hwmon: (w83791d) Convert macros to functions to avoid TOCTOU

The macro FAN_FROM_REG evaluates its arguments multiple times. When used
in lockless contexts involving 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. This guarantees that arguments
are evaluated only once (pass-by-value), preventing the race
conditions.

Additionally, in store_fan_div, move the calculation of the minimum
limit inside the update lock. This ensures that the read-modify-write
sequence operates on consistent data.

Adhere to the principle of minimal changes by only converting macros
that evaluate arguments multiple times and are used in lockless
contexts.

Link: https://lore.kernel.org/all/CALbr=LYJ_ehtp53HXEVkSpYoub+XYSTU8Rg=o1xxMJ8=[e-mail]/
Fixes: 9873964d6eb2 ("[PATCH] HWMON: w83791d: New hardware monitoring driver for the Winbond W83791D")
Cc: [e-mail]
Signed-off-by: Gui-Dong Han <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Guenter Roeck <[e-mail]>

drivers/hwmon/w83791d.c

···
 	return clamp_val((1350000 + rpm * div / 2) / (rpm * div), 1, 254);
 }
 
-#define FAN_FROM_REG(val, div)	((val) == 0 ? -1 : \
-				((val) == 255 ? 0 : \
-					1350000 / ((val) * (div))))
+static int fan_from_reg(int val, int div)
+{
+	if (val == 0)
+		return -1;
+	if (val == 255)
+		return 0;
+	return 1350000 / (val * div);
+}
 
 /* for temp1 which is 8-bit resolution, LSB = 1 degree Celsius */
 #define TEMP1_FROM_REG(val)	((val) * 1000)
···
 	struct w83791d_data *data = w83791d_update_device(dev); \
 	int nr = sensor_attr->index; \
 	return sprintf(buf, "%d\n", \
-		FAN_FROM_REG(data->reg[nr], DIV_FROM_REG(data->fan_div[nr]))); \
+		fan_from_reg(data->reg[nr], DIV_FROM_REG(data->fan_div[nr]))); \
 }
 
 show_fan_reg(fan);
···
 	if (err)
 		return err;
 
-	/* Save fan_min */
-	min = FAN_FROM_REG(data->fan_min[nr], DIV_FROM_REG(data->fan_div[nr]));
-
 	mutex_lock(&data->update_lock);
+	/* Save fan_min */
+	min = fan_from_reg(data->fan_min[nr], DIV_FROM_REG(data->fan_div[nr]));
+
 	data->fan_div[nr] = div_to_reg(nr, val);
 
 	switch (nr) {