Re: [RFC PATCH v2 05/17] drm/vkms: Avoid reading beyond LUT array

[Harry Wentland <harry.wentland@amd.com>]

On 2023-10-30 09:29, Pekka Paalanen wrote:
> On Thu, 19 Oct 2023 17:21:21 -0400
> Harry Wentland <harry.wentland@amd.com> wrote:
> 
>> When the floor LUT index (drm_fixp2int(lut_index) is the last
>> index of the array the ceil LUT index will point to an entry
>> beyond the array. Make sure we guard against it and use the
>> value of the floot LUT index.
>>
>> Blurb about LUT creation and how first element should be 0x0 and
>> last one 0xffff.
>>
>> Hold on, is that even correct? What should the ends of a LUT be?
>> How does UNORM work and how does it apply to LUTs?
> 
> Do you mean how should UNORM input value map to LUT entries for LUT
> indexing?
> 
> I suppose UNORM 16-bit converts to nominal real values as:
> - 0x0: 0.0
> - 0xffff: 1.0
> 
> And in LUT, you want 0.0 to map to the first LUT element exactly, and
> 1.0 to map to the last LUT element exactly, even if whatever
> interpolation may be in use, right?
> 
> If so, it is important to make sure that, assuming linear interpolation
> for instance, there is no "dead zone" at either end. Given high
> interpolation precision, any step away from 0.0 or 1.0 needs to imply a
> change in the real-valued output, assuming e.g. identity LUT.
> 
> If LUT has N elements, and 16-bit UNORM input value is I, then (in
> naive real-valued math, so no implicit truncation between operations)
> 
> x = I / 0xffff * (N - 1)
> ia = floor(x)
> ib = min(ia + 1, N - 1)
> 
> f = x - floor(x)
> y = (1 - f) * LUT[ia] + f * LUT[ib]
> 
> 
> Does that help?
> 

Thanks. Yes, this is what the code is doing (with this commit).

The commit description was an oversight and only reflect my initial
thoughts when coding it, before I made sure this is the right way
to go about it. I'll update it.

Harry

> In my mind, I'm thinking of a uniformly distributed LUT as a 1-D
> texture, because that's how I have implemented them in GL. There you
> have to be careful so that input values 0.0 and 1.0 map to the *center*
> of the first and last texel, and not to the edges of the texture like
> texture coordinates do. Then you can use the GL linear texture
> interpolation as-is.
> 
> 
> Thanks,
> pq
> 
> 
>> Signed-off-by: Harry Wentland <harry.wentland@amd.com>
>> Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
>> Cc: Pekka Paalanen <pekka.paalanen@collabora.com>
>> Cc: Simon Ser <contact@emersion.fr>
>> Cc: Harry Wentland <harry.wentland@amd.com>
>> Cc: Melissa Wen <mwen@igalia.com>
>> Cc: Jonas Ådahl <jadahl@redhat.com>
>> Cc: Sebastian Wick <sebastian.wick@redhat.com>
>> Cc: Shashank Sharma <shashank.sharma@amd.com>
>> Cc: Alexander Goins <agoins@nvidia.com>
>> Cc: Joshua Ashton <joshua@froggi.es>
>> Cc: Michel Dänzer <mdaenzer@redhat.com>
>> Cc: Aleix Pol <aleixpol@kde.org>
>> Cc: Xaver Hugl <xaver.hugl@gmail.com>
>> Cc: Victoria Brekenfeld <victoria@system76.com>
>> Cc: Sima <daniel@ffwll.ch>
>> Cc: Uma Shankar <uma.shankar@intel.com>
>> Cc: Naseer Ahmed <quic_naseer@quicinc.com>
>> Cc: Christopher Braga <quic_cbraga@quicinc.com>
>> Cc: Abhinav Kumar <quic_abhinavk@quicinc.com>
>> Cc: Arthur Grillo <arthurgrillo@riseup.net>
>> Cc: Hector Martin <marcan@marcan.st>
>> Cc: Liviu Dudau <Liviu.Dudau@arm.com>
>> Cc: Sasha McIntosh <sashamcintosh@google.com>
>> ---
>>  drivers/gpu/drm/vkms/vkms_composer.c | 14 ++++++++++----
>>  1 file changed, 10 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/vkms/vkms_composer.c b/drivers/gpu/drm/vkms/vkms_composer.c
>> index a0a3a6fd2926..cf1dff162920 100644
>> --- a/drivers/gpu/drm/vkms/vkms_composer.c
>> +++ b/drivers/gpu/drm/vkms/vkms_composer.c
>> @@ -123,6 +123,8 @@ static u16 apply_lut_to_channel_value(const struct vkms_color_lut *lut, u16 chan
>>  				      enum lut_channel channel)
>>  {
>>  	s64 lut_index = get_lut_index(lut, channel_value);
>> +	u16 *floor_lut_value, *ceil_lut_value;
>> +	u16 floor_channel_value, ceil_channel_value;
>>  
>>  	/*
>>  	 * This checks if `struct drm_color_lut` has any gap added by the compiler
>> @@ -130,11 +132,15 @@ static u16 apply_lut_to_channel_value(const struct vkms_color_lut *lut, u16 chan
>>  	 */
>>  	static_assert(sizeof(struct drm_color_lut) == sizeof(__u16) * 4);
>>  
>> -	u16 *floor_lut_value = (__u16 *)&lut->base[drm_fixp2int(lut_index)];
>> -	u16 *ceil_lut_value = (__u16 *)&lut->base[drm_fixp2int_ceil(lut_index)];
>> +	floor_lut_value = (__u16 *)&lut->base[drm_fixp2int(lut_index)];
>> +	if (drm_fixp2int(lut_index) == (lut->lut_length - 1))
>> +		/* We're at the end of the LUT array, use same value for ceil and floor */
>> +		ceil_lut_value = floor_lut_value;
>> +	else
>> +		ceil_lut_value = (__u16 *)&lut->base[drm_fixp2int_ceil(lut_index)];
>>  
>> -	u16 floor_channel_value = floor_lut_value[channel];
>> -	u16 ceil_channel_value = ceil_lut_value[channel];
>> +	floor_channel_value = floor_lut_value[channel];
>> +	ceil_channel_value = ceil_lut_value[channel];
>>  
>>  	return lerp_u16(floor_channel_value, ceil_channel_value,
>>  			lut_index & DRM_FIXED_DECIMAL_MASK);
>