From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1755199Ab1HCSYa (ORCPT <rfc822;w@1wt.eu>);
	Wed, 3 Aug 2011 14:24:30 -0400
Received: from mail-ew0-f46.google.com ([209.85.215.46]:36975 "EHLO
	mail-ew0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1754732Ab1HCSYY (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Wed, 3 Aug 2011 14:24:24 -0400
Date: Wed, 3 Aug 2011 22:24:17 +0400
From: Vasiliy Kulikov <segoon@openwall.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Manuel Lauss <manuel.lauss@googlemail.com>,
        Richard Weinberger <richard@nod.at>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Marc Zyngier <marc.zyngier@arm.com>, linux-kernel@vger.kernel.org
Subject: Re: + shm-fix-a-race-between-shm_exit-and-shm_init.patch added to
 -mm tree
Message-ID: <20110803182417.GA2510@albatros>
References: <20110803140456.GA14393@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20110803140456.GA14393@redhat.com>
User-Agent: Mutt/1.5.20 (2009-06-14)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Wed, Aug 03, 2011 at 16:04 +0200, Oleg Nesterov wrote:
> > From: Vasiliy Kulikov <segoon@openwall.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