FROMLIST: fuse: 32-bit user space ioctl compat for fuse device

With a 64-bit kernel build the FUSE device cannot handle ioctl requests
coming from 32-bit user space.
This is due to the ioctl command translation that generates different
command identifiers that thus cannot be used for direct comparisons
without proper manipulation.

Explicitly extract type and number from the ioctl command to enable
32-bit user space compatibility on 64-bit kernel builds.

Bug: 179164095
Link: https://lore.kernel.org/lkml/20210125153057.3623715-3-balsini@android.com/
Signed-off-by: Alessio Balsini <balsini@android.com>
Change-Id: I595517c54d551be70e83c7fcb4b62397a3615004
Signed-off-by: Alessio Balsini <balsini@google.com>

fs/fuse/dev.c

@@ -2259,37 +2259,44 @@ static int fuse_device_clone(struct fuse_conn *fc, struct file *new)
 static long fuse_dev_ioctl(struct file *file, unsigned int cmd,
 			   unsigned long arg)
 {
-	int err = -ENOTTY;
+	int res;
+	int oldfd;
+	struct fuse_dev *fud = NULL;
 
-	if (cmd == FUSE_DEV_IOC_CLONE) {
-		int oldfd;
+	if (_IOC_TYPE(cmd) != FUSE_DEV_IOC_MAGIC)
+		return -EINVAL;
 
-		err = -EFAULT;
-		if (!get_user(oldfd, (__u32 __user *) arg)) {
+	switch (_IOC_NR(cmd)) {
+	case _IOC_NR(FUSE_DEV_IOC_CLONE):
+		res = -EFAULT;
+		if (!get_user(oldfd, (__u32 __user *)arg)) {
 			struct file *old = fget(oldfd);
 
-			err = -EINVAL;
+			res = -EINVAL;
 			if (old) {
-				struct fuse_dev *fud = NULL;
-
 				/*
 				 * Check against file->f_op because CUSE
 				 * uses the same ioctl handler.
 				 */
 				if (old->f_op == file->f_op &&
-				    old->f_cred->user_ns == file->f_cred->user_ns)
+				    old->f_cred->user_ns ==
+					    file->f_cred->user_ns)
 					fud = fuse_get_dev(old);
 
 				if (fud) {
 					mutex_lock(&fuse_mutex);
-					err = fuse_device_clone(fud->fc, file);
+					res = fuse_device_clone(fud->fc, file);
 					mutex_unlock(&fuse_mutex);
 				}
 				fput(old);
 			}
 		}
+		break;
+	default:
+		res = -ENOTTY;
+		break;
 	}
-	return err;
+	return res;
 }
 
 const struct file_operations fuse_dev_operations = {

include/uapi/linux/fuse.h

@@ -777,7 +777,8 @@ struct fuse_notify_retrieve_in {
 };
 
 /* Device ioctls: */
-#define FUSE_DEV_IOC_CLONE	_IOR(229, 0, uint32_t)
+#define FUSE_DEV_IOC_MAGIC		229
+#define FUSE_DEV_IOC_CLONE		_IOR(FUSE_DEV_IOC_MAGIC, 0, uint32_t)
 
 struct fuse_lseek_in {
 	uint64_t	fh;