hey, guys.
>
> On Fri, Feb 04, 2011 at 02:04:55PM +0100, Oleg Nesterov wrote:
> > > Hmm... I can't reproduce the problem here,
> >
> > Very strange. Do you mean the test-case doesn't die? (on vanilla kernel).
>
> Heh, it turns out the second child was attaching before the first
> succeeded stopping itself, so when it gets detached for the first
> time, the first child then stops generating new exit_code.

OOPS! Yes, the test-case is racy.

> Adding a
> small delay to the parent after the first child started made it
> reliably fail on the vanially kernel.

Yes, or waitpid(WSTOPPED).

> > There is no STOPPED/TRACED transformation at all.
>
> But it is.  It happens because there is no clear distinction between
> group stop and ptrace_stop.

Ah, in this sense I agree.

> With my first series applied, it doesn't
> happen anymore because ptracer _never_ depends on or consumes group
> stop exit_code.

Yes, I know ;) Please note "[PATCH 0/1]", I specially mentioned that
your patch should fix the problem too. And yes, my patch is the hack
which doesn't even try to address the underlying problem.

> Yes, I agree it's ugly but that's what we already have.  I think we
> can still achieve well-defined behavior even with ptracer allowed to
> diddle with the task while group stop is in effect.  It may not be
> immediately intuitive but I personally think it actually would be more
> useful to do things that way, as long as we clearly lay out what are
> supported what are undefined.
>
> I think a good compromise would be guaranteeing that when the ptracer
> goes away, the tracee would put into the state the real parent can
> agree to and the real parent to be notified that it has happened.

I am not sure. The tracing should be transparent as much as possible.

> We
> are already skipping all notifications to the real parent for ptraced
> children, there's no pressing need to change that.  If there becomes a
> real pressing requirement to change that.

And this looks just wrong to me. Say, why the ptraced application
doesn't react to ^Z ? It does, just it parent have no chance to see
this. (yes, yes, we should also change do_wait).

> I could be wrong (with pretty high probability) but I don't really see
> the pressing need for a completely new API.  ptrace sure is ugly and
> quirky but it's something people are already used to.

I won't argue. And yes, in any case ptrace can't go away, we should
try to improve it anyway. The obvious problem is that almost any
"visible" improvement breaks something.

Oleg.

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