linux-kernel - [PATCH 2/2] platform/x86: alienware-wmi: fix format string overflow warning

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Date:   Thu, 20 Jul 2017 18:00:51 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Darren Hart <dvhart@...radead.org>,
        Andy Shevchenko <andy@...radead.org>
Cc:     Arnd Bergmann <arnd@...db.de>, platform-driver-x86@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH 2/2] platform/x86: alienware-wmi: fix format string overflow warning

gcc points out a possible format string overflow for a large value of 'zone':

drivers/platform/x86/alienware-wmi.c: In function 'alienware_wmi_init':
drivers/platform/x86/alienware-wmi.c:461:24: error: '%02X' directive writing between 2 and 8 bytes into a region of size 6 [-Werror=format-overflow=]
   sprintf(buffer, "zone%02X", i);
                        ^~~~
drivers/platform/x86/alienware-wmi.c:461:19: note: directive argument in the range [0, 2147483646]
   sprintf(buffer, "zone%02X", i);
                   ^~~~~~~~~~
drivers/platform/x86/alienware-wmi.c:461:3: note: 'sprintf' output between 7 and 13 bytes into a destination of size 10

This replaces the 'int' variable with an 'u8' to make sure
it always fits, renaming the variable to 'zone' for clarity.

Unfortunately, gcc-7.1.1 still warns about it with that change, which
seems to be unintended by the gcc developers. I have opened a bug
against gcc with a reduced test case. As a workaround, I also
change the format string to use "%02hhX", which shuts up the
warning in that version.

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81483
Link: https://patchwork.ozlabs.org/patch/788415/
Suggested-by: Andy Shevchenko <andy@...radead.org>
Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
v2: completely rewrite the patch after discussing with Andy
---
 drivers/platform/x86/alienware-wmi.c | 38 ++++++++++++++++++------------------
 1 file changed, 19 insertions(+), 19 deletions(-)

diff --git a/drivers/platform/x86/alienware-wmi.c b/drivers/platform/x86/alienware-wmi.c
index 0831b428c217..c56a9ecba959 100644
--- a/drivers/platform/x86/alienware-wmi.c
+++ b/drivers/platform/x86/alienware-wmi.c
@@ -255,12 +255,12 @@ static int parse_rgb(const char *buf, struct platform_zone *zone)
 
 static struct platform_zone *match_zone(struct device_attribute *attr)
 {
-	int i;
-	for (i = 0; i < quirks->num_zones; i++) {
-		if ((struct device_attribute *)zone_data[i].attr == attr) {
+	u8 zone;
+	for (zone = 0; zone < quirks->num_zones; zone++) {
+		if ((struct device_attribute *)zone_data[zone].attr == attr) {
 			pr_debug("alienware-wmi: matched zone location: %d\n",
-				 zone_data[i].location);
-			return &zone_data[i];
+				 zone_data[zone].location);
+			return &zone_data[zone];
 		}
 	}
 	return NULL;
@@ -420,7 +420,7 @@ static DEVICE_ATTR(lighting_control_state, 0644, show_control_state,
 
 static int alienware_zone_init(struct platform_device *dev)
 {
-	int i;
+	u8 zone;
 	char buffer[10];
 	char *name;
 
@@ -457,19 +457,19 @@ static int alienware_zone_init(struct platform_device *dev)
 	if (!zone_data)
 		return -ENOMEM;
 
-	for (i = 0; i < quirks->num_zones; i++) {
-		sprintf(buffer, "zone%02X", i);
+	for (zone = 0; zone < quirks->num_zones; zone++) {
+		sprintf(buffer, "zone%02hhX", zone);
 		name = kstrdup(buffer, GFP_KERNEL);
 		if (name == NULL)
 			return 1;
-		sysfs_attr_init(&zone_dev_attrs[i].attr);
-		zone_dev_attrs[i].attr.name = name;
-		zone_dev_attrs[i].attr.mode = 0644;
-		zone_dev_attrs[i].show = zone_show;
-		zone_dev_attrs[i].store = zone_set;
-		zone_data[i].location = i;
-		zone_attrs[i] = &zone_dev_attrs[i].attr;
-		zone_data[i].attr = &zone_dev_attrs[i];
+		sysfs_attr_init(&zone_dev_attrs[zone].attr);
+		zone_dev_attrs[zone].attr.name = name;
+		zone_dev_attrs[zone].attr.mode = 0644;
+		zone_dev_attrs[zone].show = zone_show;
+		zone_dev_attrs[zone].store = zone_set;
+		zone_data[zone].location = zone;
+		zone_attrs[zone] = &zone_dev_attrs[zone].attr;
+		zone_data[zone].attr = &zone_dev_attrs[zone];
 	}
 	zone_attrs[quirks->num_zones] = &dev_attr_lighting_control_state.attr;
 	zone_attribute_group.attrs = zone_attrs;
@@ -484,9 +484,9 @@ static void alienware_zone_exit(struct platform_device *dev)
 	sysfs_remove_group(&dev->dev.kobj, &zone_attribute_group);
 	led_classdev_unregister(&global_led);
 	if (zone_dev_attrs) {
-		int i;
-		for (i = 0; i < quirks->num_zones; i++)
-			kfree(zone_dev_attrs[i].attr.name);
+		u8 zone;
+		for (zone = 0; zone < quirks->num_zones; zone++)
+			kfree(zone_dev_attrs[zone].attr.name);
 	}
 	kfree(zone_dev_attrs);
 	kfree(zone_data);
-- 
2.9.0