[PATCH] drm/msm/dpu: avoid uninitialized variable use

[PATCH] drm/msm/dpu: avoid uninitialized variable use

[Arnd Bergmann]

From: Arnd Bergmann <arnd@arndb.de>

clang-21 points out a variable that is conditionally initialized
but then dereferenced:

drivers/gpu/drm/msm/disp/dpu1/dpu_plane.c:1138:6: error: variable 'crtc_state' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
 1138 |         if (plane_state->crtc)
      |             ^~~~~~~~~~~~~~~~~
drivers/gpu/drm/msm/disp/dpu1/dpu_plane.c:1142:58: note: uninitialized use occurs here
 1142 |         ret = dpu_plane_atomic_check_nosspp(plane, plane_state, crtc_state);
      |                                                                 ^~~~~~~~~~
drivers/gpu/drm/msm/disp/dpu1/dpu_plane.c:1138:2: note: remove the 'if' if its condition is always true
 1138 |         if (plane_state->crtc)
      |         ^~~~~~~~~~~~~~~~~~~~~~
 1139 |                 crtc_state = drm_atomic_get_new_crtc_state(state,
drivers/gpu/drm/msm/disp/dpu1/dpu_plane.c:1132:35: note: initialize the variable 'crtc_state' to silence this warning
 1132 |         struct drm_crtc_state *crtc_state;
      |                                          ^
      |                                           = NULL

The bug is real, but the suggestion from clang to set it to NULL is
unfortunately just as harmful as dereferencing a NULL pointer is little
better than uninitialized data.

Change the function to return an error in this case.

Fixes: 774bcfb73176 ("drm/msm/dpu: add support for virtual planes")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 drivers/gpu/drm/msm/disp/dpu1/dpu_plane.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/msm/disp/dpu1/dpu_plane.c b/drivers/gpu/drm/msm/disp/dpu1/dpu_plane.c
index 01171c535a27..4987f2f2fee0 100644
--- a/drivers/gpu/drm/msm/disp/dpu1/dpu_plane.c
+++ b/drivers/gpu/drm/msm/disp/dpu1/dpu_plane.c
@@ -1135,10 +1135,10 @@ static int dpu_plane_virtual_atomic_check(struct drm_plane *plane,
 	if (IS_ERR(plane_state))
 		return PTR_ERR(plane_state);
 
-	if (plane_state->crtc)
-		crtc_state = drm_atomic_get_new_crtc_state(state,
-							   plane_state->crtc);
+	if (!plane_state->crtc)
+		return -ENXIO;
 
+	crtc_state = drm_atomic_get_new_crtc_state(state, plane_state->crtc);
 	ret = dpu_plane_atomic_check_nosspp(plane, plane_state, crtc_state);
 	if (ret)
 		return ret;
-- 
2.39.5

Re: [PATCH] drm/msm/dpu: avoid uninitialized variable use

[Dmitry Baryshkov]

On Thu, Aug 07, 2025 at 09:19:48AM +0200, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> clang-21 points out a variable that is conditionally initialized
> but then dereferenced:
> 
> drivers/gpu/drm/msm/disp/dpu1/dpu_plane.c:1138:6: error: variable 'crtc_state' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
>  1138 |         if (plane_state->crtc)
>       |             ^~~~~~~~~~~~~~~~~
> drivers/gpu/drm/msm/disp/dpu1/dpu_plane.c:1142:58: note: uninitialized use occurs here
>  1142 |         ret = dpu_plane_atomic_check_nosspp(plane, plane_state, crtc_state);
>       |                                                                 ^~~~~~~~~~
> drivers/gpu/drm/msm/disp/dpu1/dpu_plane.c:1138:2: note: remove the 'if' if its condition is always true
>  1138 |         if (plane_state->crtc)
>       |         ^~~~~~~~~~~~~~~~~~~~~~
>  1139 |                 crtc_state = drm_atomic_get_new_crtc_state(state,
> drivers/gpu/drm/msm/disp/dpu1/dpu_plane.c:1132:35: note: initialize the variable 'crtc_state' to silence this warning
>  1132 |         struct drm_crtc_state *crtc_state;
>       |                                          ^
>       |                                           = NULL
> 
> The bug is real, but the suggestion from clang to set it to NULL is
> unfortunately just as harmful as dereferencing a NULL pointer is little
> better than uninitialized data.


