[PATCH v3 2/3] hwmon: (isl28022) Fix integer overflow in power calculation on 32-bit

[Pradhan, Sanman]

From: Sanman Pradhan <psanman@xxxxxxxxxxx>

isl28022_read_power() computes:

  *val = ((51200000L * ((long)data->gain)) /
          (long)data->shunt) * (long)regval;

On 32-bit platforms, 'long' is 32 bits. With gain=8 and shunt=10000
(the default configuration):

  (51200000 * 8) / 10000 = 40960
  40960 * 65535 = 2,684,313,600

This exceeds LONG_MAX (2,147,483,647), resulting in signed integer
overflow.

Additionally, dividing before multiplying by regval loses precision
unnecessarily.

Use u64 arithmetic with div_u64() and multiply before dividing.
The intermediate product cannot overflow u64
(worst case: 51200000 * 8 * 65535 = 26834432000000). Power is
inherently non-negative, so unsigned types are the natural fit.
Cap the result to LONG_MAX before returning it through the hwmon
callback.

Fixes: 39671a14df4f2 ("hwmon: (isl28022) new driver for ISL28022 power monitor")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Sanman Pradhan <psanman@xxxxxxxxxxx>
---
v3:
 - Use min()/div_u64() one-liner instead of clamp_val() + tmp
   variable, per review feedback
 - Add overflow justification to commit message
v2:
 - Switch from s64/div_s64() to u64/div_u64() since power is
   inherently non-negative

 drivers/hwmon/isl28022.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/hwmon/isl28022.c b/drivers/hwmon/isl28022.c
index c2e559dde63f..c5a34ceedcdb 100644
--- a/drivers/hwmon/isl28022.c
+++ b/drivers/hwmon/isl28022.c
@@ -9,6 +9,7 @@
 #include <linux/err.h>
 #include <linux/hwmon.h>
 #include <linux/i2c.h>
+#include <linux/math64.h>
 #include <linux/module.h>
 #include <linux/regmap.h>
 
@@ -185,8 +186,8 @@ static int isl28022_read_power(struct device *dev, u32 attr, long *val)
 				  ISL28022_REG_POWER, &regval);
 		if (err < 0)
 			return err;
-		*val = ((51200000L * ((long)data->gain)) /
-			(long)data->shunt) * (long)regval;
+		*val = min(div_u64(51200000ULL * data->gain * regval,
+				   data->shunt), LONG_MAX);
 		break;
 	default:
 		return -EOPNOTSUPP;
-- 
2.34.1