linux-kernel - Re: Signed divides vs shifts (Re: [Security] /dev/urandom uses uninit bytes, leaks user data)

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Date:	Mon, 17 Dec 2007 18:23:12 +0000
From:	Al Viro <viro@....linux.org.uk>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	John Reiser <jreiser@...Wagon.com>,
	Andrew Morton <akpm@...ux-foundation.org>, security@...nel.org,
	tytso@....edu,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	mpm@...enic.com, linux-sparse@...r.kernel.org
Subject: Re: Signed divides vs shifts (Re: [Security] /dev/urandom uses uninit bytes, leaks user data)

On Mon, Dec 17, 2007 at 06:55:57PM +0100, Eric Dumazet wrote:

> long *mid(long *a, long *b)
> {
> 	return ((a - b) / 2 + a);
> }

... is not actually a middle (you'd want b-a, not a-b there), but anyway

> It gave :
> mid:
>         movq    %rdi, %rdx
>         subq    %rsi, %rdx
>         sarq    $3, %rdx
>         movq    %rdx, %rax
>         shrq    $63, %rax
>         addq    %rdx, %rax
>         sarq    %rax
>         leaq    (%rdi,%rax,8), %rax
>         ret
> 
> while 
> 
> long *mid(long *a, long *b)
> {
> 	return ((a - b) / 2u + a);
> }

... undefined behaviour if a < b

> and while :
> 
> long *mid(long *a, long *b)
> {
> 	return (((unsigned long)(a - b)) / 2 + a);
> }

undefined behaviour, again.
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