Bug 788221

Description of problem:

Mounting a filesystem in a child mount namespace created with "unshare -m" results in a mount that is visible in the parent.

Version-Release number of selected component (if applicable):

# rpm -q util-linux
util-linux-2.20.1-2.2.fc16.x86_64
# rpm -q kernel
kernel-3.2.3-2.fc16.x86_64

How reproducible:

1. unshare -m /bin/bash
2. (in the child) mount -t tmpfs tmpfs /mnt
3. (in the parent) ls /mnt
  
Actual results:

The contents of /mnt in the parent are masked by the mount in the child namespace.

Expected results:

The mount in the child namespace will not affect the parent namespace.

Additional info:

I have two systems running identical kernels and identical userspace tools.  One system experiences this bug as described, the other works as expected.  

ON THE WORKING SYSTEM
=====================

In the parent namespace:

  # grep /mnt /proc/self/mountinfo
  # ls /mnt
  file1 file2 file3
  # unshare -m /bin/bash

In the child namespace:

  # mount -t tmpfs tmpfs /mnt
  # ls /mnt
  # grep /mnt /proc/self/mountinfo
  72 44 0:35 / /mnt rw,relatime - tmpfs tmpfs rw

In the parent namespace:

  # ls /mnt
  file1 file2 file3
  # grep /mnt /proc/self/mountinfo
  #

ON THE PROBLEM SYSTEM
=====================

In the parent namespace:

  # grep /mnt /proc/self/mountinfo
  # ls /mnt
  file1 file2 file3
  # unshare -m /bin/bash

In the child namespace:

  # mount -t tmpfs tmpfs /mnt
  # ls /mnt
  # grep /mnt /proc/self/mountinfo
  83 51 0:40 / /mnt rw,relatime shared:4 - tmpfs tmpfs rw

In the parent namespace:

  # ls /mnt
  # grep /mnt /proc/self/mountinfo
  84 22 0:40 / /mnt rw,relatime shared:4 - tmpfs tmpfs rw
  #

On the problem system I can make things behave as expected by running, before "unshare":

  # mount -o bind /mnt /mnt
  # mount --make-private /mnt

But it's not clear why this is necessary on one system but not the other.