Re: [PATCH] gpu: drm: i915: display: Avoid null values intel_plane_atomic_check_with_state

[Jani Nikula <jani.nikula@linux.intel.com>]

On Fri, 27 Sep 2024, Ville Syrjälä <ville.syrjala@linux.intel.com> wrote:
> On Fri, Sep 27, 2024 at 11:20:32AM +0300, Jani Nikula wrote:
>> On Fri, 27 Sep 2024, Alessandro Zanni <alessandro.zanni87@gmail.com> wrote:
>> > This fix solves multiple Smatch errors:
>> >
>> > drivers/gpu/drm/i915/display/intel_atomic_plane.c:660
>> > intel_plane_atomic_check_with_state() error:
>> > we previously assumed 'fb' could be null (see line 648)
>> >
>> > drivers/gpu/drm/i915/display/intel_atomic_plane.c:664
>> > intel_plane_atomic_check_with_state()
>> > error: we previously assumed 'fb' could be null (see line 659)
>> >
>> > drivers/gpu/drm/i915/display/intel_atomic_plane.c:671
>> > intel_plane_atomic_check_with_state()
>> > error: we previously assumed 'fb' could be null (see line 663)
>> >
>> > We should check first if fb is not null before to access its properties.
>> 
>> new_plane_state->uapi.visible && !fb should not be possible, but it's
>> probably too hard for smatch to figure out. It's not exactly trivial for
>> humans to figure out either.
>> 
>> I'm thinking something like below to help both.
>> 
>> Ville, thoughts?
>> 
>> 
>> BR,
>> Jani.
>> 
>> 
>> diff --git a/drivers/gpu/drm/i915/display/intel_atomic_plane.c b/drivers/gpu/drm/i915/display/intel_atomic_plane.c
>> index 3505a5b52eb9..d9da47aed55d 100644
>> --- a/drivers/gpu/drm/i915/display/intel_atomic_plane.c
>> +++ b/drivers/gpu/drm/i915/display/intel_atomic_plane.c
>> @@ -629,6 +629,9 @@ int intel_plane_atomic_check_with_state(const struct intel_crtc_state *old_crtc_
>>  	if (ret)
>>  		return ret;
>>  
>> +	if (drm_WARN_ON(display->drm, new_plane_state->uapi.visible && !fb))
>> +		return -EINVAL;
>> +
>
> We have probably 100 places that would need this. So it's going
> to be extremely ugly.
>
> One approach I could maybe tolerate is something like
> intel_plane_is_visible(plane_state) 
> {
> 	if (drm_WARN_ON(visible && !fb))
> 		return false;
>
> 	return plane_state->visible;
> }
>
> + s/plane_state->visible/intel_plane_is_visible(plane_state)/
>
> But is that going to help these obtuse tools?

That does help people, which is more important. :)

I think the problem is first checking if fb is NULL, and then
dereferencing it anyway.

visible always means fb != NULL, but I forget, is the reverse true? Can
we have fb != NULL and !visible? I mean could we change the fb check to
visible check?

BR,
Jani.

>
>>  	if (fb)
>>  		new_crtc_state->enabled_planes |= BIT(plane->id);
>>  
>> 
>> 
>> >
>> > Signed-off-by: Alessandro Zanni <alessandro.zanni87@gmail.com>
>> > ---
>> >  drivers/gpu/drm/i915/display/intel_atomic_plane.c | 6 +++---
>> >  1 file changed, 3 insertions(+), 3 deletions(-)
>> >
>> > diff --git a/drivers/gpu/drm/i915/display/intel_atomic_plane.c b/drivers/gpu/drm/i915/display/intel_atomic_plane.c
>> > index e979786aa5cf..1606f79b39e6 100644
>> > --- a/drivers/gpu/drm/i915/display/intel_atomic_plane.c
>> > +++ b/drivers/gpu/drm/i915/display/intel_atomic_plane.c
>> > @@ -656,18 +656,18 @@ int intel_plane_atomic_check_with_state(const struct intel_crtc_state *old_crtc_
>> >  	    intel_plane_is_scaled(new_plane_state))
>> >  		new_crtc_state->scaled_planes |= BIT(plane->id);
>> >  
>> > -	if (new_plane_state->uapi.visible &&
>> > +	if (new_plane_state->uapi.visible && fb &&
>> >  	    intel_format_info_is_yuv_semiplanar(fb->format, fb->modifier))
>> >  		new_crtc_state->nv12_planes |= BIT(plane->id);
>> >  
>> > -	if (new_plane_state->uapi.visible &&
>> > +	if (new_plane_state->uapi.visible && fb &&
>> >  	    fb->format->format == DRM_FORMAT_C8)
>> >  		new_crtc_state->c8_planes |= BIT(plane->id);
>> >  
>> >  	if (new_plane_state->uapi.visible || old_plane_state->uapi.visible)
>> >  		new_crtc_state->update_planes |= BIT(plane->id);
>> >  
>> > -	if (new_plane_state->uapi.visible &&
>> > +	if (new_plane_state->uapi.visible && fb &&
>> >  	    intel_format_info_is_yuv_semiplanar(fb->format, fb->modifier)) {
>> >  		new_crtc_state->data_rate_y[plane->id] =
>> >  			intel_plane_data_rate(new_crtc_state, new_plane_state, 0);
>> 
>> -- 
>> Jani Nikula, Intel

-- 
Jani Nikula, Intel