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

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Date:	Tue, 19 Mar 2013 05:38:36 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Engraf <david.engraf@...go.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <john.stultz@...aro.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ktime_add_ns() may overflow on 32bit architectures

On Tue, 2013-03-19 at 13:29 +0100, 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).
> 
> Best regards
> - David
> 
> Signed-off-by: David Engraf <david.engraf@...go.com>
> 

But check already exists for 64bit arches in ktime_set()



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