linux-kernel - Re: + shm-fix-a-race-between-shm_exit-and-shm

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Date:	Wed, 3 Aug 2011 22:24:17 +0400
From:	Vasiliy Kulikov <segoon@...nwall.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Manuel Lauss <manuel.lauss@...glemail.com>,
	Richard Weinberger <richard@....at>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Marc Zyngier <marc.zyngier@....com>,
	linux-kernel@...r.kernel.org
Subject: Re: + shm-fix-a-race-between-shm_exit-and-shm_init.patch added to
 -mm tree

On Wed, Aug 03, 2011 at 16:04 +0200, Oleg Nesterov wrote:
> > From: Vasiliy Kulikov <segoon@...nwall.com>
> >
> > On thread exit shm_exit_ns() is called, it uses shm_ids(ns).rw_mutex.  It
> > is initialized in shm_init(), but it is not called yet at the moment of
> > kernel threads exit.  Some kernel threads are created in
> > do_pre_smp_initcalls(), and shm_init() is called in do_initcalls().
> >
> > Static initialization of shm_ids(init_ipc_ns).rw_mutex fixes the race.
> 
> Yes, it is safe to call down_right() now.
> 
> But the code does
> 
> 	down_write(rw_mutex);
> 	if (.in_use)
> 		idr_for_each(.ipcs_idr);
> 
> and thus it relies on the static initializer anyway. it is not safe
> to do idr_for_each() before idr_init() in theory.
> 
> And since we rely on .in_use == 0, why we can't move this check
> outside of down_write/up_right to a) optimize the code and b)
> fix the problem?

Agreed.  But I second Linus that partial initialization only hides the
real problem.  And some initcall chain movement is still needed.

-- 
Vasiliy Kulikov
http://www.openwall.com - bringing security into open computing environments
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