HID: bpf: rescan the device for the group after a load/unload

When a BPF gets loaded, it was previously not possible to bind a
hid-generic device to hid-multitouch because the group was never
updated.

This change forces a rescan of the report descriptor after a bpf is
loaded/unloaded so we set up the proper group.

This was detected while Peter was trying to fix a Viewsonic device: the
HID device sending multiotuch data through a proprietary collection was
handled by hid-generic, and we don't have any way of attaching it to
hid-multitouch because the pre-scanning wasn't able to see the Contact
ID HID usage.

Suggested-by: Peter Hutterer <[e-mail]>
Reviewed-by: Peter Hutterer <[e-mail]>
Signed-off-by: Benjamin Tissoires <[e-mail]>
Signed-off-by: Jiri Kosina <[e-mail]>

drivers/hid/hid-core.c

···
 	hid->group = HID_GROUP_GENERIC;
 
 	/*
+	 * In case we are re-scanning after a BPF has been loaded,
+	 * we need to use the bpf report descriptor, not the original one.
+	 */
+	if (hid->bpf_rdesc && hid->bpf_rsize) {
+		start = hid->bpf_rdesc;
+		end = start + hid->bpf_rsize;
+	}
+
+	/*
 	 * The parsing is simpler than the one in hid_open_report() as we should
 	 * be robust against hid errors. Those errors will be raised by
 	 * hid_open_report() anyway.
···
 	int ret;
 
 	if (!hdev->bpf_rsize) {
+		/* we keep a reference to the currently scanned report descriptor */
+		const __u8  *original_rdesc = hdev->bpf_rdesc;
+
+		if (!original_rdesc)
+			original_rdesc = hdev->dev_rdesc;
+
 		/* in case a bpf program gets detached, we need to free the old one */
 		hid_free_bpf_rdesc(hdev);
 
···
 		/* call_hid_bpf_rdesc_fixup will always return a valid pointer */
 		hdev->bpf_rdesc = call_hid_bpf_rdesc_fixup(hdev, hdev->dev_rdesc,
 							   &hdev->bpf_rsize);
+
+		/* the report descriptor changed, we need to re-scan it */
+		if (original_rdesc != hdev->bpf_rdesc) {
+			hdev->group = 0;
+			hid_set_group(hdev);
+		}
 	}
 
 	if (!hid_check_device_match(hdev, hdrv, &id))