hwmon: (aspeed-pwm-tacho) Fix -Wstringop-overflow warning in aspeed_create_fan_tach_channel()

Based on the documentation below, the maximum number of Fan tach channels is 16:

Documentation/devicetree/bindings/hwmon/aspeed-pwm-tacho.txt:45:
45 - aspeed,fan-tach-ch : should specify the Fan tach input channel.

 46                 integer value in the range 0 through 15, with 0 indicating
 47                 Fan tach channel 0 and 15 indicating Fan tach channel 15.
 48                 At least one Fan tach input channel is required.

However, the compiler doesn't know that, and legitimaly warns about a potential overwrite in array u8 fan_tach_ch_source[16] in struct aspeed_pwm_tacho_data, in case index takes a value outside the boundaries of the array:

drivers/hwmon/aspeed-pwm-tacho.c:
179 struct aspeed_pwm_tacho_data {
...
184 bool fan_tach_present[16];
...
193 u8 fan_tach_ch_source[16];
...
196 };

In function ?aspeed_create_fan_tach_channel?,
inlined from ?aspeed_create_fan? at drivers/hwmon/aspeed-pwm-tacho.c:877:2,
inlined from ?aspeed_pwm_tacho_probe? at drivers/hwmon/aspeed-pwm-tacho.c:936:9:
drivers/hwmon/aspeed-pwm-tacho.c:751:49: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]

  751 |                 priv->fan_tach_ch_source[index] = pwm_source;
      |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~
drivers/hwmon/aspeed-pwm-tacho.c: In function ?aspeed_pwm_tacho_probe?:
drivers/hwmon/aspeed-pwm-tacho.c:193:12: note: at offset [48, 255] into destination object ?fan_tach_ch_source? of size 16
  193 |         u8 fan_tach_ch_source[16];
      |            ^~~~~~~~~~~~~~~~~~

Fix this by sanity checking index before using it to index arrays of size 16 elements in struct aspeed_pwm_tacho_data. Also, and just for completeness, add a pr_err() message to display in the unlikely case 0 > index >= 16.

This is probably the last remaining -Wstringop-overflow issue in the kernel, and this patch helps with the ongoing efforts to enable such compiler option globally.

Signed-off-by Gustavo A. R. Silva <[email protected]>

---
 drivers/hwmon/aspeed-pwm-tacho.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

drivers/hwmon/aspeed-pwm-tacho.c +4 -3

index 997df4b40509..092a81916325 100644
@@ -166,6 +166,8 @@
 
 #define MAX_CDEV_NAME_LEN 16
 
+#define MAX_ASPEED_FAN_TACH_CHANNELS 16
+
 struct aspeed_cooling_device {
 	char name[16];
 	struct aspeed_pwm_tacho_data *priv;

@@ -181,7 +183,7 @@ struct aspeed_pwm_tacho_data {
 	struct reset_control *rst;
 	unsigned long clk_freq;
 	bool pwm_present[8];
-	bool fan_tach_present[16];
+	bool fan_tach_present[MAX_ASPEED_FAN_TACH_CHANNELS];
 	u8 type_pwm_clock_unit[3];
 	u8 type_pwm_clock_division_h[3];
 	u8 type_pwm_clock_division_l[3];

@@ -190,7 +192,7 @@ struct aspeed_pwm_tacho_data {
 	u16 type_fan_tach_unit[3];
 	u8 pwm_port_type[8];
 	u8 pwm_port_fan_ctrl[8];
-	u8 fan_tach_ch_source[16];
+	u8 fan_tach_ch_source[MAX_ASPEED_FAN_TACH_CHANNELS];
 	struct aspeed_cooling_device *cdev[8];
 	const struct attribute_group *groups[3];
 };

@@ -746,10 +748,14 @@ static void aspeed_create_fan_tach_channel(struct aspeed_pwm_tacho_data *priv,
 
 	for (val = 0; val < count; val++) {
 		index = fan_tach_ch[val];
-		aspeed_set_fan_tach_ch_enable(priv->regmap, index, true);

Quoted 9 lines · Gustavo A. R. Silva

Based on the documentation below, the maximum number of Fan tach channels is 16:

Documentation/devicetree/bindings/hwmon/aspeed-pwm-tacho.txt:45:
45 - aspeed,fan-tach-ch : should specify the Fan tach input channel.

  46                 integer value in the range 0 through 15, with 0 indicating
  47                 Fan tach channel 0 and 15 indicating Fan tach channel 15.
  48                 At least one Fan tach input channel is required.

Quoted 4 lines

However, the compiler doesn't know that, and legitimaly warns about a potential overwrite in array u8 fan_tach_ch_source[16] in struct aspeed_pwm_tacho_data, in case index takes a value outside the boundaries of the array:

All that doesn't warrant introducing checkpatch warnings.

Quoted 21 lines

drivers/hwmon/aspeed-pwm-tacho.c:
179 struct aspeed_pwm_tacho_data {
...
184 bool fan_tach_present[16];
...
193 u8 fan_tach_ch_source[16];
...
196 };

In function ?aspeed_create_fan_tach_channel?,
inlined from ?aspeed_create_fan? at drivers/hwmon/aspeed-pwm-tacho.c:877:2,
inlined from ?aspeed_pwm_tacho_probe? at drivers/hwmon/aspeed-pwm-tacho.c:936:9:
drivers/hwmon/aspeed-pwm-tacho.c:751:49: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]

   751 |                 priv->fan_tach_ch_source[index] = pwm_source;
       |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~
drivers/hwmon/aspeed-pwm-tacho.c: In function ?aspeed_pwm_tacho_probe?:
drivers/hwmon/aspeed-pwm-tacho.c:193:12: note: at offset [48, 255] into destination object ?fan_tach_ch_source? of size 16
   193 |         u8 fan_tach_ch_source[16];
       |            ^~~~~~~~~~~~~~~~~~

Quoted 9 lines

Fix this by sanity checking index before using it to index arrays of size 16 elements in struct aspeed_pwm_tacho_data. Also, and just for completeness, add a pr_err() message to display in the unlikely case 0 > index >= 16.

This is probably the last remaining -Wstringop-overflow issue in the kernel, and this patch helps with the ongoing efforts to enable such compiler option globally.

I am sorry, but this description almost completely misses the point. The real issue is that the values in aspeed,fan-tach-ch are not range checked, which can cause real problems if is elements are set to values larger than 15. This is a real problem and has nothing to do with string overflows.

This should use dev_err() (and, yes, that means dev needs to be passed as argument), and the function should return -EINVAL if this is encountered. Also, error handling should come first.

		if (index >= MAX_ASPEED_FAN_TACH_CHANNELS) {
			dev_err(dev, "Invalid Fan Tach input channel %u\n.", index);
			return -EINVAL;
		}