Re: [PATCH v3] timer: prevent overflow in apply_slack

[Jiri Bohac <jbohac@suse.cz>]

On Tue, Apr 29, 2014 at 07:22:25PM +0200, Thomas Gleixner wrote:
> > > +	mask = (1LL << bit) - 1;
> 
> This should be 1UL, shouldn't it?

yes, good catch, thanks!




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 = (1UL << bit) - 1;
 
 	expires_limit = expires_limit & ~(mask);
 
-- 
Jiri Bohac <jbohac@suse.cz>
SUSE Labs, SUSE CZ