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

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Message-Id: <8D90F8B2-EA29-4EB9-9807-294CE0D5523B@dilger.ca>
Date:	Sat, 13 Nov 2010 14:38:19 -0700
From:	Andreas Dilger <adilger.kernel@...ger.ca>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Vasiliy Kulikov <segooon@...il.com>,
	kernel-janitors@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: select: fix information leak to userspace

On 2010-11-12, at 13:08, Andrew Morton wrote:
> On Wed, 10 Nov 2010 23:38:02 +0300
> Vasiliy Kulikov <segooon@...il.com> wrote:
>> On some architectures __kernel_suseconds_t is int.
> 
> On sparc and parisc.  On all other architectures this patch is a waste
> of cycles.
> 
> --- a/fs/select.c~fs-select-fix-information-leak-to-userspace-fix
> +++ a/fs/select.c
> @@ -306,7 +306,8 @@ static int poll_select_copy_remaining(st
> 		rts.tv_sec = rts.tv_nsec = 0;
> 
> 	if (timeval) {
> -		memset(&rtv, 0, sizeof(rtv));
> +		if (sizeof(rtv) > sizeof(rtv.tv_sec) + sizeof(rtv.tv_usec))
> +			memset(&rtv, 0, sizeof(rtv));
> 		rtv.tv_sec = rts.tv_sec;
> 		rtv.tv_usec = rts.tv_nsec / NSEC_PER_USEC;
> 
> _
> 
> 
> The `if' gets eliminated at compile time.  With this approach we add
> four bytes of text to the sparc64 build and zero bytes of text to the
> x86_64 build.

It's nice to have comments (or at least a good commit message) for unusual code like this, so that in the future it is clear when this kind of workaround can be removed (e.g. if the time_t is changed to always be a 64-bit value for Y2038 issues, even on 32-bit arches). 

Cheers, Andreas





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