linux-kernel - Re: [PATCH v2] timer: prevent overflow in apply

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Date:	Tue, 29 Apr 2014 19:22:25 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jiri Bohac <jbohac@...e.cz>
cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] timer: prevent overflow in apply_slack

On Tue, 29 Apr 2014, Jiri Bohac wrote:

> Thomas, does this make sense now, with the new description?

Yep, except
 
> 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@...e.cz>
> > Suggested-by: Deborah Townsend <dstownse@...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;

This should be 1UL, shouldn't it?

> >  	expires_limit = expires_limit & ~(mask);
> >  
> 
> Thanks,
> 
> -- 
> Jiri Bohac <jbohac@...e.cz>
> SUSE Labs, SUSE CZ
> 
> 
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