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

[Lyude Paul]

On Tue, 2020-11-03 at 14:53 -0500, Ilia Mirkin wrote:
> On Tue, Nov 3, 2020 at 2:47 PM Lyude Paul <lyude@xxxxxxxxxx> wrote:
> > 
> > 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.
> 
> The value can be anything, but it has to be something. The switch is
> on "unknown & 0x0c", so only 4 cases are possible, which are
> enumerated in the switch.

oops, you're completely right lol. will figure out what the unreachable macro in
the kernel is and use that in a respin of this patch

> 
>   -ilia
> 

-- 
Sincerely,
   Lyude Paul (she/her)
   Software Engineer at Red Hat
   
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