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

From: Eric Dumazet
Date: Tue Mar 19 2013 - 08:38:43 EST

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@xxxxxxxxx>

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

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