[PATCH] hwmon: (lm87) Convert macros to functions to avoid TOCTOU

[Gui-Dong Han]

The macro FAN_FROM_REG evaluates its arguments multiple times. When used
in lockless contexts involving shared driver data, this causes
Time-of-Check to Time-of-Use (TOCTOU) race conditions.

Convert the macro to a static function. This guarantees that arguments
are evaluated only once (pass-by-value), preventing the race
conditions.

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=5z8B-g@xxxxxxxxxxxxxx/
Signed-off-by: Gui-Dong Han <hanguidong02@xxxxxxxxx>
---
Based on the discussion in the link, I will submit a series of patches to
address TOCTOU issues in the hwmon subsystem by converting macros to
functions or adjusting locking where appropriate.
---
 drivers/hwmon/lm87.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/drivers/hwmon/lm87.c b/drivers/hwmon/lm87.c
index d2d970e73c61..37bf2d1d3d09 100644
--- a/drivers/hwmon/lm87.c
+++ b/drivers/hwmon/lm87.c
@@ -116,8 +116,14 @@ static u8 LM87_REG_TEMP_LOW[3] = { 0x3A, 0x38, 0x2C };
 				 (((val) < 0 ? (val) - 500 : \
 				   (val) + 500) / 1000))
 
-#define FAN_FROM_REG(reg, div)	((reg) == 255 || (reg) == 0 ? 0 : \
-				 (1350000 + (reg)*(div) / 2) / ((reg) * (div)))
+static int fan_from_reg(int reg, int div)
+{
+	if (reg == 255 || reg == 0)
+		return 0;
+
+	return (1350000 + reg * div / 2) / (reg * div);
+}
+
 #define FAN_TO_REG(val, div)	((val) * (div) * 255 <= 1350000 ? 255 : \
 				 (1350000 + (val)*(div) / 2) / ((val) * (div)))
 
@@ -465,7 +471,7 @@ static ssize_t fan_input_show(struct device *dev,
 	struct lm87_data *data = lm87_update_device(dev);
 	int nr = to_sensor_dev_attr(attr)->index;
 
-	return sprintf(buf, "%d\n", FAN_FROM_REG(data->fan[nr],
+	return sprintf(buf, "%d\n", fan_from_reg(data->fan[nr],
 		       FAN_DIV_FROM_REG(data->fan_div[nr])));
 }
 
@@ -475,7 +481,7 @@ static ssize_t fan_min_show(struct device *dev, struct device_attribute *attr,
 	struct lm87_data *data = lm87_update_device(dev);
 	int nr = to_sensor_dev_attr(attr)->index;
 
-	return sprintf(buf, "%d\n", FAN_FROM_REG(data->fan_min[nr],
+	return sprintf(buf, "%d\n", fan_from_reg(data->fan_min[nr],
 		       FAN_DIV_FROM_REG(data->fan_div[nr])));
 }
 
@@ -534,7 +540,7 @@ static ssize_t fan_div_store(struct device *dev,
 		return err;
 
 	mutex_lock(&data->update_lock);
-	min = FAN_FROM_REG(data->fan_min[nr],
+	min = fan_from_reg(data->fan_min[nr],
 			   FAN_DIV_FROM_REG(data->fan_div[nr]));
 
 	switch (val) {
-- 
2.43.0