Re: [PATCH] drm/edid: Fix uninitialized variable in drm_cvt_modes()

[Lyude Paul]

Sorry! Thought I had responded to this but apparently not, comments down below

On Thu, 2020-10-22 at 14:04 -0400, Ilia Mirkin wrote:
> On Thu, Oct 22, 2020 at 12:55 PM Lyude Paul <lyude@xxxxxxxxxx> wrote:
> > 
> > Noticed this when trying to compile with -Wall on a kernel fork. We
> > potentially
> > don't set width here, which causes the compiler to complain about width
> > potentially being uninitialized in drm_cvt_modes(). So, let's fix that.
> > 
> > Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
> > 
> > Cc: <stable@xxxxxxxxxxxxxxx> # v5.9+
> > Fixes: 3f649ab728cd ("treewide: Remove uninitialized_var() usage")
> > Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
> > ---
> >  drivers/gpu/drm/drm_edid.c | 8 +++++++-
> >  1 file changed, 7 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
> > index 631125b46e04..2da158ffed8e 100644
> > --- a/drivers/gpu/drm/drm_edid.c
> > +++ b/drivers/gpu/drm/drm_edid.c
> > @@ -3094,6 +3094,7 @@ static int drm_cvt_modes(struct drm_connector
> > *connector,
> > 
> >         for (i = 0; i < 4; i++) {
> >                 int width, height;
> > +               u8 cvt_aspect_ratio;
> > 
> >                 cvt = &(timing->data.other_data.data.cvt[i]);
> > 
> > @@ -3101,7 +3102,8 @@ static int drm_cvt_modes(struct drm_connector
> > *connector,
> >                         continue;
> > 
> >                 height = (cvt->code[0] + ((cvt->code[1] & 0xf0) << 4) + 1) *
> > 2;
> > -               switch (cvt->code[1] & 0x0c) {
> > +               cvt_aspect_ratio = cvt->code[1] & 0x0c;
> > +               switch (cvt_aspect_ratio) {
> >                 case 0x00:
> >                         width = height * 4 / 3;
> >                         break;
> > @@ -3114,6 +3116,10 @@ static int drm_cvt_modes(struct drm_connector
> > *connector,
> >                 case 0x0c:
> >                         width = height * 15 / 9;
> >                         break;
> > +               default:
> 
> What value would cvt->code[1] have such that this gets hit?
> 
> Or is this a "compiler is broken, so let's add more code" situation?
> If so, perhaps the code added could just be enough to silence the
> compiler (unreachable, etc)?

I mean, this information comes from the EDID which inherently means it's coming
from an untrusted source so the value could be literally anything as long as the
EDID has a valid checksum. Note (assuming I'm understanding this code
correctly): 

drm_add_edid_modes() → add_cvt_modes() → drm_for_each_detailed_block() →
do_cvt_mode() → drm_cvt_modes()

So afaict this isn't a broken compiler but a legitimate uninitialized variable.
> 
>   -ilia
> 

-- 
Sincerely,
   Lyude Paul (she/her)
   Software Engineer at Red Hat
   
Note: I deal with a lot of emails and have a lot of bugs on my plate. If you've
asked me a question, are waiting for a review/merge on a patch, etc. and I
haven't responded in a while, please feel free to send me another email to check
on my status. I don't bite!