eCryptfs: Invalidate dcache entries when lower i_nlink is zero

commit 5556e7e6 upstream.

Consider eCryptfs dcache entries to be stale when the corresponding
lower inode's i_nlink count is zero. This solves a problem caused by the
lower inode being directly modified, without going through the eCryptfs
mount, leaving stale eCryptfs dentries cached and the eCryptfs inode's
i_nlink count not being cleared.
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Reported-by: Richard Weinberger <richard@nod.at>
[bwh: Backported to 3.2:
 - Test d_revalidate pointer directly rather than a DCACHE_OP flag
 - Open-code d_inode()
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

fs/ecryptfs/dentry.c

@@ -55,8 +55,7 @@ static int ecryptfs_d_revalidate(struct dentry *dentry, struct nameidata *nd)
 
 	lower_dentry = ecryptfs_dentry_to_lower(dentry);
 	lower_mnt = ecryptfs_dentry_to_lower_mnt(dentry);
-	if (!lower_dentry->d_op || !lower_dentry->d_op->d_revalidate)
-		goto out;
+	if (lower_dentry->d_op && lower_dentry->d_op->d_revalidate) {
 		if (nd) {
 			dentry_save = nd->path.dentry;
 			vfsmount_save = nd->path.mnt;
@@ -68,13 +67,14 @@ static int ecryptfs_d_revalidate(struct dentry *dentry, struct nameidata *nd)
 			nd->path.dentry = dentry_save;
 			nd->path.mnt = vfsmount_save;
 		}
+	}
 	if (dentry->d_inode) {
-		struct inode *lower_inode =
-			ecryptfs_inode_to_lower(dentry->d_inode);
+		struct inode *inode = dentry->d_inode;
 
-		fsstack_copy_attr_all(dentry->d_inode, lower_inode);
+		fsstack_copy_attr_all(inode, ecryptfs_inode_to_lower(inode));
+		if (!inode->i_nlink)
+			return 0;
 	}
-out:
 	return rc;
 }