linux-kernel - Re: [PATCH 1/2] copy_process: fix CLONE_PARENT && ->exit

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Date:	Wed, 25 Feb 2009 22:20:39 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Roland McGrath <roland@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Chris Evans <scarybeasts@...il.com>,
	David Howells <dhowells@...hat.com>,
	Don Howard <dhoward@...hat.com>,
	Eugene Teo <eugene@...hat.com>,
	Michael Kerrisk <mtk.manpages@...glemail.com>,
	Tavis Ormandy <taviso@....lonestar.org>,
	Vitaly Mayatskikh <vmayatsk@...hat.com>, stable@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] copy_process: fix CLONE_PARENT && ->exit_signal
	interaction

On 02/25, Linus Torvalds wrote:
>
> > As I think I said before, I don't really know what the actual use case is
> > for CLONE_PARENT without CLONE_THREAD.  So it's easy to approve changing
> > its behavior, but I do vaguely worry about who expected what behavior before.
>
> I think changing it is wrong.

Perhaps. As I said, I don't know what is the expected behaviour. And in fact
I can't think of the "obviously good" behaviour.

> I can easily see somebody using CLONE_PARENT to get the correct getppid
> semantics in the thread, and then setting the signal to zero to not make
> the parent see the thread go away.

->exit_signal == 0 doesn't mean the thread silently goes away, it becomes
a zombie (even if ->parent ignores SIGCHLD). We don't send the signal, but
that is all.

And if ->parent execs, we reset ->exit_signal to SIGCHLD anyway.

> So at the very least it should accept zero for "no signal".

perhaps. I don't know. But I am not sure this is always right.

> And quite
> frankly, it would be good to try to see if there are other alternatives.

Agreed. I thought about checking ->xxx_exec_id's in copy_process(),
but doesn't look very nice...


(can't resist... hopefully now it is clear this should have beeen discussed
 outside of the closed lists from the very beginning ;)

Oleg.

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