Re: [PATCH 1/2] [RFC] time: Fix problem with large timespecs & ktime_get_update_offsets

[John Stultz]

On 07/31/2012 11:52 PM, Thomas Gleixner wrote:
> On Tue, 31 Jul 2012, John Stultz wrote:
> > There's currently a slight difference in ktime_get_update_offsets()
> > vs ktime_get() which can result in boot time crashes when booting
> > with insane CMOS clock values larger then ~2264.
> >
> > ktime_get() does basically the following:
> >          return timespec_to_ktime(timespec_add(xtime, wall_to_monotonic))
> >
> > Where as ktime_get_update_offsets does approximately:
> >          return ktime_sub(timespec_to_ktime(xtime), realtime_offset);
> >
> > The problem is, at boot we set xtime = year 8200 and
> > wall_to_monotonic = year -8200,  ktime_get adds both values, mostly
> > nulling the difference out (leaving only how long the system has been
> > up), then converts that relatively small value to a ktime_t properly
> > without losing any information.
> >
> > ktime_get_update_offsets however, since it converts xtime (again set
> > to some value greater then year 8200), to a ktime, it gets clamped at
> > KTIME_MAX, then we subtract realtime_offset, which is _also_ clamped
> > at KTIME_MAX, resulting in us always returning almost[1] zero. This
> > causes us to stop expiring timers.
> >
> > Now, one of the reasons Thomas and I changed the logic was that using
> > the precalculated realtime_offset was slightly more efficient then
> > re-adding xtime and wall_to_monotonic's components separately. But
> > how valuable this unmeasured slight efficiency is vs extra
> > robustness for crazy time values is questionable.
> >
> > So switch back to the ktime_get implementation for
> > ktime_get_update_offsets
> NAK.
>
> You're papering over the real problem: Using crap values without
> sanity checking them in the first place.
>
> All your patch does is papering over the problem. Heck, year 8200 is
> obvious bullshit, so we can detect and reject it.

Ok, sounds good. I'll drop this one and just keep the sanity checking patch.

thanks
-john

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