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

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Date:	Mon, 24 Nov 2014 20:35:45 -0800
From:	John Stultz <john.stultz@...aro.org>
To:	lkml <linux-kernel@...r.kernel.org>
Cc:	John Stultz <john.stultz@...aro.org>,
	"pang.xunlei" <pang.xunlei@...aro.org>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>
Subject: [PATCH] time: Fix sign bug in ntp mult overflow warning

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;
-- 
1.9.1

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