linux-kernel - Re: [PATCH v2] drm/edid: Distribute switch variables for initialization

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Message-ID: <CAKwvOd=POMeVJmu4pU78kLqZf=K-1s9kn7MagjGF8_BdztRTNA@mail.gmail.com>
Date:   Fri, 6 Mar 2020 09:37:20 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Daniel Vetter <daniel@...ll.ch>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        David Airlie <airlied@...ux.ie>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] drm/edid: Distribute switch variables for initialization

On Fri, Mar 6, 2020 at 9:36 AM Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>
> On Fri, Mar 6, 2020 at 9:32 AM Kees Cook <keescook@...omium.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=y) 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.

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 ‘drm_edid_to_eld’:
> > 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=44916
> >
> > Signed-off-by: Kees Cook <keescook@...omium.org>
>
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
>
> > ---
> > v2: move into function block instead being switch-local (Ville Syrjälä)
> > ---
> >  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) >= 3) {
> >                 int i, start, end;
> > +               int sad_count;
> >
> >                 if (cea_db_offsets(cea, &start, &end)) {
> >                         start = 0;
> > @@ -4392,8 +4393,6 @@ static void drm_edid_to_eld(struct drm_connector *connector, struct edid *edid)
> >                         dbl = cea_db_payload_len(db);
> >
> >                         switch (cea_db_tag(db)) {
> > -                               int sad_count;
> > -
> >                         case AUDIO_BLOCK:
> >                                 /* Audio Data Block, contains SADs */
> >                                 sad_count = min(dbl / 3, 15 - total_sad_count);
> > --
> > 2.20.1
> >
> >
> > --
> > Kees Cook
> >
> > --
> > You received this message because you are subscribed to the Google Groups "Clang Built Linux" group.
> > To unsubscribe from this group and stop receiving emails from it, send an email to clang-built-linux+unsubscribe@...glegroups.com.
> > To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/202003060930.DDCCB6659%40keescook.
>
>
>
> --
> Thanks,
> ~Nick Desaulniers



-- 
Thanks,
~Nick Desaulniers