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From:   "Chrisanthus, Anitha" <anitha.chrisanthus@intel.com>
To:     Kees Cook <keescook@chromium.org>
CC:     "Dea, Edmund J" <edmund.j.dea@intel.com>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        "dri-devel@lists.freedesktop.org" <dri-devel@lists.freedesktop.org>,
        Sam Ravnborg <sam@ravnborg.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-hardening@vger.kernel.org" <linux-hardening@vger.kernel.org>
Subject: RE: [PATCH] drm/kmb: Avoid warnings on impossible plane_id
Thread-Topic: [PATCH] drm/kmb: Avoid warnings on impossible plane_id
Thread-Index: AQHXmd2XU9vEPkR5QkaaPDsb+p2SVauEr1kw
Date:   Wed, 25 Aug 2021 20:44:29 +0000
Message-ID: <BY5PR11MB4182ABD607EB99C1F68777928CC69@BY5PR11MB4182.namprd11.prod.outlook.com>
References: <20210825181807.1138053-1-keescook@chromium.org>
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Hi Kees,
Thanks for your patch.=20
The switch statement is needed for multiple planes which is already approve=
d in this patch series.
https://patchwork.kernel.org/project/dri-devel/patch/20210728003126.1425028=
-13-anitha.chrisanthus@intel.com/

This patch has dependencies on the previous patches in this series, and we =
are waiting for reviews on those before this can be checked in.

Thanks,
Anitha

> -----Original Message-----
> From: Kees Cook <keescook@chromium.org>
> Sent: Wednesday, August 25, 2021 11:18 AM
> To: Chrisanthus, Anitha <anitha.chrisanthus@intel.com>
> Cc: Kees Cook <keescook@chromium.org>; Dea, Edmund J
> <edmund.j.dea@intel.com>; David Airlie <airlied@linux.ie>; Daniel Vetter
> <daniel@ffwll.ch>; dri-devel@lists.freedesktop.org; Sam Ravnborg
> <sam@ravnborg.org>; linux-kernel@vger.kernel.org; linux-
> hardening@vger.kernel.org
> Subject: [PATCH] drm/kmb: Avoid warnings on impossible plane_id
>=20
> KMB_MAX_PLANES is defined as 1, yet kmb_plane_atomic_disable() had code
> for writing beyond 1. It is gated by a WARN_ON() that would skip it,
> though, but under some compiler versions, poor Dead Code Elimination
> wasn't optimizing away the unused switch cases, leading to array bounds
> warnings when building with -Warray-bounds:
>=20
> drivers/gpu/drm/kmb/kmb_plane.c:135:20: warning: array subscript 3 is
> above array bounds of 'struct layer_status[1]' [-Warray-bounds]
> drivers/gpu/drm/kmb/kmb_plane.c:132:20: warning: array subscript 2 is
> above array bounds of 'struct layer_status[1]' [-Warray-bounds]
> drivers/gpu/drm/kmb/kmb_plane.c:129:20: warning: array subscript 1 is
> above array bounds of 'struct layer_status[1]' [-Warray-bounds]
>=20
> Instead, just remove the switch statement entirely and adjust the index
> type to match the struct "id" member.
>=20
> Cc: Anitha Chrisanthus <anitha.chrisanthus@intel.com>
> Cc: Edmund Dea <edmund.j.dea@intel.com>
> Cc: David Airlie <airlied@linux.ie>
> Cc: Daniel Vetter <daniel@ffwll.ch>
> Cc: dri-devel@lists.freedesktop.org
> Fixes: 7f7b96a8a0a1 ("drm/kmb: Add support for KeemBay Display")
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  drivers/gpu/drm/kmb/kmb_plane.c | 18 ++----------------
>  1 file changed, 2 insertions(+), 16 deletions(-)
>=20
> diff --git a/drivers/gpu/drm/kmb/kmb_plane.c
> b/drivers/gpu/drm/kmb/kmb_plane.c
> index ecee6782612d..3d46e756f2fe 100644
> --- a/drivers/gpu/drm/kmb/kmb_plane.c
> +++ b/drivers/gpu/drm/kmb/kmb_plane.c
> @@ -113,7 +113,7 @@ static void kmb_plane_atomic_disable(struct
> drm_plane *plane,
>  				     struct drm_atomic_state *state)
>  {
>  	struct kmb_plane *kmb_plane =3D to_kmb_plane(plane);
> -	int plane_id =3D kmb_plane->id;
> +	unsigned char plane_id =3D kmb_plane->id;
>  	struct kmb_drm_private *kmb;
>=20
>  	kmb =3D to_kmb(plane->dev);
> @@ -121,21 +121,7 @@ static void kmb_plane_atomic_disable(struct
> drm_plane *plane,
>  	if (WARN_ON(plane_id >=3D KMB_MAX_PLANES))
>  		return;
>=20
> -	switch (plane_id) {
> -	case LAYER_0:
> -		kmb->plane_status[plane_id].ctrl =3D LCD_CTRL_VL1_ENABLE;
> -		break;
> -	case LAYER_1:
> -		kmb->plane_status[plane_id].ctrl =3D LCD_CTRL_VL2_ENABLE;
> -		break;
> -	case LAYER_2:
> -		kmb->plane_status[plane_id].ctrl =3D LCD_CTRL_GL1_ENABLE;
> -		break;
> -	case LAYER_3:
> -		kmb->plane_status[plane_id].ctrl =3D LCD_CTRL_GL2_ENABLE;
> -		break;
> -	}
> -
> +	kmb->plane_status[plane_id].ctrl =3D LCD_CTRL_VL1_ENABLE;
>  	kmb->plane_status[plane_id].disable =3D true;
>  }
>=20
> --
> 2.30.2