drm/vc4: Add a pad field to align drm_vc4_submit_cl to 64 bits.

I had originally asked Stefan Schake to drop the pad field from the
syncobj changes that just landed, because I couldn't come up with a
reason to align to 64 bits.

Talking with Dave Airlie about the new v3d driver's submit ioctl, we
came up with a reason: sizeof() on 64-bit platforms may align to 64
bits, in which case the userspace will be submitting the aligned size
and the final 32 bits won't be zero-padded by the kernel.  If
userspace doesn't zero-fill, then a future ABI change adding a 32-bit
field at the end could potentially cause the kernel to read undefined
data from old userspace (our userspace happens to use structure
initialization that zero-fills, but as a general rule we try not to
rely on that in the kernel).

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20180430235927.28712-1-eric@anholt.net
Reviewed-by: Stefan Schake <stschake@gmail.com>

drivers/gpu/drm/vc4/vc4_gem.c

@@ -1132,6 +1132,11 @@ vc4_submit_cl_ioctl(struct drm_device *dev, void *data,
 		return -EINVAL;
 	}
 
+	if (args->pad2 != 0) {
+		DRM_DEBUG("Invalid pad: 0x%08x\n", args->pad2);
+		return -EINVAL;
+	}
+
 	exec = kcalloc(1, sizeof(*exec), GFP_KERNEL);
 	if (!exec) {
 		DRM_ERROR("malloc failure on exec struct\n");

include/uapi/drm/vc4_drm.h

@@ -193,6 +193,8 @@ struct drm_vc4_submit_cl {
 	 * render job. 0 means ignore.
 	 */
 	__u32 out_sync;
+
+	__u32 pad2;
 };
 
 /**