Re: [PATCH] drm: Pad drm_mode_get_connector to 64-bit boundary

[Jani Nikula <jani.nikula@linux.intel.com>]

On Wed, 16 Oct 2013, Chris Wilson <chris@chris-wilson.co.uk> wrote:
> Pavel Roskin reported that DRM_IOCTL_MODE_GETCONNECTOR was overwritting
> the 4 bytes beyond the end of its structure with a 32-bit userspace
> running on a 64-bit kernel. This is due to the padding gcc inserts as
> the drm_mode_get_connector struct includes a u64 and its size is not a
> natural multiple of u64s.
>
> 64-bit kernel:
>
> sizeof(drm_mode_get_connector)=80, alignof=8
> sizeof(drm_mode_get_encoder)=20, alignof=4
> sizeof(drm_mode_modeinfo)=68, alignof=4
>
> 32-bit userspace:
>
> sizeof(drm_mode_get_connector)=76, alignof=4
> sizeof(drm_mode_get_encoder)=20, alignof=4
> sizeof(drm_mode_modeinfo)=68, alignof=4
>
> Fortuituously we can insert explicit padding to the tail of our
> structures without breaking ABI.
>
> Reported-by: Pavel Roskin <proski@gnu.org>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Dave Airlie <airlied@redhat.com>
> Cc: dri-devel@lists.freedesktop.org
> Cc: stable@vger.kernel.org
> ---
>  include/uapi/drm/drm_mode.h | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
> index 550811712f78..28acbaf4a81e 100644
> --- a/include/uapi/drm/drm_mode.h
> +++ b/include/uapi/drm/drm_mode.h
> @@ -223,6 +223,8 @@ struct drm_mode_get_connector {
>  	__u32 connection;
>  	__u32 mm_width, mm_height; /**< HxW in millimeters */
>  	__u32 subpixel;
> +
> +	__u32 pad;

Would a small comment here be in order?

Jani.

>  };
>  
>  #define DRM_MODE_PROP_PENDING	(1<<0)
> -- 
> 1.8.4.rc3
>
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-- 
Jani Nikula, Intel Open Source Technology Center