linux-kernel - Re: [PATCH] drm/msm/dpu: Fix bit-shifting UB in DPU_HW

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Date:   Wed, 22 Mar 2023 13:48:45 -0700
From:   Rob Clark <robdclark@...il.com>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     Geert Uytterhoeven <geert+renesas@...der.be>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>, Vinod Koul <vkoul@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Kuogee Hsieh <quic_khsieh@...cinc.com>,
        linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        freedreno@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/msm/dpu: Fix bit-shifting UB in DPU_HW_VER() macro

On Tue, Mar 21, 2023 at 8:08 AM Randy Dunlap <rdunlap@...radead.org> wrote:
>
>
>
> On 3/6/23 01:06, Geert Uytterhoeven wrote:
> > With gcc-5 and CONFIG_UBSAN_SHIFT=y:
> >
> >     drivers/gpu/drm/msm/msm_mdss.c: In function 'msm_mdss_enable':
> >     drivers/gpu/drm/msm/msm_mdss.c:296:2: error: case label does not reduce to an integer constant
> >       case DPU_HW_VER_800:
> >       ^
> >     drivers/gpu/drm/msm/msm_mdss.c:299:2: error: case label does not reduce to an integer constant
> >       case DPU_HW_VER_810:
> >       ^
> >     drivers/gpu/drm/msm/msm_mdss.c:300:2: error: case label does not reduce to an integer constant
> >       case DPU_HW_VER_900:
> >       ^
> >
> > This happens because for major revisions 8 or greather, the non-sign bit
> > of the major revision number is shifted into bit 31 of a signed integer,
> > which is undefined behavior.
> >
> > Fix this by casting the major revision number to unsigned int.
> >
> > Fixes: efcd0107727c4f04 ("drm/msm/dpu: add support for SM8550")
> > Fixes: 4a352c2fc15aec1e ("drm/msm/dpu: Introduce SC8280XP")
> > Fixes: 100d7ef6995d1f86 ("drm/msm/dpu: add support for SM8450")
> > Signed-off-by: Geert Uytterhoeven <geert+renesas@...der.be>
>
> Reviewed-by: Randy Dunlap <rdunlap@...radead.org>

Reviewed-by: Rob Clark <robdclark@...il.com>

>
> Thanks.
>
> > ---
> >  drivers/gpu/drm/msm/disp/dpu1/dpu_hw_catalog.h | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/msm/disp/dpu1/dpu_hw_catalog.h b/drivers/gpu/drm/msm/disp/dpu1/dpu_hw_catalog.h
> > index ddab9caebb18c40d..bbd3cbdd77956c5d 100644
> > --- a/drivers/gpu/drm/msm/disp/dpu1/dpu_hw_catalog.h
> > +++ b/drivers/gpu/drm/msm/disp/dpu1/dpu_hw_catalog.h
> > @@ -19,8 +19,9 @@
> >   */
> >  #define MAX_BLOCKS    12
> >
> > -#define DPU_HW_VER(MAJOR, MINOR, STEP) (((MAJOR & 0xF) << 28)    |\
> > -             ((MINOR & 0xFFF) << 16)  |\
> > +#define DPU_HW_VER(MAJOR, MINOR, STEP)                       \
> > +             ((((unsigned int)MAJOR & 0xF) << 28) |  \
> > +             ((MINOR & 0xFFF) << 16) |               \
> >               (STEP & 0xFFFF))
> >
> >  #define DPU_HW_MAJOR(rev)            ((rev) >> 28)
>
> --
> ~Randy