misc/187778: Two ZFS filesystems mounted on / at same time

[Cory Smelosky]

>Number:         187778
>Category:       misc
>Synopsis:       Two ZFS filesystems mounted on / at same time
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Thu Mar 20 06:00:02 UTC 2014
>Closed-Date:
>Last-Modified:
>Originator:     Cory Smelosky
>Release:        10.0
>Organization:
>Environment:
FreeBSD marie 10.0-RELEASE FreeBSD 10.0-RELEASE #0 r260789: Thu Jan 16 22:34:59 UTC 2014     root at snap.freebsd.org:/usr/obj/usr/src/sys/GENERIC  amd64
>Description:
I managed to successfully mount two ZFS filesets at the same mountpoint: /

root at marie:~ # zfs mount green/ROOT/default
root at marie:~ # df -h
Filesystem                    Size    Used   Avail Capacity  Mounted on
marie/ROOT/default             69G    2.7G     67G     4%    /
devfs                         1.0K    1.0K      0B   100%    /dev
marie/tmp                      67G     40K     67G     0%    /tmp
marie/usr/home                 67G     39K     67G     0%    /usr/home
marie/usr/ports                68G    1.2G     67G     2%    /usr/ports
marie/usr/src                  67G    359M     67G     1%    /usr/src
marie/var                      67G    464M     67G     1%    /var
marie/var/crash                67G     32K     67G     0%    /var/crash
marie/var/log                  67G     67K     67G     0%    /var/log
marie/var/mail                 67G     31K     67G     0%    /var/mail
marie/var/tmp                  67G     32K     67G     0%    /var/tmp
Storage                       1.3T    168K    1.3T     0%    /mnt/Storage
Storage/Storage               2.4T    1.2T    1.3T    47%    /mnt/Storage/Storage
Storage/VirtualBox             50G     34G     16G    67%    /mnt/Storage/VirtualBox
marie-virt                    1.8T     32K    1.8T     0%    /marie-virt
marie-virt/jails              1.8T     48K    1.8T     0%    /marie-virt/jails
marie-virt/jails/basejail     1.8T    175M    1.8T     0%    /marie-virt/jails/basejail
marie-virt/jails/newjail      1.8T    1.8M    1.8T     0%    /marie-virt/jails/newjail
marie-virt/jails/green        1.8T    135M    1.8T     0%    /marie-virt/jails/green
procfs                        4.0K    4.0K      0B   100%    /marie-virt/jails/green/proc
/marie-virt/jails/basejail    1.8T    175M    1.8T     0%    /marie-virt/jails/green/basejail
devfs                         1.0K    1.0K      0B   100%    /marie-virt/jails/green/dev
fdescfs                       1.0K    1.0K      0B   100%    /marie-virt/jails/green/dev/fd
green/ROOT/default             18G    1.0G     17G     6%    /
root at marie:~ # zfs unmount green/ROOT/default
internal error: failed to initialize ZFS library

I am aware this is a case of PEBKAC, but this should not be possible.
>How-To-Repeat:
Use zroot + mount disk image of another zroot at the same time without changing mountpoints.
>Fix:


>Release-Note:
>Audit-Trail:
>Unformatted: