Mount volumes rprivate for archival and other use cases

People have reported following problem.

- docker run -ti --name=foo -v /dev/:/dev/ fedora bash
- docker cp foo:/bin/bash /tmp

Once the cp operation is complete, it unmounted /dev/pts on the host. /dev/pts
is a submount of /dev/. This is completely unexpected. Following is the
reson for this behavior.

containerArchivePath() call mountVolumes() which goes through all the mounts
points of a container and mounts them in daemon mount namespace in
/var/lib/docker/devicemapper/mnt/<containerid>/rootfs dir. And once we have
extracted the data required, these are unmounted using UnmountVolumes().

Mounts are done using recursive bind (rbind). And these are unmounted using
lazy mount option on top level mount. (detachMounted()). That means if there
are submounts under top level mounts, these mount events will propagate and
they were "shared" mounts with host, it will unmount the submount on host
as well.

For example, try following.

- Prepare a parent and child mount point.
  $ mkdir /root/foo
  $ mount --bind /root/foo /root/foo 
  $ mount --make-rshared /root/foo
  
- Prepare a child mount 

  $ mkdir /root/foo/foo1
  $ mount --bind /root/foo/foo1 /root/foo/foo1
 
- Bind mount foo at bar

  $ mkdir /root/bar
  $ mount --rbind /root/foo /root/bar
  
- Now lazy unmount /root/bar and it will unmount /root/foo/foo1 as well.

  $ umount -l /root/bar

This is not unintended. We just wanted to unmount /root/bar and anything
underneath but did not have intentions of unmounting anything on source.

So far this was not a problem as docker daemon was running in a seprate
mount namespace where all propagation was "slave". That means any unmounts
in docker daemon namespace did not propagate to host namespace. 

But now we are running docker daemon in host namespace so that it is possible
to mount some volumes "shared" with container. So that if container mounts
something it propagates to host namespace as well. 

Given mountVolumes() seems to be doing only temporary mounts to read some
data, there does not seem to be a need to mount these shared/slave. Just
mount these private so that on unmount, nothing propagates and does not
have unintended consequences. 

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>

daemon/container_operations_unix.go

@@ -246,6 +246,19 @@ func (daemon *Daemon) mountVolumes(container *container.Container) error {
 		if err := mount.Mount(m.Source, dest, "bind", opts); err != nil {
 			return err
 		}
+
+		// mountVolumes() seems to be called for temporary mounts
+		// outside the container. Soon these will be unmounted with
+		// lazy unmount option and given we have mounted the rbind,
+		// all the submounts will propagate if these are shared. If
+		// daemon is running in host namespace and has / as shared
+		// then these unmounts will propagate and unmount original
+		// mount as well. So make all these mounts rprivate.
+		// Do not use propagation property of volume as that should
+		// apply only when mounting happen inside the container.
+		if err := mount.MakeRPrivate(dest); err != nil {
+			return err
+		}
 	}
 
 	return nil