ceph: avoid accessing / when mounting a subpath

Accessing / causes failuire if the client has caps that restrict path
Signed-off-by: Yan, Zheng <zyan@redhat.com>

fs/ceph/super.c

@@ -396,11 +396,13 @@ static int parse_mount_options(struct ceph_mount_options **pfsopt,
 	 */
 	dev_name_end = strchr(dev_name, '/');
 	if (dev_name_end) {
+		if (strlen(dev_name_end) > 1) {
 			fsopt->server_path = kstrdup(dev_name_end, GFP_KERNEL);
 			if (!fsopt->server_path) {
 				err = -ENOMEM;
 				goto out;
 			}
+		}
 	} else {
 		dev_name_end = dev_name + strlen(dev_name);
 	}
@@ -788,16 +790,11 @@ static struct dentry *open_root_dentry(struct ceph_fs_client *fsc,
 		struct inode *inode = req->r_target_inode;
 		req->r_target_inode = NULL;
 		dout("open_root_inode success\n");
-		if (ceph_ino(inode) == CEPH_INO_ROOT &&
-		    fsc->sb->s_root == NULL) {
 		root = d_make_root(inode);
 		if (!root) {
 			root = ERR_PTR(-ENOMEM);
 			goto out;
 		}
-		} else {
-			root = d_obtain_root(inode);
-		}
 		ceph_init_dentry(root);
 		dout("open_root_inode success, root dentry is %p\n", root);
 	} else {
@@ -825,17 +822,24 @@ static struct dentry *ceph_real_mount(struct ceph_fs_client *fsc)
 	mutex_lock(&fsc->client->mount_mutex);
 
 	if (!fsc->sb->s_root) {
+		const char *path;
 		err = __ceph_open_session(fsc->client, started);
 		if (err < 0)
 			goto out;
 
-		dout("mount opening root\n");
-		root = open_root_dentry(fsc, "", started);
+		if (!fsc->mount_options->server_path) {
+			path = "";
+			dout("mount opening path \\t\n");
+		} else {
+			path = fsc->mount_options->server_path + 1;
+			dout("mount opening path %s\n", path);
+		}
+		root = open_root_dentry(fsc, path, started);
 		if (IS_ERR(root)) {
 			err = PTR_ERR(root);
 			goto out;
 		}
-		fsc->sb->s_root = root;
+		fsc->sb->s_root = dget(root);
 		first = 1;
 
 		err = ceph_fs_debugfs_init(fsc);
@@ -843,19 +847,6 @@ static struct dentry *ceph_real_mount(struct ceph_fs_client *fsc)
 			goto fail;
 	}
 
-	if (!fsc->mount_options->server_path) {
-		root = fsc->sb->s_root;
-		dget(root);
-	} else {
-		const char *path = fsc->mount_options->server_path + 1;
-		dout("mount opening path %s\n", path);
-		root = open_root_dentry(fsc, path, started);
-		if (IS_ERR(root)) {
-			err = PTR_ERR(root);
-			goto fail;
-		}
-	}
-
 	fsc->mount_state = CEPH_MOUNT_MOUNTED;
 	dout("mount success\n");
 	mutex_unlock(&fsc->client->mount_mutex);