[PATCH][next] hwmon: (aspeed-pwm-tacho) Fix -Wstringop-overflow warning in aspeed_create_fan_tach_channel()

[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.

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

diff --git a/drivers/hwmon/aspeed-pwm-tacho.c b/drivers/hwmon/aspeed-pwm-tacho.c
index 997df4b40509..092a81916325 100644
--- a/drivers/hwmon/aspeed-pwm-tacho.c
+++ b/drivers/hwmon/aspeed-pwm-tacho.c
@@ -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);
-		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);
+		if (index < MAX_ASPEED_FAN_TACH_CHANNELS) {
+			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);
+		} else {
+			pr_err("Invalid Fan Tach input channel %u\n.", index);
+		}
 	}
 }
 
-- 
2.34.1