[PATCH] rtc: rtc-ds1672: fix unintended sign extension

[PATCH] rtc: rtc-ds1672: fix unintended sign extension

[Colin King]

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

Shifting a u8 by 24 will cause the value to be promoted to an integer. If
the top bit of the u8 is set then the following conversion to an unsigned
long will sign extend the value causing the upper 32 bits to be set in
the result.

Fix this by casting the u8 value to an unsigned long before the shift.

Detected by CoverityScan, CID#138801 ("Unintended sign extension")

Fixes: edf1aaa31fc5 ("[PATCH] RTC subsystem: DS1672 driver")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
---
 drivers/rtc/rtc-ds1672.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/rtc/rtc-ds1672.c b/drivers/rtc/rtc-ds1672.c
index 9caaccccaa57..b1ebca099b0d 100644
--- a/drivers/rtc/rtc-ds1672.c
+++ b/drivers/rtc/rtc-ds1672.c
@@ -58,7 +58,8 @@ static int ds1672_get_datetime(struct i2c_client *client, struct rtc_time *tm)
 		"%s: raw read data - counters=%02x,%02x,%02x,%02x\n",
 		__func__, buf[0], buf[1], buf[2], buf[3]);
 
-	time = (buf[3] << 24) | (buf[2] << 16) | (buf[1] << 8) | buf[0];
+	time = ((unsigned long)buf[3] << 24) | (buf[2] << 16) |
+	       (buf[1] << 8) | buf[0];
 
 	rtc_time_to_tm(time, tm);
 
-- 
2.20.1

Re: [PATCH] rtc: rtc-ds1672: fix unintended sign extension

[Alexandre Belloni]

On 05/02/2019 18:04:49+0000, Colin King wrote:
> From: Colin Ian King <colin.king@canonical.com>
> 
> Shifting a u8 by 24 will cause the value to be promoted to an integer. If
> the top bit of the u8 is set then the following conversion to an unsigned
> long will sign extend the value causing the upper 32 bits to be set in
> the result.
> 
> Fix this by casting the u8 value to an unsigned long before the shift.
> 
> Detected by CoverityScan, CID#138801 ("Unintended sign extension")
> 
> Fixes: edf1aaa31fc5 ("[PATCH] RTC subsystem: DS1672 driver")
> Signed-off-by: Colin Ian King <colin.king@canonical.com>
> ---
>  drivers/rtc/rtc-ds1672.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
Applied, thanks.

-- 
Alexandre Belloni, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com