Re: [PATCH v2] hwmon: (cros_ec) Handle temperature conversion overflows

[Guenter Roeck <linux@roeck-us.net>]

On 7/28/26 03:17, Thomas Weißschuh wrote:
> The calculations converting between the different temperature units can
> overflow on 32-bit systems, resulting in incorrect data.
> 
> Detect these overflows and handle them.
> 
> The code is written in a way that there is a single conversion function
> and the compiler can recognize when overflows are impossible (on 64-bit
> and in cros_ec_hwmon_temp_to_millicelsius()). If the compiler detects
> this, it will optimize away the overflow checks.
> 
> Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
> ---
> Changes in v2:
> - Drop already applied patch 1.
> - Also handle overflow of u32 -> long.
> - Clarify commit message wrt compiler optimizations.
> - Use __always_inline over __flatten to allow the compiler to optimize
>    away more unnecessary overflow checks.
> - Link to v1: https://patch.msgid.link/20260630-cros_ec-hwmon-overflow-v1-0-3d2ecd3eb0f2@weissschuh.net
> ---
>   drivers/hwmon/cros_ec_hwmon.c | 31 ++++++++++++++++++++++++++-----
>   1 file changed, 26 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/hwmon/cros_ec_hwmon.c b/drivers/hwmon/cros_ec_hwmon.c
> index 1337b646e022..004a8180b665 100644
> --- a/drivers/hwmon/cros_ec_hwmon.c
> +++ b/drivers/hwmon/cros_ec_hwmon.c
> @@ -5,11 +5,13 @@
>    *  Copyright (C) 2024 Thomas Weißschuh <linux@weissschuh.net>
>    */
>   
> +#include <linux/build_bug.h>
>   #include <linux/cleanup.h>
>   #include <linux/device.h>
>   #include <linux/hwmon.h>
>   #include <linux/math.h>
>   #include <linux/module.h>
> +#include <linux/overflow.h>
>   #include <linux/platform_device.h>
>   #include <linux/platform_data/cros_ec_commands.h>
>   #include <linux/platform_data/cros_ec_proto.h>
> @@ -151,14 +153,28 @@ static bool cros_ec_hwmon_is_error_temp(u8 temp)
>   /* This differs slightly from the variant in units.h to avoid rounding inconsistencies. */
>   #define CROS_EC_HWMON_ABSOLUTE_ZERO_MILLICELSIUS (-273000)
>   
> -static long cros_ec_hwmon_kelvin_to_millicelsius(long t)
> +static __always_inline bool cros_ec_hwmon_kelvin_to_millicelsius_overflow(long t, long *ret)
>   {
> -	return t * MILLIDEGREE_PER_DEGREE + CROS_EC_HWMON_ABSOLUTE_ZERO_MILLICELSIUS;
> +	if (check_mul_overflow(t, MILLIDEGREE_PER_DEGREE, ret))
> +		return true;
> +
> +	if (check_add_overflow(*ret, CROS_EC_HWMON_ABSOLUTE_ZERO_MILLICELSIUS, ret))
> +		return true;
> +
> +	return false;

	return check_add_overflow(*ret, CROS_EC_HWMON_ABSOLUTE_ZERO_MILLICELSIUS, ret);
>   }
>   
>   static long cros_ec_hwmon_temp_to_millicelsius(u8 temp)

The maximum value of "temp" is 255.

>   {
> -	return cros_ec_hwmon_kelvin_to_millicelsius((((long)temp) + EC_TEMP_SENSOR_OFFSET));

255 + 200 = 455.
455 * 1000 = 455000 (not even counting the conversion from kelvin to C).

How could this ever overflow ?

> +	long ret;
> +
> +	if (check_add_overflow(temp, EC_TEMP_SENSOR_OFFSET, &ret))
> +		BUILD_BUG();
> +
> +	if (cros_ec_hwmon_kelvin_to_millicelsius_overflow(ret, &ret))
> +		BUILD_BUG();
> +
> +	return ret;
>   }
>   
>   static bool cros_ec_hwmon_attr_is_temp_threshold(u32 attr)
> @@ -236,8 +252,13 @@ static int cros_ec_hwmon_read(struct device *dev, enum hwmon_sensor_types type,
>   			ret = cros_ec_hwmon_read_temp_threshold(priv->cros_ec, channel,
>   								cros_ec_hwmon_attr_to_thres(attr),
>   								&threshold);
> -			if (ret == 0)
> -				*val = cros_ec_hwmon_kelvin_to_millicelsius(threshold);
> +			if (ret == 0) {
> +				if (overflows_type(threshold, long))
> +					*val = LONG_MAX;
> +
> +				if (cros_ec_hwmon_kelvin_to_millicelsius_overflow(threshold, val))
> +					*val = LONG_MAX;
> +			}

I agree with Sashiko - it does not make sense to call cros_ec_hwmon_kelvin_to_millicelsius_overflow()
if it is already known that threshold overflowed.

Personally I think this is way too complicated. An overflowing temperature above 255 degrees C
does not make any sense, and returning LONG_MAX as temperature seems a bit odd.
I would just have checked for some reasonable limit, such as
			if (threshold > 255 + 273)
				*val = 255000;
			else
				*val = cros_ec_hwmon_kelvin_to_millicelsius(threshold);

If it does make sense to return LONG_MAX as temperature for some reason, that should
be explained.

Thanks,
Guenter