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

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Message-ID: <20140429111356.GA23888@midget.suse.cz>
Date:	Tue, 29 Apr 2014 13:13:56 +0200
From:	Jiri Bohac <jbohac@...e.cz>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [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@...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;
>  
>  	expires_limit = expires_limit & ~(mask);
>  

Thanks,

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

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