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

[Clemens Ladisch <clemens@ladisch.de>]

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