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SubjectRe: [PATCH v2] timer: prevent overflow in apply_slack
Thomas, does this make sense now, with the new description?

On Fri, Apr 18, 2014 at 05:23:11PM +0200, Jiri Bohac wrote:
> On architectures with sizeof(int) < sizeof (long), the
> computation of mask inside apply_slack() can be undefined if the
> computed bit is > 32.
>
> E.g. with: expires = 0xffffe6f5 and slack = 25, we get:
>
> expires_limit = 0x20000000e
> bit = 33
> mask = (1 << 33) - 1 /* undefined */
>
> On x86, mask becomes 1 and and the slack is not applied properly.
> On s390, mask is -1, expires is set to 0 and the timer fires immediately.
>
> Signed-off-by: Jiri Bohac <jbohac@suse.cz>
> Suggested-by: Deborah Townsend <dstownse@us.ibm.com>
>
> diff --git a/kernel/timer.c b/kernel/timer.c
> index 87bd529..4c36c91 100644
> --- a/kernel/timer.c
> +++ b/kernel/timer.c
> @@ -838,7 +838,7 @@ unsigned long apply_slack(struct timer_list *timer, unsigned long expires)
>
> bit = find_last_bit(&mask, BITS_PER_LONG);
>
> - mask = (1 << bit) - 1;
> + mask = (1LL << bit) - 1;
>
> expires_limit = expires_limit & ~(mask);
>

Thanks,

--
Jiri Bohac <jbohac@suse.cz>
SUSE Labs, SUSE CZ



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