[PATCH] autofs4: check for directory re-create in lookup

This problem was identified and fixed some time ago by Jeff Moyer but it fell
through the cracks somehow.

It is possible that a user space application could remove and re-create a
directory during a request.  To avoid returning a failure from lookup
incorrectly when our current dentry is unhashed we need to check if another
positive, hashed dentry matching this one exists and if so return it instead
of a fail.

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Ian Kent <raven@themaw.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/autofs4/root.c

@@ -655,14 +655,29 @@ static struct dentry *autofs4_lookup(struct inode *dir, struct dentry *dentry, s
 
 	/*
 	 * If this dentry is unhashed, then we shouldn't honour this
-	 * lookup even if the dentry is positive.  Returning ENOENT here
-	 * doesn't do the right thing for all system calls, but it should
-	 * be OK for the operations we permit from an autofs.
+	 * lookup.  Returning ENOENT here doesn't do the right thing
+	 * for all system calls, but it should be OK for the operations
+	 * we permit from an autofs.
 	 */
 	if (dentry->d_inode && d_unhashed(dentry)) {
+		/*
+		 * A user space application can (and has done in the past)
+		 * remove and re-create this directory during the callback.
+		 * This can leave us with an unhashed dentry, but a
+		 * successful mount!  So we need to perform another
+		 * cached lookup in case the dentry now exists.
+		 */
+		struct dentry *parent = dentry->d_parent;
+		struct dentry *new = d_lookup(parent, &dentry->d_name);
+		if (new != NULL)
+			dentry = new;
+		else
+			dentry = ERR_PTR(-ENOENT);
+
 		if (unhashed)
 			dput(unhashed);
-		return ERR_PTR(-ENOENT);
+
+		return dentry;
 	}
 
 	if (unhashed)