misc/173136: (unionfs) mounting above the NFS read-only share panic

[Marek Salwerowicz]

>Number:         173136
>Category:       misc
>Synopsis:       (unionfs) mounting above the NFS read-only share panic
>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:   Sat Oct 27 00:00:00 UTC 2012
>Closed-Date:
>Last-Modified:
>Originator:     Marek Salwerowicz
>Release:        9.1-PRERELEASE amd64
>Organization:
>Environment:
FreeBSD s14 9.1-PRERELEASE FreeBSD 9.1-PRERELEASE #0: Thu Oct  4 23:46:19 CEST 2012     ato at volt.iem.pw.edu.pl:/tmp/obj/usr/src/sys/DISKLESS  amd64
>Description:
I am using a diskless mode of FreeBSD with root ('/') read-only mounted from my server.
The /tmp directory is mounted as tmpfs.

After creating eg. /tmp/istgt directory, mounting it (using mount_unionfs) above the eg. /usr/local/etc/istgt directory and editing existing file (istgt.conf), I receive a kernel panic

s14% mount
194.29.146.3:/ on / (nfs, read-only)
devfs on /dev (devfs, local)
/dev/md0 on /etc (ufs, local)
procfs on /proc (procfs, local)
tmpfs on /tmp (tmpfs, local)
tmpfs on /var (tmpfs, local)

output from machine console:
https://www.dropbox.com/s/e3wocs9tx0fip6e/20121026434.jpg
https://www.dropbox.com/s/jzwjrda5itoyuez/20121026436.jpg

>How-To-Repeat:
[assume that /tmp is tmpfs, and / is mounted read-only from NFS server]

% sudo mkdir /tmp/istgt
% sudo mount_unionfs /tmp/istgt /usr/local/etc/istgt
% vim /usr/local/etc/istgt/istgt.conf
(edit file and save it, using :wq)


>Fix:


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