linux-kernel - Re: [PATCH 1/2] time: Fix NTP adjustment mult overflow.

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Message-ID: <CALAqxLVtYeHX0qBkOFjf58opp9ympxbjFa2DsaYx-KOthqQS=w@mail.gmail.com>
Date:	Tue, 21 Oct 2014 09:11:04 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	"pang.xunlei" <pang.xunlei@...aro.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] time: Fix NTP adjustment mult overflow.

On Tue, Oct 21, 2014 at 3:19 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Thu, 9 Oct 2014, pang.xunlei wrote:
>
> First of all: Please use proper line breaks in the changelog.
>
>> The mult memember of struct clocksource should always be a large u32
>> number when calculated through __clocksource_updatefreq_scale(). The
>> value of (cs->mult+cs->maxadj) may have a chance to reach very near
>> 0xFFFFFFFF.
>
> And what's the actual problem with reaching a value near 0xFFFFFFFF?
>
>> For instance, 555MHz oscillator: cs->mult is 0xE6A17102,
>> cs->maxadj is 0x195E8EFD, cs->mult+cs->maxadj is 0xFFFFFFFF. Such
>> oscillators would probably exist on some processors like MIPS which
>> use CP0 compare/count CPU clock as the clock source.
>
> Again, what's the problem?

So the problem is that with an adjustment the mult value might
overflow, becoming very small, bascially causing time to stop
increasing. This is mentioned below, but I agree we're burying the
headline a bit.


>> Clocksource might encounter large frequency adjustment due to the
>> hardware unstability, environment temperature, software deviation,
>> NTP algorithm accuracy, etc. When NTP slewes the clock, kernel goes
>> through update_wall_time()->...->timekeeping_apply_adjustment():
>> tk->tkr.mult += mult_adj; Unfortunately, tk->tkr.mult may overflow
>> after this operation, though such cases are next to impossible to
>> happen in practice.
>
> So you adding this just for correctness reasons, not because you
> observed the problem in practice?

This is my understanding.

I'll work with Xunlei to make further clarifications to the changelog
to make this more explicit.

Thanks for your feedback!
-john
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