linux-kernel - [PATCH] rtc: Fix overflow when converting time64_t to rtc

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Message-Id: <a63f2b482281004df854d8345cb18368224880b6.1514200190.git.baolin.wang@linaro.org>
Date:   Mon, 25 Dec 2017 19:10:37 +0800
From:   Baolin Wang <baolin.wang@...aro.org>
To:     a.zummo@...ertech.it, alexandre.belloni@...e-electrons.com
Cc:     arnd@...db.de, broonie@...nel.org, linux-rtc@...r.kernel.org,
        linux-kernel@...r.kernel.org, baolin.wang@...aro.org
Subject: [PATCH] rtc: Fix overflow when converting time64_t to rtc_time

If we convert one large time values to rtc_time, in the original formula
'days * 86400' can be overflowed in 'unsigned int' type to make the formula
get one incorrect remain seconds value. Thus we can use div_s64_rem()
function to avoid this situation.

Signed-off-by: Baolin Wang <baolin.wang@...aro.org>
---
 drivers/rtc/rtc-lib.c |    6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/drivers/rtc/rtc-lib.c b/drivers/rtc/rtc-lib.c
index 1ae7da5..ad5bb21 100644
--- a/drivers/rtc/rtc-lib.c
+++ b/drivers/rtc/rtc-lib.c
@@ -52,13 +52,11 @@ int rtc_year_days(unsigned int day, unsigned int month, unsigned int year)
  */
 void rtc_time64_to_tm(time64_t time, struct rtc_time *tm)
 {
-	unsigned int month, year;
-	unsigned long secs;
+	unsigned int month, year, secs;
 	int days;
 
 	/* time must be positive */
-	days = div_s64(time, 86400);
-	secs = time - (unsigned int) days * 86400;
+	days = div_s64_rem(time, 86400, &secs);
 
 	/* day of the week, 1970-01-01 was a Thursday */
 	tm->tm_wday = (days + 4) % 7;
-- 
1.7.9.5