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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Fri, 6 Mar 2020 09:37:20 -0800
Message-ID: <CAKwvOd=POMeVJmu4pU78kLqZf=K-1s9kn7MagjGF8_BdztRTNA@mail.gmail.com>
Subject: Re: [PATCH v2] drm/edid: Distribute switch variables for initialization
To:     Kees Cook <keescook@chromium.org>
Cc:     Daniel Vetter <daniel@ffwll.ch>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        David Airlie <airlied@linux.ie>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Fri, Mar 6, 2020 at 9:36 AM Nick Desaulniers <ndesaulniers@google.com> w=
rote:
>
> On Fri, Mar 6, 2020 at 9:32 AM Kees Cook <keescook@chromium.org> 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
>
> That's not good, have you filed a bug against Clang yet?  It should at
> least warn when the corresponding stack init flag is set.

D'oh, link is below.

>
> > 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, lift such variables up into the next code
> > block.
> >
> > 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
> > executed [-Wswitch-unreachable]
> >  4395 |     int sad_count;
> >       |         ^~~~~~~~~
> >
> > [1] https://bugs.llvm.org/show_bug.cgi?id=3D44916
> >
> > Signed-off-by: Kees Cook <keescook@chromium.org>
>
> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
>
> > ---
> > v2: move into function block instead being switch-local (Ville Syrj=C3=
=A4l=C3=A4)
> > ---
> >  drivers/gpu/drm/drm_edid.c | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
> > index 805fb004c8eb..46cee78bc175 100644
> > --- a/drivers/gpu/drm/drm_edid.c
> > +++ b/drivers/gpu/drm/drm_edid.c
> > @@ -4381,6 +4381,7 @@ static void drm_edid_to_eld(struct drm_connector =
*connector, struct edid *edid)
> >
> >         if (cea_revision(cea) >=3D 3) {
> >                 int i, start, end;
> > +               int sad_count;
> >
> >                 if (cea_db_offsets(cea, &start, &end)) {
> >                         start =3D 0;
> > @@ -4392,8 +4393,6 @@ 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:
> >                                 /* Audio Data Block, contains SADs */
> >                                 sad_count =3D min(dbl / 3, 15 - total_s=
ad_count);
> > --
> > 2.20.1
> >
> >
> > --
> > Kees Cook
> >
> > --
> > You received this message because you are subscribed to the Google Grou=
ps "Clang Built Linux" group.
> > To unsubscribe from this group and stop receiving emails from it, send =
an email to clang-built-linux+unsubscribe@googlegroups.com.
> > To view this discussion on the web visit https://groups.google.com/d/ms=
gid/clang-built-linux/202003060930.DDCCB6659%40keescook.
>
>
>
> --
> Thanks,
> ~Nick Desaulniers


--=20
Thanks,
~Nick Desaulniers