Re: [PATCH 15/22] hwmon: applesmc: fix format string overflow

From: Guenter Roeck
Date: Fri Jul 14 2017 - 10:07:06 EST

On 07/14/2017 05:07 AM, Arnd Bergmann wrote:
gcc-7 warns that the key might exceed five bytes for lage index

drivers/hwmon/applesmc.c: In function 'applesmc_show_fan_position':
drivers/hwmon/applesmc.c:906:18: error: '%d' directive writing between 1 and 5 bytes into a region of size 4 [-Werror=format-overflow=]
sprintf(newkey, FAN_ID_FMT, to_index(attr));
drivers/hwmon/applesmc.c:906:18: note: directive argument in the range [0, 65535]
drivers/hwmon/applesmc.c:906:2: note: 'sprintf' output between 5 and 9 bytes into a destination of size 5

As the key is required to be four characters plus trailing zero,
we know that the index has to be small here. I'm using snprintf()
to avoid the warning. This would truncate the string instead of

Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>

I submitted a more comprehensive patch a couple of days ago. There are other similar
sprintf() calls in the driver which gcc doesn't report.


drivers/hwmon/applesmc.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/hwmon/applesmc.c b/drivers/hwmon/applesmc.c
index 0af7fd311979..515163b9a89f 100644
--- a/drivers/hwmon/applesmc.c
+++ b/drivers/hwmon/applesmc.c
@@ -903,7 +903,7 @@ static ssize_t applesmc_show_fan_position(struct device *dev,
char newkey[5];
u8 buffer[17];
- sprintf(newkey, FAN_ID_FMT, to_index(attr));
+ snprintf(newkey, sizeof(newkey), FAN_ID_FMT, to_index(attr));
ret = applesmc_read_key(newkey, buffer, 16);
buffer[16] = 0;