From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754541Ab0KXM3A (ORCPT ); Wed, 24 Nov 2010 07:29:00 -0500 Received: from mail-gx0-f174.google.com ([209.85.161.174]:47345 "EHLO mail-gx0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753841Ab0KXM27 (ORCPT ); Wed, 24 Nov 2010 07:28:59 -0500 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=date:from:to:cc:subject:message-id:references:mime-version :content-type:content-disposition:content-transfer-encoding :in-reply-to:user-agent; b=an1nO6xKWAFQkMPSW+ZvQEUxAAouQUl8l5zwEriCRpylQ1Xz5nTvTHc0l+804u02Bf 3DnxV0xXHUL7vB6b7FmWtsrCVIC+fSm6wN1rNbmCCFl7TqxQA/DrXgKSJiNjq3SCFlmZ vNz5PJmjQQ2Y7C398nXK2fNJRF22yAWYU+UAU= Date: Wed, 24 Nov 2010 20:32:13 +0800 From: =?utf-8?Q?Am=C3=A9rico?= Wang To: =?utf-8?Q?P=C3=A1draig?= Brady Cc: =?utf-8?Q?Am=C3=A9rico?= Wang , Andreas Dilger , wharms@bfs.de, Eric Dumazet , Andrew Morton , Vasiliy Kulikov , kernel-janitors@vger.kernel.org, Alexander Viro , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Jakub Jelinek Subject: Re: [PATCH v2] fs: select: fix information leak to userspace Message-ID: <20101124123213.GB2922@hack> References: <8D90F8B2-EA29-4EB9-9807-294CE0D5523B@dilger.ca> <20101114092533.GB5323@albatros> <20101114180643.593d19ac.akpm@linux-foundation.org> <1289848341.2607.125.camel@edumazet-laptop> <20101123140111.GA3816@hack> <4CEBD37E.5060107@bfs.de> <203E1F2A-2D04-4B7F-8D1B-9DC24522CB5E@dilger.ca> <4CECECA2.6070301@draigBrady.com> <20101124110535.GD5139@cr0.nay.redhat.com> <4CECFB19.1090003@draigBrady.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <4CECFB19.1090003@draigBrady.com> User-Agent: Mutt/1.5.19 (2009-01-05) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Nov 24, 2010 at 11:46:33AM +0000, Pádraig Brady wrote: >On 24/11/10 11:05, Américo Wang wrote: >> On Wed, Nov 24, 2010 at 10:44:50AM +0000, Pádraig Brady wrote: >>> On 23/11/10 18:02, Andreas Dilger wrote: >>>> On 2010-11-23, at 07:45, walter harms wrote: >>>>> Maybe we can convince the gcc people to make 0 padding default. That will not solve the problems for other compilers but when they claim "works like gcc" we can press then to support this also. I can imagine that this will close some other subtle leaks also. >>>> >>>> It makes the most sense to tackle this at the GCC level, since the added overhead of doing memset(0) on the whole struct may be non-trivial for commonly-used and/or large structures. Since GCC is already explicitly zeroing the _used_ fields in the struct, it can much more easily determine whether there is padding in the structure, and zero those few bytes as needed. >>> >>> Zero padding structs is part of C90. Details here: >>> http://www.pixelbeat.org/programming/gcc/auto_init.html >> >> Nope. >> >>> >>> gcc doesn't zero pad when _all_ elements are specified. >>> >> >> That is what gcc does, not what C standard specifies. > >Looks like gcc is following the standard exactly. > >C90 - 6.5.7 >C99 - 6.7.8 > > If there are fewer initializers in a brace-enclosed list than > there are elements or members of an aggregate ... the remainder > of the aggregate shall be initialized implicitly the same as > objects that have static storage duration. > Depends on if "the remainder of the aggregate" includes padding bytes or not.