Re: [PATCH] HID: pidff: bound the unit exponent rescaling loop

["Tomasz Pakuła" <tomasz.pakula.oficjalny@gmail.com>]

On Thu, 2026-10-29 at 00:04 +0800, Shengzhuo Wei wrote:
> pidff_rescale_time() walks the device-provided unit exponent one step
> at a time, allowing extreme values to cause billions of iterations
> while ff->mutex is held. In a UHID test with an INT_MIN exponent,
> EVIOCSFF took 3.1 to 4.3 seconds to upload a finite-duration effect.
> 
> Stop rescaling once scaled_time becomes zero, since further
> multiplication or division by 10 cannot change the result. The u32
> value becomes zero within 32 multiplications by 10, and the u16
> input within 5 divisions. This bounds the loops while preserving
> their results for all exponents, including wraparound.
> 
> Fixes: 8713107221a8 ("HID: pidff: Rescale time values to match field units")
> Cc: stable@vger.kernel.org
> Signed-off-by: Shengzhuo Wei <me@cherr.cc>
> ---
>  drivers/hid/usbhid/hid-pidff.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/hid/usbhid/hid-pidff.c b/drivers/hid/usbhid/hid-pidff.c
> index 22951b7ecd17..1a2fffcf204e 100644
> --- a/drivers/hid/usbhid/hid-pidff.c
> +++ b/drivers/hid/usbhid/hid-pidff.c
> @@ -354,9 +354,9 @@ static u32 pidff_rescale_time(u16 time, struct hid_field *field)
>  	int exponent = field->unit_exponent;
>  
>  	pr_debug("time field exponent: %d\n", exponent);
> -	for (; exponent < FF_TIME_EXPONENT; exponent++)
> +	for (; scaled_time && exponent < FF_TIME_EXPONENT; exponent++)
>  		scaled_time *= 10;
> -	for (; exponent > FF_TIME_EXPONENT; exponent--)
> +	for (; scaled_time && exponent > FF_TIME_EXPONENT; exponent--)
>  		scaled_time /= 10;
>  
>  	pr_debug("time calculated from %d to %d\n", time, scaled_time);
> 
> ---
> base-commit: d135ab453a7400a34930bdda1ef4bdc8acfac80e
> change-id: 20261028-b4-pidff-rescale-exponent-1d5c4e218e6d
> 
> Best regards,
> --  
> Shengzhuo Wei <me@cherr.cc>

Tested this locally and works nice.

Should we maybe add a warning to the driver that the time exponent might
be out of a sane range? Sane range being, say, 10 to 0? This of course
would have to happen during init stage not to spam dmesg.

Regardless of that:
-- 
Reviewed-by: Tomasz Pakuła <tomasz.pakula.oficjalny@gmail.com>