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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Fri, 6 Mar 2020 09:36:30 -0800
Message-ID: <CAKwvOdn4q4OWzvhAMHFf441DNrmO00ye_H_MnoegP7jw3YAWqA@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 9:32 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 they
> 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 silent

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.

> 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=A4=
l=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 *c=
onnector, 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 *c=
onnector, 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_sad=
_count);
> --
> 2.20.1
>
>
> --
> Kees Cook
>
> --
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--=20
Thanks,
~Nick Desaulniers