linux-kernel - [PATCH] time: Cast tv_nsec to u64 for proper shifting in update

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Message-Id: <1399648287-15178-1-git-send-email-boris.ostrovsky@oracle.com>
Date:	Fri,  9 May 2014 11:11:27 -0400
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com
Cc:	linux-kernel@...r.kernel.org, stefani@...bold.net,
	luto@...capital.net, konrad.wilk@...cle.com,
	boris.ostrovsky@...cle.com, stable@...r.kernel.org
Subject: [PATCH] time: Cast tv_nsec to u64 for proper shifting in update_vsyscall()

With tk->wall_to_monotonic.tv_nsec being a 32-bit value on 32-bit
systems, (tk->wall_to_monotonic.tv_nsec << tk->shift) in update_vsyscall()
may lose upper bits or, worse, add them since compiler will do this:
	(u64)(tk->wall_to_monotonic.tv_nsec << tk->shift)
instead of
	((u64)tk->wall_to_monotonic.tv_nsec << tk->shift)

So if, for example, tv_nsec is 0x800000 and shift is 8 we will end up
with 0xffffffff80000000 instead of 0x80000000. And then we are stuck in
the subsequent 'while' loop.

We need explicit cast.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc: stable@...r.kernel.org
---
 arch/x86/kernel/vsyscall_gtod.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kernel/vsyscall_gtod.c b/arch/x86/kernel/vsyscall_gtod.c
index f9c6e56..9531fbb 100644
--- a/arch/x86/kernel/vsyscall_gtod.c
+++ b/arch/x86/kernel/vsyscall_gtod.c
@@ -43,7 +43,7 @@ void update_vsyscall(struct timekeeper *tk)
 	vdata->monotonic_time_sec	= tk->xtime_sec
 					+ tk->wall_to_monotonic.tv_sec;
 	vdata->monotonic_time_snsec	= tk->xtime_nsec
-					+ (tk->wall_to_monotonic.tv_nsec
+					+ ((u64)tk->wall_to_monotonic.tv_nsec
 						<< tk->shift);
 	while (vdata->monotonic_time_snsec >=
 					(((u64)NSEC_PER_SEC) << tk->shift)) {
-- 
1.7.1

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