linux-kernel - Re: [PATCH] drm/edid: Fix uninitialized variable in drm_cvt

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Date:   Thu, 22 Oct 2020 14:04:45 -0400
From:   Ilia Mirkin <imirkin@...m.mit.edu>
To:     Lyude Paul <lyude@...hat.com>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        Leon Romanovsky <leon@...nel.org>,
        David Airlie <airlied@...ux.ie>, Chao Yu <chao@...nel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        "# 3.9+" <stable@...r.kernel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Kalle Valo <kvalo@...eaurora.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] drm/edid: Fix uninitialized variable in drm_cvt_modes()

On Thu, Oct 22, 2020 at 12:55 PM Lyude Paul <lyude@...hat.com> 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@...hat.com>
>
> Cc: <stable@...r.kernel.org> # v5.9+
> Fixes: 3f649ab728cd ("treewide: Remove uninitialized_var() usage")
> Signed-off-by: Lyude Paul <lyude@...hat.com>
> ---
>  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)?

  -ilia