[4.2.y-ckt stable] Patch "debugfs: Make automount point inodes permanently empty" has been added to the 4.2.y-ckt tree

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    debugfs: Make automount point inodes permanently empty

to the linux-4.2.y-queue branch of the 4.2.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-4.2.y-queue

This patch is scheduled to be released in version 4.2.8-ckt9.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 4.2.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

---8<------------------------------------------------------------

>From 8acf947921209bef0f2cb5980d63ba6003302a50 Mon Sep 17 00:00:00 2001
From: Seth Forshee <seth.forshee at canonical.com>
Date: Wed, 9 Mar 2016 09:18:07 -0600
Subject: debugfs: Make automount point inodes permanently empty

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.

Signed-off-by: Seth Forshee <seth.forshee at canonical.com>
Acked-by: Serge Hallyn <serge.hallyn at canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 fs/debugfs/inode.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
index 9c8d233..6d0ead7 100644
--- a/fs/debugfs/inode.c
+++ b/fs/debugfs/inode.c
@@ -457,7 +457,7 @@ struct dentry *debugfs_create_automount(const char *name,
 	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;
--
2.7.4