Patch "debugfs: Make automount point inodes permanently empty" has been added to the 4.5-stable tree

This is a note to let you know that I've just added the patch titled

debugfs: Make automount point inodes permanently empty

to the 4.5-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
debugfs-make-automount-point-inodes-permanently-empty.patch
and it can be found in the queue-4.5 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Seth Forshee <[email protected]>
Date: Wed, 9 Mar 2016 09:18:07 -0600
Subject: debugfs: Make automount point inodes permanently empty

From: Seth Forshee <[email protected]>

commit 87243deb88671f70def4c52dfa7ca7830707bd31 upstream.

Starting with 4.1 the tracing subsystem has its own filesystem which is automounted in the tracing subdirectory of debugfs. Prior to this debugfs could be bind mounted in a cloned mount namespace, but if tracefs has been mounted under debugfs this now fails because there is a locked child mount. This creates a regression for container software which bind mounts debugfs to satisfy the assumption of some userspace software.

In other pseudo filesystems such as proc and sysfs we're already creating mountpoints like this in such a way that no dirents can be created in the directories, allowing them to be exceptions to some MNT_LOCKED tests. In fact we're already do this for the tracefs mountpoint in sysfs.

Do the same in debugfs_create_automount(), since the intention here is clearly to create a mountpoint. This fixes the regression, as locked child mounts on permanently empty directories do not cause a bind mount to fail.

---
 fs/debugfs/inode.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

@@ -457,7 +457,7 @@ struct dentry *debugfs_create_automount(
 	if (unlikely(!inode))
 		return failed_creating(dentry);
 
-	inode->i_mode = S_IFDIR | S_IRWXU | S_IRUGO | S_IXUGO;
+	make_empty_dir_inode(inode);
 	inode->i_flags |= S_AUTOMOUNT;
 	inode->i_private = data;
 	dentry->d_fsdata = (void *)f;

Patches currently in stable-queue which might be from [email protected] are

queue-4.5/debugfs-make-automount-point-inodes-permanently-empty.patch