Bug 745782

Description of problem:
One of the first things that is required when setting up a container with a
private root filesystem, is to unmount all filesystems inherited from the host
OS.

For most filesystems this works without trouble, but for autofs this is not the
case.

For demonstration, consider an autofs map with one direct mount and one
indirect mount:

# cat /etc/auto.master 
/net -hosts
/-        /etc/auto.marrow

# cat /etc/auto.marrow 
/mnt/demo marrow.example.com:/var/lib/libvirt/images

When autofs initially starts these mount points are visible

# grep -E ' (/net|/mnt)' /proc/mounts 
-hosts /net autofs
rw,relatime,fd=6,pgrp=2938,timeout=300,minproto=5,maxproto=5,indirect 0 0
/etc/auto.marrow /mnt/demo autofs
rw,relatime,fd=12,pgrp=2938,timeout=300,minproto=5,maxproto=5,direct 0 0


The attached demo program creates a container and attempts to unmount the
requested filesystem inside the container.


With a direct mount, which has not yet been triggered it fails:

# ./autofsdemo /mnt/demo
We are the parent
We are the container!
Found mount point 1 /mnt/demo auto.marrow autofs rw,relatime,fd=13,pgrp=1363,timeout=300,minproto=5,maxproto=5,direct
Umount point 1 /mnt/demo
Could not umount /mnt/demo: Operation not permitted


If I then trigger the mount 

# ls /mnt/demo
debian-6.0.2.1-amd64-netinst.iso  debian6-x86_64.img  f16_x86_64.img  migtest.img  rhel6x86_64.img  sanlock  tck

# grep /mnt/demo /proc/mounts 
auto.marrow /mnt/demo autofs rw,relatime,fd=13,pgrp=1363,timeout=300,minproto=5,maxproto=5,direct 0 0
marrow.gsslab.fab.redhat.com:/var/lib/libvirt/images/ /mnt/demo nfs rw,relatime,vers=3,rsize=524288,wsize=524288,namlen=255,hard,proto=tcp,timeo=600,retrans=2,sec=sys,mountaddr=10.33.8.114,mountvers=3,mountport=35386,mountproto=udp,local_lock=none,addr=10.33.8.114 0 0

# ./autofsdemo /mnt/demo
We are the parent
We are the container!
Found mount point 1 /mnt/demo auto.marrow autofs rw,relatime,fd=13,pgrp=1363,timeout=300,minproto=5,maxproto=5,direct
Found mount point 2 /mnt/demo marrow.gsslab.fab.redhat.com:/var/lib/libvirt/images/ nfs rw,relatime,vers=3,rsize=524288,wsize=524288,namlen=255,hard,proto=tcp,timeo=600,retrans=2,sec=sys,mountaddr=10.33.8.114,mountvers=3,mountport=35386,mountproto=udp,local_lock=none,addr=10.33.8.114
Umount point 2 /mnt/demo
Could not umount /mnt/demo: Operation not permitted

We see I can't even unmount the NFS server mount, let alone the autofs mount.

I believe indirect mounts will have similar problems, but I am currently unable to check that, pending resolution of  bug 745781

Attempting to use umount2() with MNT_DETACH also fails, so I can't simply hide the mounts from the container either.


Version-Release number of selected component (if applicable):
kernel-3.1.0-0.rc9.git0.0.fc16.x86_64
autofs-5.0.6-2.fc16.x86_64

How reproducible:
Always

Steps to Reproduce:
1. See above + attached demo program