linux-kernel - [PATCH 3/4] time: Avoid potential shift-overflow with large shift values

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Message-Id: <1345595449-34965-4-git-send-email-john.stultz@linaro.org>
Date:	Tue, 21 Aug 2012 20:30:48 -0400
From:	John Stultz <john.stultz@...aro.org>
To:	linux-kernel <linux-kernel@...r.kernel.org>
Cc:	John Stultz <john.stultz@...aro.org>,
	Ingo Molnar <mingo@...nel.org>,
	Prarit Bhargava <prarit@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andreas Schwab <schwab@...ux-m68k.org>
Subject: [PATCH 3/4] time: Avoid potential shift-overflow with large shift values

Andreas Schwab noted that the 1 << tk->shift could overflow
if the shift value was greater then 30, since 1 would be
a 32bit long on 32bit architectures.

This patch uses 1ULL instead to ensure we don't overflow on
the shift.

This issue was introduced by 1e75fa8be9fb61e1af46b5b3b176347a4c958ca1

Cc: Ingo Molnar <mingo@...nel.org>
Cc: Prarit Bhargava <prarit@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Andreas Schwab <schwab@...ux-m68k.org>
Reported-by: Andreas Schwab <schwab@...ux-m68k.org>
Signed-off-by: John Stultz <john.stultz@...aro.org>
---
 kernel/time/timekeeping.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 1dbf80e..a5a9389 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1184,9 +1184,9 @@ static void update_wall_time(void)
 	* the vsyscall implementations are converted to use xtime_nsec
 	* (shifted nanoseconds), this can be killed.
 	*/
-	remainder = tk->xtime_nsec & ((1 << tk->shift) - 1);
+	remainder = tk->xtime_nsec & ((1ULL << tk->shift) - 1);
 	tk->xtime_nsec -= remainder;
-	tk->xtime_nsec += 1 << tk->shift;
+	tk->xtime_nsec += 1ULL << tk->shift;
 	tk->ntp_error += remainder << tk->ntp_error_shift;
 
 	/*
-- 
1.7.9.5

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