fuse: allow O_PATH fd for FUSE_DEV_IOC_BACKING_OPEN

Only f_path is used from backing files registered with
FUSE_DEV_IOC_BACKING_OPEN, so it makes sense to allow O_PATH descriptors.

O_PATH files have an empty f_op, so don't check read_iter/write_iter.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

Author: Miklos Szeredi

fs/fuse/passthrough.c

@@ -228,16 +228,13 @@ int fuse_backing_open(struct fuse_conn *fc, struct fuse_backing_map *map)
 	if (map->flags || map->padding)
 		goto out;
 
-	file = fget(map->fd);
+	file = fget_raw(map->fd);
 	res = -EBADF;
 	if (!file)
 		goto out;
 
-	res = -EOPNOTSUPP;
-	if (!file->f_op->read_iter || !file->f_op->write_iter)
-		goto out_fput;
-
 	backing_sb = file_inode(file)->i_sb;
+	pr_info("%s: %x:%pD %i\n", __func__, backing_sb->s_dev, file, backing_sb->s_stack_depth);
 	res = -ELOOP;
 	if (backing_sb->s_stack_depth >= fc->max_stack_depth)
 		goto out_fput;