linux-hardening - RE: [PATCH] drm/kmb: Avoid warnings on impossible plane

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Message-ID: <BY5PR11MB4182ABD607EB99C1F68777928CC69@BY5PR11MB4182.namprd11.prod.outlook.com>
Date:   Wed, 25 Aug 2021 20:44:29 +0000
From:   "Chrisanthus, Anitha" <anitha.chrisanthus@...el.com>
To:     Kees Cook <keescook@...omium.org>
CC:     "Dea, Edmund J" <edmund.j.dea@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        Sam Ravnborg <sam@...nborg.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>
Subject: RE: [PATCH] drm/kmb: Avoid warnings on impossible plane_id

Hi Kees,
Thanks for your patch. 
The switch statement is needed for multiple planes which is already approved 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@...omium.org>
> Sent: Wednesday, August 25, 2021 11:18 AM
> To: Chrisanthus, Anitha <anitha.chrisanthus@...el.com>
> Cc: Kees Cook <keescook@...omium.org>; Dea, Edmund J
> <edmund.j.dea@...el.com>; David Airlie <airlied@...ux.ie>; Daniel Vetter
> <daniel@...ll.ch>; dri-devel@...ts.freedesktop.org; Sam Ravnborg
> <sam@...nborg.org>; linux-kernel@...r.kernel.org; linux-
> hardening@...r.kernel.org
> Subject: [PATCH] drm/kmb: Avoid warnings on impossible plane_id
> 
> 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:
> 
> 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]
> 
> Instead, just remove the switch statement entirely and adjust the index
> type to match the struct "id" member.
> 
> Cc: Anitha Chrisanthus <anitha.chrisanthus@...el.com>
> Cc: Edmund Dea <edmund.j.dea@...el.com>
> Cc: David Airlie <airlied@...ux.ie>
> Cc: Daniel Vetter <daniel@...ll.ch>
> Cc: dri-devel@...ts.freedesktop.org
> Fixes: 7f7b96a8a0a1 ("drm/kmb: Add support for KeemBay Display")
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  drivers/gpu/drm/kmb/kmb_plane.c | 18 ++----------------
>  1 file changed, 2 insertions(+), 16 deletions(-)
> 
> 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 = to_kmb_plane(plane);
> -	int plane_id = kmb_plane->id;
> +	unsigned char plane_id = kmb_plane->id;
>  	struct kmb_drm_private *kmb;
> 
>  	kmb = to_kmb(plane->dev);
> @@ -121,21 +121,7 @@ static void kmb_plane_atomic_disable(struct
> drm_plane *plane,
>  	if (WARN_ON(plane_id >= KMB_MAX_PLANES))
>  		return;
> 
> -	switch (plane_id) {
> -	case LAYER_0:
> -		kmb->plane_status[plane_id].ctrl = LCD_CTRL_VL1_ENABLE;
> -		break;
> -	case LAYER_1:
> -		kmb->plane_status[plane_id].ctrl = LCD_CTRL_VL2_ENABLE;
> -		break;
> -	case LAYER_2:
> -		kmb->plane_status[plane_id].ctrl = LCD_CTRL_GL1_ENABLE;
> -		break;
> -	case LAYER_3:
> -		kmb->plane_status[plane_id].ctrl = LCD_CTRL_GL2_ENABLE;
> -		break;
> -	}
> -
> +	kmb->plane_status[plane_id].ctrl = LCD_CTRL_VL1_ENABLE;
>  	kmb->plane_status[plane_id].disable = true;
>  }
> 
> --
> 2.30.2