linux-kernel - Re: [PATCH 1/4] pid_ns: zap_pid_ns_processes: fix the ->child

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Date:	Wed, 27 Aug 2008 20:36:19 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	"Serge E. Hallyn" <serue@...ibm.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Robert Rex <robert.rex@...sol.com>,
	Roland McGrath <roland@...hat.com>,
	Sukadev Bhattiprolu <sukadev@...ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] pid_ns: zap_pid_ns_processes: fix the ->child_reaper changing

On 08/26, Serge E. Hallyn wrote:
>
> Quoting Oleg Nesterov (oleg@...sign.ru):
> > zap_pid_ns_processes() sets pid_ns->child_reaper = NULL, this is wrong.
> > ...
> > Even if there are no childs, it is not good that forget_original_parent()
> > uses reaper == NULL.
> > ...
> 
> Well while it looked correct to me all along, I couldn't get the
> testcase to cause an oops.

Perhaps you can start several instances or increase the delay, I
forgot that usleep() uses usecs, not msecs. In any case I agree,
the test-case is oversimplified, but since it worked for me I didn't
try to make it more "stubborn".

> Acked-by: Serge Hallyn <serue@...ibm.com>

Thanks!

> > +	/*
> > +	 * We can not clear ->child_reaper or leave it alone.
> > +	 * There may by stealth EXIT_DEAD tasks on ->children,
> > +	 * forget_original_parent() must move them somewhere.
>
> Actually this comment is a little bit misleading - the null
> deref will happen regardless of whether there are children,
> right?

Yes, the comment could be better ;)

But no, we won't oops without children. Please note that
forget_original_parent() uses reaper inside the
list_for_each_entry_safe(father->children) loop. ->children is
empty, reaper is not used at all.

But please also note the "not good" part of the changelog above.


Thanks to all for review!

Oleg.

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