Messages in this thread | | | Date | Tue, 2 Dec 2014 23:00:26 +0800 | Subject | Re: [PATCH] time: Fix sign bug in ntp mult overflow warning | From | Xunlei Pang <> |
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On 2 December 2014 at 17:32, Jeremiah Mahler <jmmahler@gmail.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 >> overflow") a new check was added to watch for adjustments that could >> cause a mult overflow. >> >> Unfortunately the check compares a signed with unsigned value and >> ignored the case where the adjustment was negative, which causes >> spurious warn-ons on some systems (and seems like it would result in >> problematic time adjustments there as well, due to the early >> return). >> >> Thus this patch adds a check to make sure the adjustment is positive >> before we check for an overflow, and resovles the issue in my >> testing. >> >> Cc: pang.xunlei <pang.xunlei@linaro.org> >> Cc: Fengguang Wu <fengguang.wu@intel.com> >> Cc: Thomas Gleixner <tglx@linutronix.de> >> Cc: Ingo Molnar <mingo@kernel.org> >> Reported-by: Fengguang Wu <fengguang.wu@intel.com> >> Debugged-by: pang.xunlei <pang.xunlei@linaro.org> >> Signed-off-by: John Stultz <john.stultz@linaro.org> >> --- >> kernel/time/timekeeping.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c >> index 29a7d67..2dc0646 100644 >> --- a/kernel/time/timekeeping.c >> +++ b/kernel/time/timekeeping.c >> @@ -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
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