linux-kernel - Re: [PATCH RFC] exec: avoid possible undefined behavior in count()

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Date:	Mon, 7 Jan 2013 13:44:22 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Xi Wang <xi.wang@...il.com>
Cc:	linux-kernel@...r.kernel.org, Jason Baron <jbaron@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH RFC] exec: avoid possible undefined behavior in count()

On Sun,  6 Jan 2013 00:29:05 -0500
Xi Wang <xi.wang@...il.com> wrote:

> The tricky problem is this check:
> 
> 	if (i++ >= max)
> 
> icc (mis)optimizes this check as:
> 
> 	if (++i > max)
> 
> The check now becomes a no-op since max is MAX_ARG_STRINGS (0x7FFFFFFF).
> 
> This is "allowed" by the C standard, assuming i++ never overflows,
> because signed integer overflow is undefined behavior.  This optimization
> effectively reverts the previous commit 362e6663ef ("exec.c, compat.c:
> fix count(), compat_count() bounds checking") that tries to fix the check.
> 
> This patch simply moves ++ after the check.
> 
> ...
>
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -434,8 +434,9 @@ static int count(struct user_arg_ptr argv, int max)
>  			if (IS_ERR(p))
>  				return -EFAULT;
>  
> -			if (i++ >= max)
> +			if (i >= max)
>  				return -E2BIG;
> +			++i;
>  
>  			if (fatal_signal_pending(current))
>  				return -ERESTARTNOHAND;

I have no problem working around a compiler bug when the workaround is
so small and simple.  For clarity and accuracy I renamed the patch to
"fs/exec.c: work around icc miscompilation".  

However I'd also like to be able to add "this bug has been reported to
the icc developers and will be fixed in version X.Y"?
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