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

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Message-ID: <20071217190810.GE8181@ftp.linux.org.uk>
Date:	Mon, 17 Dec 2007 19:08:10 +0000
From:	Al Viro <viro@....linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Eric Dumazet <dada1@...mosbay.com>, security@...nel.org,
	tytso@....edu, Herbert Xu <herbert@...dor.apana.org.au>,
	John Reiser <jreiser@...Wagon.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-sparse@...r.kernel.org, mpm@...enic.com,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [Security] Signed divides vs shifts (Re: /dev/urandom uses uninit bytes, leaks user data)

On Mon, Dec 17, 2007 at 10:28:38AM -0800, Linus Torvalds wrote:
> [ So Al, when you said that
> 
> 	(a-b)
> 
>   is equivalent to
> 
> 	((char *)a-(char *)b)/4
> 
>   for a "int *" a and b, you're right in the sense that the *result* is 
>   the same, but the code generation likely isn't. The "a-b" thing can (and 

Sure.  And yes, I very much do prefer code that uses C as it ought to be
used and doesn't play games with casts from hell, etc.  For a lot of reasons,
both correctness- and efficiency-related.

We _do_ have such turds.  In spades.  And such places are potential timebombs,
since well-intentioned idiotic patch ("I've read in lecture notes that sizeof
is better than explicit constant, so replacement surely can only improve the
things and the best part is, I don't need to understand what I'm doing")
turns an ugly FPOS into equally ugly FPOS that silently doesn't work ;-/

[sorry about the rant, I'm about 3/4 through the drivers/net colonoscopy,
with >300Kb of patches and a pile of assorted bugs so far - and then there's
drivers/scsi to deal with.  Endianness stuff, mostly...]
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