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

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Message-ID: <CALAqxLW-Jc63WPcHYhT8SsWf-kex6=4JcNX7hUD6r7fd8B+0iQ@mail.gmail.com>
Date:	Thu, 23 Oct 2014 21:22:39 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Xunlei Pang <pang.xunlei@...aro.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] time: Fix NTP adjustment mult overflow.

On Wed, Oct 22, 2014 at 5:37 AM, Xunlei Pang <pang.xunlei@...aro.org> wrote:
> 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, so it may overflow when doing NTP positive adjustment, see
> the following detail:
> 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.
>
>
> This patch avoids mult overflow by judging the overflow case before adding
> mult_adj to mult, also adds the
> WARNING message when capturing such case.
>
> Signed-off-by: pang.xunlei <pang.xunlei@...aro.org>

I reworded this a bit further, but its in my queue for 3.19.

thanks
-john
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