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

[David Engraf]

Am 19.03.2013 13:38, schrieb Eric Dumazet:
> 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()

Yes, but not for 32bit arches. 64-bit arches doesn't run into this 
problem because ktime_add_ns() can directly calculate the result without 
calling do_div() and ktime_set().

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