linux-kernel - Re: + prctl-add-pr_setget_child_reaper-to-allow-simple-process-supervision .patch added to -mm tree

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Message-ID: <20110820153331.GA22577@redhat.com>
Date:	Sat, 20 Aug 2011 17:33:31 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Kay Sievers <kay.sievers@...y.org>
Cc:	Lennart Poettering <mzxreary@...inter.de>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-man@...r.kernel.org, roland@...k.frob.com,
	torvalds@...ux-foundation.org
Subject: Re: +
	prctl-add-pr_setget_child_reaper-to-allow-simple-process-supervision
	.patch added to -mm tree

On 08/19, Oleg Nesterov wrote:
>
> On 08/19, Kay Sievers wrote:
> >
> > version 4:
>
> Looks correct... But I don't trust myself. Especially after I missed
> the problem with klthreads ;)

And I missed another problem. So. We do need to check that
father != init_task even if we check has_child_subreaper.

Suppose that a kernel thread execs the usermode task T which
does prctl(REAPER). Suppose that its grandchild C exits, it
should be reparented to T.

If T is alive - everything fine, the lookup finds T with
->is_sub_reaper set.

If T has exited - everything fine again, C->parent was already
reparented to pid_ns->child_reaper (or another sub-reaper).

But! If T exits, there is a window between setting PF_EXITING
and forget_original_parent() which should re-parent C->parent.
If C exits in this window, it will see PF_EXITING and continue
the lookup, but it will never reach pid_ns->child_reaper.

(if we could check ->exit_state instead of PF_EXITING, everything
 would be fine).

And cough... there is another, not that subtle problem ;) That
task T can _clear_ ->is_child_subreaper after forking the child.
But since this obviously can't clear C->has_child_subreaper, we
can't trust it.

So. please add this check back. I insisted you should remove it,
but I was wrong. Otherwise looks correct.

Damn. And why do we check PF_EXITING but not exit_state? this is
because we have to drop tasklist for exit_task_namespaces(), see
762a24beed3f3ab93224bd447710e6c36fcf1968. However, there were a
lot of changes since then. Afaics we can change do_notify_parent()
to use task_active_pid_ns(tsk->parent) and then we can call
exit_task_namespaces() before exit_notify(). In this case we can
change exit_notify/forget_original_parent to reparent and set
exit_state under tasklist, this also saves unlock+lock.

Oleg.

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