ubifs, race between link and unlink/rename?

[hooanon05@yahoo.co.jp]

Hello,

Is there a race condition in ubifs?
Here is a scenario.

Process A		Process B
----------------------+---------------------------
create("dirA/fileA"); |
unlink("dirA/fileA"); |	link("dirA/fileA", "dirB/fileB");
		      | unlink("dirB/fileB");
----------------------+---------------------------

In link(2), dirA->i_mutex is not held. So unlink("dirA/fileA") can run
concurrently (after lookup).
While ubifs acquires ubifs_inode->ui_mutex, it doesn't check i_nlink.
When unlink("dirA/fileA") wins ui_mutex race and link() loses,
- ubifs_unlink("dirA/fileA") will call ubifs_jnl_update/ubifs_add_orphan()
- ubifs_link() will operate the inode with i_nlink == 0 (finally it will
  be 1)
- ubifs_unlink("dirB/fileB") will call ubifs_add_orphan() again for the
  same inode
- and it will produce "orphaned twice" error.
- (ubifs_add_orphan() is called by ubifs_rename/ubifs_jnl_rename() too)

If this scenario is possible, it may happen in every FS.
To check i_nlink in vfs_link (like this) may be one option, but it might
be better to check in ubifs since ubifs_add_orphan() is for i_nlink == 0
and ubifs specific.

diff --git a/fs/namei.c b/fs/namei.c
index b207821..820c386 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2409,9 +2409,12 @@ int vfs_link(struct dentry *old_dentry, struct inode *dir, struct dentry *new_de
 	if (error)
 		return error;
 
+	error = -ENOENT;
 	mutex_lock(&inode->i_mutex);
-	DQUOT_INIT(dir);
-	error = dir->i_op->link(old_dentry, dir, new_dentry);
+	if (inode->i_nlink) {
+		DQUOT_INIT(dir);
+		error = dir->i_op->link(old_dentry, dir, new_dentry);
+	}
 	mutex_unlock(&inode->i_mutex);
 	if (!error)
 		fsnotify_link(dir, inode, new_dentry);


J. R. Okajima