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From:   Daniel Vetter <daniel@ffwll.ch>
Date:   Tue, 3 Mar 2020 09:21:26 +0100
Message-ID: <CAKMK7uHRppv==G+Ep4S48dPMKZ9EwZGOt3WwWGXJiv+bXR-0SA@mail.gmail.com>
Subject: Re: [PATCH] drm/edid: Distribute switch variables for initialization
To:     Kees Cook <keescook@chromium.org>
Cc:     Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        David Airlie <airlied@linux.ie>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Alexander Potapenko <glider@google.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Tue, Mar 3, 2020 at 5:39 AM Kees Cook <keescook@chromium.org> wrote:
>
> On Wed, Feb 19, 2020 at 10:22:29PM -0800, Kees Cook wrote:
> > Variables declared in a switch statement before any case statements
> > cannot be automatically initialized with compiler instrumentation (as
> > they are not part of any execution flow). With GCC's proposed automatic
> > stack variable initialization feature, this triggers a warning (and the=
y
> > don't get initialized). Clang's automatic stack variable initialization
> > (via CONFIG_INIT_STACK_ALL=3Dy) doesn't throw a warning, but it also
> > doesn't initialize such variables[1]. Note that these warnings (or sile=
nt
> > skipping) happen before the dead-store elimination optimization phase,
> > so even when the automatic initializations are later elided in favor of
> > direct initializations, the warnings remain.
> >
> > To avoid these problems, move such variables into the "case" where
> > they're used or lift them up into the main function body.
> >
> > drivers/gpu/drm/drm_edid.c: In function =E2=80=98drm_edid_to_eld=E2=80=
=99:
> > drivers/gpu/drm/drm_edid.c:4395:9: warning: statement will never be exe=
cuted [-Wswitch-unreachable]
> >  4395 |     int sad_count;
> >       |         ^~~~~~~~~
> >
> > [1] https://bugs.llvm.org/show_bug.cgi?id=3D44916
> >
> > Signed-off-by: Kees Cook <keescook@chromium.org>
>
> Ping. Can someone pick this up, please?

Whatever the reasons, but your original patch didn't make it through
to dri-devel. Can you pls resubmit?

Thanks, Daniel

>
> Thanks!
>
> -Kees
>
> > ---
> >  drivers/gpu/drm/drm_edid.c |    5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
> > index 805fb004c8eb..2941b65b427f 100644
> > --- a/drivers/gpu/drm/drm_edid.c
> > +++ b/drivers/gpu/drm/drm_edid.c
> > @@ -4392,9 +4392,9 @@ static void drm_edid_to_eld(struct drm_connector =
*connector, struct edid *edid)
> >                       dbl =3D cea_db_payload_len(db);
> >
> >                       switch (cea_db_tag(db)) {
> > -                             int sad_count;
> > +                     case AUDIO_BLOCK: {
> >
> > -                     case AUDIO_BLOCK:
> > +                             int sad_count;
> >                               /* Audio Data Block, contains SADs */
> >                               sad_count =3D min(dbl / 3, 15 - total_sad=
_count);
> >                               if (sad_count >=3D 1)
> > @@ -4402,6 +4402,7 @@ static void drm_edid_to_eld(struct drm_connector =
*connector, struct edid *edid)
> >                                              &db[1], sad_count * 3);
> >                               total_sad_count +=3D sad_count;
> >                               break;
> > +                     }
> >                       case SPEAKER_BLOCK:
> >                               /* Speaker Allocation Data Block */
> >                               if (dbl >=3D 1)
> >
>
> --
> Kees Cook


--=20
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch