hwmon: (vt8231) Convert macros to functions to avoid TOCTOU

The macro FAN_FROM_REG evaluates its arguments multiple times. When used
with shared driver data, this leads to Time-of-Check to Time-of-Use
(TOCTOU) race conditions, potentially causing divide-by-zero errors.

Convert the macro to a static function to ensure arguments are evaluated
only once.

Additionally, in fan_div_store, move the reading of the old register
value and the calculation of the minimum limit inside the update lock.
This ensures that the read-modify-write sequence operates on consistent
data, preventing race conditions during fan divider updates.

Link: https://lore.kernel.org/all/CALbr=LYJ_ehtp53HXEVkSpYoub+XYSTU8Rg=o1xxMJ8=5z8B-g@mail.gmail.com/
Signed-off-by: Gui-Dong Han <hanguidong02@gmail.com>
Link: https://lore.kernel.org/r/20251124165900.4713-1-hanguidong02@gmail.com
[groeck: Dropped unnecessary line split]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Author: 2045castor

drivers/hwmon/vt8231.c

@@ -138,7 +138,12 @@ static inline u8 FAN_TO_REG(long rpm, int div)
 	return clamp_val(1310720 / (rpm * div), 1, 255);
 }
 
-#define FAN_FROM_REG(val, div) ((val) == 0 ? 0 : 1310720 / ((val) * (div)))
+static int fan_from_reg(int val, int div)
+{
+	if (val == 0)
+		return 0;
+	return 1310720 / (val * div);
+}
 
 struct vt8231_data {
 	unsigned short addr;
@@ -561,7 +566,7 @@ static ssize_t fan_show(struct device *dev, struct device_attribute *attr,
 	struct sensor_device_attribute *sensor_attr = to_sensor_dev_attr(attr);
 	int nr = sensor_attr->index;
 	struct vt8231_data *data = vt8231_update_device(dev);
-	return sprintf(buf, "%d\n", FAN_FROM_REG(data->fan[nr],
+	return sprintf(buf, "%d\n", fan_from_reg(data->fan[nr],
 				DIV_FROM_REG(data->fan_div[nr])));
 }
 
@@ -571,7 +576,7 @@ static ssize_t fan_min_show(struct device *dev, struct device_attribute *attr,
 	struct sensor_device_attribute *sensor_attr = to_sensor_dev_attr(attr);
 	int nr = sensor_attr->index;
 	struct vt8231_data *data = vt8231_update_device(dev);
-	return sprintf(buf, "%d\n", FAN_FROM_REG(data->fan_min[nr],
+	return sprintf(buf, "%d\n", fan_from_reg(data->fan_min[nr],
 			DIV_FROM_REG(data->fan_div[nr])));
 }
 
@@ -613,16 +618,17 @@ static ssize_t fan_div_store(struct device *dev,
 	struct sensor_device_attribute *sensor_attr = to_sensor_dev_attr(attr);
 	unsigned long val;
 	int nr = sensor_attr->index;
-	int old = vt8231_read_value(data, VT8231_REG_FANDIV);
-	long min = FAN_FROM_REG(data->fan_min[nr],
-				 DIV_FROM_REG(data->fan_div[nr]));
+	int old;
+	long min;
 	int err;
 
 	err = kstrtoul(buf, 10, &val);
 	if (err)
 		return err;
 
 	mutex_lock(&data->update_lock);
+	old = vt8231_read_value(data, VT8231_REG_FANDIV);
+	min = fan_from_reg(data->fan_min[nr], DIV_FROM_REG(data->fan_div[nr]));
 	switch (val) {
 	case 1:
 		data->fan_div[nr] = 0;