dcache: close d_move race in d_splice_alias

d_splice_alias will d_move an IS_ROOT() directory dentry into place if
one exists.  This should be safe as long as the dentry remains IS_ROOT,
but I can't see what guarantees that: once we drop the i_lock all we
hold here is the i_mutex on an unrelated parent directory.

Instead copy the logic of d_materialise_unique.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 6 insertions, 1 deletions

diff --git a/fs/dcache.c b/fs/dcache.c
index 8bdae36a095f..8c09db9bb2a4 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2676,9 +2676,14 @@ struct dentry *d_splice_alias(struct inode *inode, struct dentry *dentry)
                 new = __d_find_alias(inode, 1);
                 if (new) {
                         BUG_ON(!(new->d_flags & DCACHE_DISCONNECTED));
+                        write_seqlock(&rename_lock);
+                        __d_materialise_dentry(dentry, new);
+                        write_sequnlock(&rename_lock);
+                        __d_drop(new);
+                        _d_rehash(new);
+                        spin_unlock(&new->d_lock);
                         spin_unlock(&inode->i_lock);
                         security_d_instantiate(new, inode);
-                        d_move(new, dentry);
                         iput(inode);
                 } else {
                         /* already taking inode->i_lock, so d_add() by hand */