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>
[ luis: backported to 3.16:
  - use dentry->d_inode instead of d_inode(dentry)
  - adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

fs/ecryptfs/dentry.c

@@ -45,20 +45,20 @@
 static int ecryptfs_d_revalidate(struct dentry *dentry, unsigned int flags)
 {
 	struct dentry *lower_dentry = ecryptfs_dentry_to_lower(dentry);
-	int rc;
-
-	if (!(lower_dentry->d_flags & DCACHE_OP_REVALIDATE))
-		return 1;
+	int rc = 1;
 
 	if (flags & LOOKUP_RCU)
 		return -ECHILD;
 
+	if (lower_dentry->d_flags & DCACHE_OP_REVALIDATE)
 		rc = lower_dentry->d_op->d_revalidate(lower_dentry, flags);
+
 	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;
 	}
 	return rc;
 }