Btrfs: rework error handling in btrfs_mount()

Commits 6c41761f and 45ea6095 introduced the possibility of NULL pointer
dereference on error paths, also we would leave all devices busy and
leak fs_info with all sub-structures on error when trying to mount an
already mounted fs to a different directory.

Fix this by doing all allocations before trying to open any of the
devices, adjust error path for mount-already-mounted-fs case.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

fs/btrfs/super.c

@@ -891,7 +891,6 @@ static struct dentry *btrfs_mount(struct file_system_type *fs_type, int flags,
 	struct super_block *s;
 	struct dentry *root;
 	struct btrfs_fs_devices *fs_devices = NULL;
-	struct btrfs_root *tree_root = NULL;
 	struct btrfs_fs_info *fs_info = NULL;
 	fmode_t mode = FMODE_READ;
 	char *subvol_name = NULL;
@@ -920,15 +919,6 @@ static struct dentry *btrfs_mount(struct file_system_type *fs_type, int flags,
 	if (error)
 		return ERR_PTR(error);
 
-	error = btrfs_open_devices(fs_devices, mode, fs_type);
-	if (error)
-		return ERR_PTR(error);
-
-	if (!(flags & MS_RDONLY) && fs_devices->rw_devices == 0) {
-		error = -EACCES;
-		goto error_close_devices;
-	}
-
 	/*
 	 * Setup a dummy root and fs_info for test/set super.  This is because
 	 * we don't actually fill this stuff out until open_ctree, but we need
@@ -936,28 +926,36 @@ static struct dentry *btrfs_mount(struct file_system_type *fs_type, int flags,
 	 * then open_ctree will properly initialize everything later.
 	 */
 	fs_info = kzalloc(sizeof(struct btrfs_fs_info), GFP_NOFS);
-	if (!fs_info) {
-		error = -ENOMEM;
-		goto error_close_devices;
-	}
-	tree_root = kzalloc(sizeof(struct btrfs_root), GFP_NOFS);
-	if (!tree_root) {
+	if (!fs_info)
+		return ERR_PTR(-ENOMEM);
+
+	fs_info->tree_root = kzalloc(sizeof(struct btrfs_root), GFP_NOFS);
+	if (!fs_info->tree_root) {
 		error = -ENOMEM;
-		goto error_close_devices;
+		goto error_fs_info;
 	}
-	fs_info->tree_root = tree_root;
+	fs_info->tree_root->fs_info = fs_info;
 	fs_info->fs_devices = fs_devices;
-	tree_root->fs_info = fs_info;
 
 	fs_info->super_copy = kzalloc(BTRFS_SUPER_INFO_SIZE, GFP_NOFS);
 	fs_info->super_for_commit = kzalloc(BTRFS_SUPER_INFO_SIZE, GFP_NOFS);
 	if (!fs_info->super_copy || !fs_info->super_for_commit) {
 		error = -ENOMEM;
+		goto error_fs_info;
+	}
+
+	error = btrfs_open_devices(fs_devices, mode, fs_type);
+	if (error)
+		goto error_fs_info;
+
+	if (!(flags & MS_RDONLY) && fs_devices->rw_devices == 0) {
+		error = -EACCES;
 		goto error_close_devices;
 	}
 
 	bdev = fs_devices->latest_bdev;
-	s = sget(fs_type, btrfs_test_super, btrfs_set_super, tree_root);
+	s = sget(fs_type, btrfs_test_super, btrfs_set_super,
+		 fs_info->tree_root);
 	if (IS_ERR(s)) {
 		error = PTR_ERR(s);
 		goto error_close_devices;
@@ -966,7 +964,8 @@ static struct dentry *btrfs_mount(struct file_system_type *fs_type, int flags,
 	if (s->s_root) {
 		if ((flags ^ s->s_flags) & MS_RDONLY) {
 			deactivate_locked_super(s);
-			return ERR_PTR(-EBUSY);
+			error = -EBUSY;
+			goto error_close_devices;
 		}
 
 		btrfs_close_devices(fs_devices);
@@ -997,6 +996,7 @@ static struct dentry *btrfs_mount(struct file_system_type *fs_type, int flags,
 
 error_close_devices:
 	btrfs_close_devices(fs_devices);
+error_fs_info:
 	free_fs_info(fs_info);
 	return ERR_PTR(error);
 }