linux-kernel - [PATCH] time: Avoid signed overflow in timekeeping_delta_to

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Date:   Mon, 14 Nov 2016 14:42:49 -0500
From:   Chris Metcalf <cmetcalf@...lanox.com>
To:     John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Laurent Vivier <lvivier@...hat.com>,
        David Gibson <david@...son.dropbear.id.au>,
        "Christopher S . Hall" <christopher.s.hall@...el.com>,
        linux-kernel@...r.kernel.org
Cc:     Chris Metcalf <cmetcalf@...lanox.com>
Subject: [PATCH] time: Avoid signed overflow in timekeeping_delta_to_ns()

This bugfix was originally made in commit 35a4933a8959 ("time:
Avoid signed overflow in timekeeping_get_ns()").  When the code was
refactored in commit 6bd58f09e1d8 ("time: Add cycles to nanoseconds
translation") the signed overflow fix was lost.  Re-introduce it.

Signed-off-by: Chris Metcalf <cmetcalf@...lanox.com>
---
I happened to be looking for an unrelated fix, found this code,
realized the tip code didn't match the fixed code, and
backtracked to where it had gone away.

 kernel/time/timekeeping.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 37dec7e3db43..57926bc7b7f3 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -304,8 +304,7 @@ static inline s64 timekeeping_delta_to_ns(struct tk_read_base *tkr,
 {
 	s64 nsec;
 
-	nsec = delta * tkr->mult + tkr->xtime_nsec;
-	nsec >>= tkr->shift;
+	nsec = (delta * tkr->mult + tkr->xtime_nsec) >> tkr->shift;
 
 	/* If arch requires, add in get_arch_timeoffset() */
 	return nsec + arch_gettimeoffset();
-- 
2.7.2