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

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Message-ID: <ouan6tfmoefwuvs7wmhpzjdwmxfhqh3ad26j5tmwdugnq7ieda@ddw6dfqtq27g>
Date: Thu, 7 Aug 2025 11:09:38 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@....qualcomm.com>
To: Arnd Bergmann <arnd@...nel.org>
Cc: Rob Clark <robin.clark@....qualcomm.com>,
        Dmitry Baryshkov <lumag@...nel.org>, David Airlie <airlied@...il.com>,
        Simona Vetter <simona@...ll.ch>, Nathan Chancellor <nathan@...nel.org>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>,
        Arnd Bergmann <arnd@...db.de>, Abhinav Kumar <abhinav.kumar@...ux.dev>,
        Jessica Zhang <jessica.zhang@....qualcomm.com>,
        Sean Paul <sean@...rly.run>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
        Bill Wendling <morbo@...gle.com>,
        Justin Stitt <justinstitt@...gle.com>,
        Antonino Maniscalco <antomani103@...il.com>,
        Konrad Dybcio <konrad.dybcio@....qualcomm.com>,
        Jun Nie <jun.nie@...aro.org>, linux-arm-msm@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, freedreno@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH] drm/msm/dpu: avoid uninitialized variable use

On Thu, Aug 07, 2025 at 09:19:48AM +0200, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.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@...db.de>
> ---
>  drivers/gpu/drm/msm/disp/dpu1/dpu_plane.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 

-- 
With best wishes
Dmitry