Having no plane->crtc is a valid setting and it is handled inside
drm_atomic_helper_check_plane_state() by setting plane_state->visible =
false and returning early. Setting crtc_state to NULL is a correct fix.
Could you please send it?

> 
> Change the function to return an error in this case.
> 
> Fixes: 774bcfb73176 ("drm/msm/dpu: add support for virtual planes")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  drivers/gpu/drm/msm/disp/dpu1/dpu_plane.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 

-- 
With best wishes
Dmitry

Re: [PATCH] drm/msm/dpu: avoid uninitialized variable use

[Nathan Chancellor]

On Thu, Aug 07, 2025 at 11:09:38AM +0300, Dmitry Baryshkov wrote:
> Having no plane->crtc is a valid setting and it is handled inside
> drm_atomic_helper_check_plane_state() by setting plane_state->visible =
> false and returning early. Setting crtc_state to NULL is a correct fix.
> Could you please send it?

I sent this fix three weeks ago, could this be applied?

https://lore.kernel.org/20250715-drm-msm-fix-const-uninit-warning-v1-1-d6a366fd9a32@kernel.org/

Cheers,
Nathan

Re: [PATCH] drm/msm/dpu: avoid uninitialized variable use

[Dmitry Baryshkov]

On Thu, Aug 07, 2025 at 07:34:44AM -0700, Nathan Chancellor wrote:
> On Thu, Aug 07, 2025 at 11:09:38AM +0300, Dmitry Baryshkov wrote:
> > Having no plane->crtc is a valid setting and it is handled inside
> > drm_atomic_helper_check_plane_state() by setting plane_state->visible =
> > false and returning early. Setting crtc_state to NULL is a correct fix.
> > Could you please send it?
> 
> I sent this fix three weeks ago, could this be applied?

It will be picked up for -rc2 (hopefully)

> 
> https://lore.kernel.org/20250715-drm-msm-fix-const-uninit-warning-v1-1-d6a366fd9a32@kernel.org/
> 
> Cheers,
> Nathan

-- 
With best wishes
Dmitry

Re: [PATCH] drm/msm/dpu: avoid uninitialized variable use

[Nathan Chancellor]

On Sat, Aug 09, 2025 at 11:20:34AM +0300, Dmitry Baryshkov wrote:
> On Thu, Aug 07, 2025 at 07:34:44AM -0700, Nathan Chancellor wrote:
> > On Thu, Aug 07, 2025 at 11:09:38AM +0300, Dmitry Baryshkov wrote:
> > > Having no plane->crtc is a valid setting and it is handled inside
> > > drm_atomic_helper_check_plane_state() by setting plane_state->visible =
> > > false and returning early. Setting crtc_state to NULL is a correct fix.
> > > Could you please send it?
> > 
> > I sent this fix three weeks ago, could this be applied?
> 
> It will be picked up for -rc2 (hopefully)

Great, thank you! Even if it slips to -rc3, no worries, as long it is in
the pipeline to get there.

Cheers,
Nathan

Re: [PATCH] drm/msm/dpu: avoid uninitialized variable use

[Arnd Bergmann]

On Thu, Aug 7, 2025, at 10:09, Dmitry Baryshkov wrote:
> On Thu, Aug 07, 2025 at 09:19:48AM +0200, Arnd Bergmann wrote:
>>
>> The bug is real, but the suggestion from clang to set it to NULL is
>> unfortunately just as harmful as dereferencing a NULL pointer is little
>> better than uninitialized data.
>
>
> Having no plane->crtc is a valid setting and it is handled inside
> drm_atomic_helper_check_plane_state() by setting plane_state->visible =
> false and returning early. Setting crtc_state to NULL is a correct fix.
> Could you please send it?

Ah, I see. I saw the crtc_state dereference in

  WARN_ON(plane_state->crtc && plane_state->crtc != crtc_state->crtc);

but that is indeed guarded by the plane_state->crtc check.

Nathan's patch is sufficient then.

    Arnd