On Tue, Jan 21, 2025 at 06:52:03AM -0800, Guenter Roeck 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.
>
> Analyzed-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Suggested-by: David Laight <david.laight.linux@xxxxxxxxx>
> Cc: David Laight <david.laight.linux@xxxxxxxxx>
> Cc: Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx>
> Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> Signed-off-by: Guenter Roeck <linux@xxxxxxxxxxxx>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@xxxxxxxxx>
I'm pushing this soon to drm-intel-next, unless Linus want to take
this one directly to his tree
Thanks,
Rodrigo.
> ---
> v2: Simplify code to always return target_min after a warning,
> and also warn if source_min == source_max.
>
> drivers/gpu/drm/i915/display/intel_backlight.c | 5 +++--
> 1 file changed, 3 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..ca588bed82b9 100644
> --- a/drivers/gpu/drm/i915/display/intel_backlight.c
> +++ b/drivers/gpu/drm/i915/display/intel_backlight.c
> @@ -40,8 +40,9 @@ static u32 scale(u32 source_val,
> {
> u64 target_val;
>
> - WARN_ON(source_min > source_max);
> - WARN_ON(target_min > target_max);
> + if (WARN_ON(source_min >= source_max) ||
> + WARN_ON(target_min > target_max))
> + return target_min;
>
> /* defensive */
> source_val = clamp(source_val, source_min, source_max);
> --
> 2.45.2
>
