linux-kernel - [PATCH][next] rtc: msc313: Fix unintentional sign extension issue on left shift of a u16

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Date:   Tue, 28 Sep 2021 13:39:06 +0100
From:   Colin King <colin.king@...onical.com>
To:     Daniel Palmer <daniel@...ngy.jp>,
        Romain Perier <romain.perier@...il.com>,
        Alessandro Zummo <a.zummo@...ertech.it>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Nobuhiro Iwamatsu <iwamatsu@...auri.org>,
        linux-arm-kernel@...ts.infradead.org, linux-rtc@...r.kernel.org
Cc:     kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH][next] rtc: msc313: Fix unintentional sign extension issue on left shift of a u16

From: Colin Ian King <colin.king@...onical.com>

Shifting the u16 value returned by readw by 16 bits to the left
will be promoted to a 32 bit signed int and then sign-extended
to an unsigned long. If the top bit of the readw is set then
the shifted value will be sign extended and the top 32 bits of
the result will be set.

Fixes: be7d9c9161b9 ("rtc: Add support for the MSTAR MSC313 RTC")
Signed-off-by: Colin Ian King <colin.king@...onical.com>
---
 drivers/rtc/rtc-msc313.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/rtc/rtc-msc313.c b/drivers/rtc/rtc-msc313.c
index 5f178d29cfd8..7430244ad867 100644
--- a/drivers/rtc/rtc-msc313.c
+++ b/drivers/rtc/rtc-msc313.c
@@ -53,7 +53,7 @@ static int msc313_rtc_read_alarm(struct device *dev, struct rtc_wkalrm *alarm)
 	unsigned long seconds;
 
 	seconds = readw(priv->rtc_base + REG_RTC_MATCH_VAL_L)
-			| (readw(priv->rtc_base + REG_RTC_MATCH_VAL_H) << 16);
+			| ((unsigned long)readw(priv->rtc_base + REG_RTC_MATCH_VAL_H) << 16);
 
 	rtc_time64_to_tm(seconds, &alarm->time);
 
-- 
2.32.0