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, pass `dev` as
argument to function `aspeed_create_fan_tach_channel()`, and add an error
message in case `index` is out-of-bounds, in which case return `-EINVAL`.

Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Link: https://lore.kernel.org/r/ZVPQJIP26dIzRAr6@work
[groeck: Fixed continuation line alignment]
Signed-off-by: Guenter Roeck <[e-mail]>

drivers/hwmon/aspeed-pwm-tacho.c

···
 
 #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;
···
 	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];
···
 	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];
 };
···
 	aspeed_set_pwm_port_fan_ctrl(priv, pwm_port, INIT_FAN_CTRL);
 }
 
-static void aspeed_create_fan_tach_channel(struct aspeed_pwm_tacho_data *priv,
-					   u8 *fan_tach_ch,
-					   int count,
-					   u8 pwm_source)
+static int aspeed_create_fan_tach_channel(struct device *dev,
+					  struct aspeed_pwm_tacho_data *priv,
+					  u8 *fan_tach_ch,
+					  int count,
+					  u8 pwm_source)
 {
 	u8 val, index;
 
 	for (val = 0; val < count; val++) {
 		index = fan_tach_ch[val];
+		if (index >= MAX_ASPEED_FAN_TACH_CHANNELS) {
+			dev_err(dev, "Invalid Fan Tach input channel %u\n.", index);
+			return -EINVAL;
+		}
 		aspeed_set_fan_tach_ch_enable(priv->regmap, index, true);
 		priv->fan_tach_present[index] = true;
 		priv->fan_tach_ch_source[index] = pwm_source;
 		aspeed_set_fan_tach_ch_source(priv->regmap, index, pwm_source);
 	}
+
+	return 0;
 }
 
 static int
···
 					fan_tach_ch, count);
 	if (ret)
 		return ret;
-	aspeed_create_fan_tach_channel(priv, fan_tach_ch, count, pwm_port);
+
+	ret = aspeed_create_fan_tach_channel(dev, priv, fan_tach_ch, count, pwm_port);
+	if (ret)
+		return ret;
 
 	return 0;
 }