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

From: "Américo Wang" <xiyou.wangcong@gmail.com>
To: "Pádraig Brady" <P@draigBrady.com>
Cc: "Andreas Dilger" <adilger.kernel@dilger.ca>,
	wharms@bfs.de, "Américo Wang" <xiyou.wangcong@gmail.com>,
	"Eric Dumazet" <eric.dumazet@gmail.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Vasiliy Kulikov" <segoon@openwall.com>,
	kernel-janitors@vger.kernel.org,
	"Alexander Viro" <viro@zeniv.linux.org.uk>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	"Jakub Jelinek" <jakub@redhat.com>
Subject: Re: [PATCH v2] fs: select: fix information leak to userspace
Date: Wed, 24 Nov 2010 19:05:35 +0800	[thread overview]
Message-ID: <20101124110535.GD5139@cr0.nay.redhat.com> (raw)
In-Reply-To: <4CECECA2.6070301@draigBrady.com>

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.