linux-kernel - Re: [PATCH] time: Fix sign bug in ntp mult overflow warning

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Message-ID: <20141202213104.GA14149@hudson.localdomain>
Date:	Tue, 2 Dec 2014 13:31:04 -0800
From:	Jeremiah Mahler <jmmahler@...il.com>
To:	Xunlei Pang <pang.xunlei@...aro.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] time: Fix sign bug in ntp mult overflow warning

Xunlei,

On Tue, Dec 02, 2014 at 11:00:26PM +0800, Xunlei Pang wrote:
> On 2 December 2014 at 17:32, Jeremiah Mahler <jmmahler@...il.com> wrote:
> > John,
> >
> > On Mon, Nov 24, 2014 at 08:35:45PM -0800, John Stultz wrote:
> >> In commit 6067dc5a8c2b ("time: Avoid possible NTP adjustment mult
> >> @@ -1330,7 +1330,7 @@ static __always_inline void timekeeping_apply_adjustment(struct timekeeper *tk,
[...]
> >>        *
> >>        * XXX - TODO: Doc ntp_error calculation.
> >>        */
> >> -     if (tk->tkr.mult + mult_adj < mult_adj) {
> >> +     if ((mult_adj > 0) && (tk->tkr.mult + mult_adj < mult_adj)) {
> >>               /* NTP adjustment caused clocksource mult overflow */
> >>               WARN_ON_ONCE(1);
> >>               return;
> >
> > This change does quiet the warning but I think it does so for the wrong
> > reason.
> >
> > mult_adj is a signed number and tk->tkr.mult is an unsigned number.
> > Adding the check that (mult_adj > 0) limits the test to only positive
> > numbers.  A positive number plus a positive number will never be less
> > than either of the two positive numbers.  The test is always false.
> 
> Hi Jeremiah,
> 
> The result of a positive number plus a positive number may overflow,
> for example, u32  (0xFFFF_FFF0 + 0x20) will be 0x10,
> that's what this patch is dealing with.
> 
> Thanks,
> Xunlei
> >
> > --
> > - Jeremiah Mahler

Oh, got it.  That makes sense now.  Thanks :)

-- 
- Jeremiah Mahler
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