linux-kernel - Re: Compat sysinfo syscall (kernel/sys.c) relying on undefined behavior?

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Message-ID: <5406BF88.8040102@ladisch.de>
Date:	Wed, 03 Sep 2014 09:13:12 +0200
From:	Clemens Ladisch <clemens@...isch.de>
To:	Scotty Bauer <sbauer@....utah.edu>, linux-kernel@...r.kernel.org
Subject: Re: Compat sysinfo syscall (kernel/sys.c) relying on undefined behavior?

Scotty Bauer wrote:
> In the compat version of sysinfo, kernel/sys.c we see the following:
>
>     /* Check to see if any memory value is too large for 32-bit and scale
>      *  down if needed
>      */
>     if ((s.totalram >> 32) || (s.totalswap >> 32)) {

This code is supposed to check if any of the bits in the upper half of
these 64-bit values are set.

> s.totalram is a u32

Oops.

> the behavior is undefined.

For a constant shift amount, gcc happens to generate correct code, i.e.,
the result is zero.  (If the shift amount were not a constant, x86
processors would use only its lowest five bits, and the result would be
wrong.)

Anyway, it's not a good idea to rely on gcc's implementation of this
undefined behaviour; the code should have used upper_32_bits() instead.
Please write a patch.

Regards,
Clemens
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