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

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Date:	Wed, 24 Nov 2010 10:44:50 +0000
From:	Pádraig Brady <P@...igBrady.com>
To:	Andreas Dilger <adilger.kernel@...ger.ca>
CC:	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 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

gcc doesn't zero pad when _all_ elements are specified.

So perhaps just:

-        struct timespec rts;
-        struct timeval rtv;
+        struct timespec rts = {0,};
+        struct timeval rtv = {0,};

One could also move the rtv declaration
to the scope where it's used.

cheers,
Pádraig.
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