linux-kernel - Re: [PATCH v2] fs: select: fix information leak to userspace

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Message-ID: <20101124110535.GD5139@cr0.nay.redhat.com>
Date:	Wed, 24 Nov 2010 19:05:35 +0800
From:	Américo Wang <xiyou.wangcong@...il.com>
To:	Pádraig Brady <P@...igBrady.com>
Cc:	Andreas Dilger <adilger.kernel@...ger.ca>, wharms@....de,
	Américo Wang <xiyou.wangcong@...il.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	kernel-janitors@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jakub Jelinek <jakub@...hat.com>
Subject: Re: [PATCH v2] fs: select: fix information leak to userspace

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.
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