linux-kernel - Re: [PATCH] ktime_add_ns() may overflow on 32bit architectures

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Date:	Mon, 08 Apr 2013 13:20:03 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	David Engraf <david.engraf@...go.com>
CC:	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ktime_add_ns() may overflow on 32bit architectures

On 03/19/2013 05:29 AM, David Engraf wrote:
> Hello,
>
> I've triggered an overflow when using ktime_add_ns() on a 32bit 
> architecture not supporting CONFIG_KTIME_SCALAR.
>
> When passing a very high value for u64 nsec, e.g. 7881299347898368000 
> the do_div() function converts this value to seconds (7881299347) 
> which is still to high to pass to the ktime_set() function as long. 
> The result in my case is a negative value.
>
> The problem on my system occurs in the tick-sched.c, 
> tick_nohz_stop_sched_tick() when time_delta is set to 
> timekeeping_max_deferment(). The check for time_delta < KTIME_MAX is 
> valid, thus ktime_add_ns() is called with a too large value resulting 
> in a negative expire value. This leads to an endless loop in the 
> ticker code:
>
> time_delta: 7881299347898368000
> expires = ktime_add_ns(last_update, time_delta)
> expires: negative value
>
> This error doesn't occurs on 64bit or architectures supporting 
> CONFIG_KTIME_SCALAR (e.g. ARM, x86-32).

Sorry, this fell through the cracks. I see Andrew caught it, but I've 
queued for 3.10 in my tree as well.

This should be tagged for -stable as well, no?

thanks
-john

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