hwmon: (corsair-psu) Fix linear11 calculation

In corsairpsu_linear11_to_int(), the mantissa is extracted using bitwise
operations and cast to s16 before being shifted left:

static int corsairpsu_linear11_to_int(const u16 val, const int scale)
{
    ...
    const int mant = (((s16)(val & 0x7ff)) << 5) >> 5;
    ...
}

Due to C integer promotion rules, the masked value (which is always
positive) is promoted to a 32-bit integer before the left shift. As a
result, the sign bit is never extended to bit 31 of the promoted integer.

When the device hardware reports a negative temperature in Linear11 format
(such as an ambient temperature probe reporting sub-zero), the negative
mantissa is parsed incorrectly as a massive positive value. For example,
-1 becomes 2047, which scales to 2047 degrees Celsius.

Fix the problem by type casting the result of the left shift operation
to s16.

Another problem is left-shifting of negative values. In C, the result of
left-shifting negative values is undefined. Use a multiplication instead
to avoid the problem.

Also use a local s64 variable to store temporary results, change
the return value type from int to long, and clamp the final value
to LONG_MIN and LONG_MAX to avoid under- and overflow issues while
retaining as much information as possible.

Reported-by: Sashiko <sashiko-bot@kernel.org>
Cc: Wilken Gottwalt <wilken.gottwalt@posteo.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Wilken Gottwalt <wilken.gottwalt@posteo.net>
Link: https://lore.kernel.org/r/20260804034811.2385506-1-linux@roeck-us.net
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

diff --git a/drivers/hwmon/corsair-psu.c b/drivers/hwmon/corsair-psu.c
index ebcaddf337d807bcf26cb38759fe99c10fbbd65b..0cf072f86acbfab1dad1a82f0e752fe5f99f68f2 100644 (file)
--- a/drivers/hwmon/corsair-psu.c
+++ b/drivers/hwmon/corsair-psu.c
@@ -137,13 +137,18 @@ struct corsairpsu_data {
 };
 
 /* some values are SMBus LINEAR11 data which need a conversion */
-static int corsairpsu_linear11_to_int(const u16 val, const int scale)
+static long corsairpsu_linear11_to_long(const u16 val, const int scale)
 {
        const int exp = ((s16)val) >> 11;
-       const int mant = (((s16)(val & 0x7ff)) << 5) >> 5;
-       const int result = mant * scale;
+       const int mant = ((s16)((val & 0x7ff) << 5)) >> 5;
+       s64 result = mant * scale;
 
-       return (exp >= 0) ? (result << exp) : (result >> -exp);
+       if (exp >= 0)
+               result *= (int)(1UL << exp);
+       else
+               result >>= -exp;
+
+       return clamp(result, LONG_MIN, LONG_MAX);
 }
 
 /* the micro-controller uses percentage values to control pwm */
@@ -263,13 +268,13 @@ static int corsairpsu_get_value(struct corsairpsu_data *priv, u8 cmd, u8 rail, l
        case PSU_CMD_RAIL_AMPS:
        case PSU_CMD_TEMP0:
        case PSU_CMD_TEMP1:
-               *val = corsairpsu_linear11_to_int(tmp & 0xFFFF, 1000);
+               *val = corsairpsu_linear11_to_long(tmp & 0xFFFF, 1000);
                break;
        case PSU_CMD_FAN:
-               *val = corsairpsu_linear11_to_int(tmp & 0xFFFF, 1);
+               *val = corsairpsu_linear11_to_long(tmp & 0xFFFF, 1);
                break;
        case PSU_CMD_FAN_PWM_ENABLE:
-               *val = corsairpsu_linear11_to_int(tmp & 0xFFFF, 1);
+               *val = corsairpsu_linear11_to_long(tmp & 0xFFFF, 1);
                /*
                 * 0 = automatic mode, means the micro-controller controls the fan using a plan
                 *     which can be modified, but changing this plan is not supported by this
@@ -283,12 +288,12 @@ static int corsairpsu_get_value(struct corsairpsu_data *priv, u8 cmd, u8 rail, l
                        *val = 2;
                break;
        case PSU_CMD_FAN_PWM:
-               *val = corsairpsu_linear11_to_int(tmp & 0xFFFF, 1);
+               *val = corsairpsu_linear11_to_long(tmp & 0xFFFF, 1);
                *val = corsairpsu_dutycycle_to_pwm(*val);
                break;
        case PSU_CMD_RAIL_WATTS:
        case PSU_CMD_TOTAL_WATTS:
-               *val = corsairpsu_linear11_to_int(tmp & 0xFFFF, 1000000);
+               *val = corsairpsu_linear11_to_long(tmp & 0xFFFF, 1000000);
                break;
        case PSU_CMD_TOTAL_UPTIME:
        case PSU_CMD_UPTIME: