[PATCH] drm/i915/backlight: Return immediately when scale() finds invalid parameters

[Jani Nikula]

On Mon, 20 Jan 2025, Guenter Roeck <linux at roeck-us.net> wrote:
> The scale() functions detects invalid parameters, but continues
> its calculations anyway. This causes bad results if negative values
> are used for unsigned operations. Worst case, a division by 0 error
> will be seen if source_min == source_max.
>
> On top of that, after v6.13, the sequence of WARN_ON() followed by clamp()
> may result in a build error with gcc 13.x.
>
> drivers/gpu/drm/i915/display/intel_backlight.c: In function 'scale':
> include/linux/compiler_types.h:542:45: error:
> 	call to '__compiletime_assert_415' declared with attribute error:
> 	clamp() low limit source_min greater than high limit source_max
>
> This happens if the compiler decides to rearrange the code as follows.
>
>         if (source_min > source_max) {
>                 WARN(..);
>                 /* Do the clamp() knowing that source_min > source_max */
>                 source_val = clamp(source_val, source_min, source_max);
>         } else {
>                 /* Do the clamp knowing that source_min <= source_max */
>                 source_val = clamp(source_val, source_min, source_max);
>         }
>
> Fix the problem by evaluating the return values from WARN_ON and returning
> immediately after a warning. While at it, fix divide by zero error seen
> if source_min == source_max.

Thanks for the effort in tracking this down.

> Analyzed-by: Linus Torvalds <torvalds at linux-foundation.org>
> Suggested-by: Linus Torvalds <torvalds at linux-foundation.org>
> Suggested-by: David Laight <david.laight.linux at gmail.com>
> Cc: David Laight <david.laight.linux at gmail.com>
> Cc: Jani Nikula <jani.nikula at linux.intel.com>
> Cc: Andy Shevchenko <andriy.shevchenko at linux.intel.com>
> Signed-off-by: Guenter Roeck <linux at roeck-us.net>
> ---
>  drivers/gpu/drm/i915/display/intel_backlight.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_backlight.c b/drivers/gpu/drm/i915/display/intel_backlight.c
> index 3f81a726cc7d..ad49bd4a1c12 100644
> --- a/drivers/gpu/drm/i915/display/intel_backlight.c
> +++ b/drivers/gpu/drm/i915/display/intel_backlight.c
> @@ -40,8 +40,11 @@ static u32 scale(u32 source_val,
>  {
>  	u64 target_val;
>  
> -	WARN_ON(source_min > source_max);
> -	WARN_ON(target_min > target_max);
> +	if (WARN_ON(target_min > target_max))
> +		return target_min;
> +
> +	if (WARN_ON(source_min > source_max) || source_min == source_max)
> +		return target_min + (target_max - target_min) / 2;

There's really no need to be this fancy, though. Years down the line
someone's going to think that mean value calculation is something we
need to preserve, but we don't. I'd just return target_max everywhere.
And source_min == source_max could be a warn too.


BR,
Jani.


>  
>  	/* defensive */
>  	source_val = clamp(source_val, source_min, source_max);

-- 
Jani Nikula, Intel