Re: [PATCH] drm/edid: Distribute switch variables for initialization

[Ville Syrjälä]

On Mon, Mar 02, 2020 at 08:39:37PM -0800, Kees Cook 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 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
> > 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 âdrm_edid_to_eldâ:
> > 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@xxxxxxxxxxxx>
> 
> Ping. Can someone pick this up, please?
> 
> 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 = cea_db_payload_len(db);
> >  
> >  			switch (cea_db_tag(db)) {
> > -				int sad_count;
> > +			case AUDIO_BLOCK: {

I've never been a fan of {} inside switch statements. I'd just
move this one level up.

> >  
> > -			case AUDIO_BLOCK:
> > +				int sad_count;
> >  				/* Audio Data Block, contains SADs */
> >  				sad_count = min(dbl / 3, 15 - total_sad_count);
> >  				if (sad_count >= 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 += sad_count;
> >  				break;
> > +			}
> >  			case SPEAKER_BLOCK:
> >  				/* Speaker Allocation Data Block */
> >  				if (dbl >= 1)
> > 
> 
> -- 
> Kees Cook
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-- 
Ville SyrjÃlÃ
Intel