[3.16.y-ckt stable] Patch "mnt: Update fs_fully_visible to test for permanently empty directories" has been added to staging queue

[Luis Henriques]

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

    mnt: Update fs_fully_visible to test for permanently empty directories

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt15.

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

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From e7b1a5e38e5d5154fb040493a2535ccae0c0fcae Mon Sep 17 00:00:00 2001
From: "Eric W. Biederman" <ebiederm at xmission.com>
Date: Wed, 13 May 2015 20:51:09 -0500
Subject: mnt: Update fs_fully_visible to test for permanently empty
 directories

commit 7236c85e1be51a9e25ba0f6e087a66ca89605a49 upstream.

fs_fully_visible attempts to make fresh mounts of proc and sysfs give
the mounter no more access to proc and sysfs than if they could have
by creating a bind mount.  One aspect of proc and sysfs that makes
this particularly tricky is that there are other filesystems that
typically mount on top of proc and sysfs.  As those filesystems are
mounted on empty directories in practice it is safe to ignore them.
However testing to ensure filesystems are mounted on empty directories
has not been something the in kernel data structures have supported so
the current test for an empty directory which checks to see
if nlink <= 2 is a bit lacking.

proc and sysfs have recently been modified to use the new empty_dir
infrastructure to create all of their dedicated mount points.  Instead
of testing for S_ISDIR(inode->i_mode) && i_nlink <= 2 to see if a
directory is empty, test for is_empty_dir_inode(inode).  That small
change guaranteess mounts found on proc and sysfs really are safe to
ignore, because the directories are not only empty but nothing can
ever be added to them.  This guarantees there is nothing to worry
about when mounting proc and sysfs.

Signed-off-by: "Eric W. Biederman" <ebiederm at xmission.com>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 fs/namespace.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index efd13c26ad9c..b50d8daa7cb0 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3073,9 +3073,8 @@ static bool fs_fully_visible(struct file_system_type *type, int *new_mnt_flags)
 			/* Only worry about locked mounts */
 			if (!(mnt->mnt.mnt_flags & MNT_LOCKED))
 				continue;
-			if (!S_ISDIR(inode->i_mode))
-				goto next;
-			if (inode->i_nlink > 2)
+			/* Is the directory permanetly empty? */
+			if (!is_empty_dir_inode(inode))
 				goto next;
 		}
 		/* Preserve the locked attributes */