linux-kernel - Re: C language lawyers needed

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20080827220652.GD28946@ZenIV.linux.org.uk>
Date:	Wed, 27 Aug 2008 23:06:52 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Roland Dreier <rdreier@...co.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: C language lawyers needed

On Wed, Aug 27, 2008 at 02:43:05PM -0700, Roland Dreier wrote:
> Can anyone explain why building the current kernel gives
> 
>     drivers/net/mlx4/mcg.c: In function 'mlx4_multicast_attach':
>     drivers/net/mlx4/mcg.c:217: warning: integer overflow in expression
> 
> (with gcc (Ubuntu 4.3.1-9ubuntu1) 4.3.1 on x86-64), while the patch
> below gets rid of the warning?  I can't see what is overflowing in the
> expression that is warned out.
> 
> MGM_BLCK_LB_BIT is 30, and 1 << 30 is not negative or close to
> overflowing.  And MGM_QPN_MASK is 0x00FFFFFF, which is also nowhere near
> to integer overflow.
> 
> A fairly small test case that I don't understand either is:
> 
> unsigned foo(int x)
> {
> 	return (((x & 0xffffff) | (1 << 30)) & 0xff000000) >> 24;
> }
> 
> just running "gcc -c" (ie no extra warnings enabled) on that produces
> the same:
> 
>     b.c: In function 'foo':
>     b.c:3: warning: integer overflow in expression
> 
> I'm sure there's some promotion rule or something that makes sense of
> this, but it's a mystery to me...

There isn't.  It's gcc being buggy and inconsistent.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/