linux-kernel - Re: [PATCH 1/2] [RFC] time: Fix problem with large timespecs & ktime_get_update

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Message-ID: <50195E20.90704@linaro.org>
Date:	Wed, 01 Aug 2012 09:49:36 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Prarit Bhargava <prarit@...hat.com>,
	Zhouping Liu <zliu@...hat.com>, CAI Qian <caiqian@...hat.com>
Subject: Re: [PATCH 1/2] [RFC] time: Fix problem with large timespecs & ktime_get_update_offsets

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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