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

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Message-ID: <20141202093228.GA8792@hudson.localdomain>
Date:	Tue, 2 Dec 2014 01:32:28 -0800
From:	Jeremiah Mahler <jmmahler@...il.com>
To:	John Stultz <john.stultz@...aro.org>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	"pang.xunlei" <pang.xunlei@...aro.org>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] time: Fix sign bug in ntp mult overflow warning

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@...aro.org>
> Cc: Fengguang Wu <fengguang.wu@...el.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> Reported-by: Fengguang Wu <fengguang.wu@...el.com>
> Debugged-by: pang.xunlei <pang.xunlei@...aro.org>
> Signed-off-by: John Stultz <john.stultz@...aro.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.

-- 
- Jeremiah Mahler
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