dcache: d_splice_alias mustn't create directory aliases

Currently if d_splice_alias finds a directory with an alias that is not
IS_ROOT or not DCACHE_DISCONNECTED, it creates a duplicate directory.

Duplicate directory dentries are unacceptable; it is better just to
error out.

(In the case of a local filesystem the most likely case is filesystem
corruption: for example, perhaps two directories point to the same child
directory, and the other parent has already been found and cached.)

Note that distributed filesystems may encounter this case in normal
operation if a remote host moves a directory to a location different
from the one we last cached in the dcache.  For that reason, such
filesystems should instead use d_materialise_unique, which tries to move
the old directory alias to the right place instead of erroring out.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/dcache.c

@@ -2653,6 +2653,9 @@ static void __d_materialise_dentry(struct dentry *dentry, struct dentry *anon)
  * DCACHE_DISCONNECTED), then d_move that in place of the given dentry
  * and return it, else simply d_add the inode to the dentry and return NULL.
  *
+ * If a non-IS_ROOT directory is found, the filesystem is corrupt, and
+ * we should error out: directories can't have multiple aliases.
+ *
  * This is needed in the lookup routine of any filesystem that is exportable
  * (via knfsd) so that we can build dcache paths to directories effectively.
  *
@@ -2673,9 +2676,13 @@ struct dentry *d_splice_alias(struct inode *inode, struct dentry *dentry)
 
 	if (inode && S_ISDIR(inode->i_mode)) {
 		spin_lock(&inode->i_lock);
-		new = __d_find_alias(inode, 1);
+		new = __d_find_any_alias(inode);
 		if (new) {
-			BUG_ON(!(new->d_flags & DCACHE_DISCONNECTED));
+			if (!IS_ROOT(new) || !(new->d_flags & DCACHE_DISCONNECTED)) {
+				spin_unlock(&inode->i_lock);
+				dput(new);
+				return ERR_PTR(-EIO);
+			}
 			write_seqlock(&rename_lock);
 			__d_materialise_dentry(dentry, new);
 			write_sequnlock(&rename_lock);