freebsd stable - Nov 2005 - Mount related panic with FreeBSD 6?

Hi,
I got this panic on freshly installed FreeBSD 6. I did this df -h and
noticed that /mnt/oldroot/home is somewhat mangled. The /mnt/oldroot is
root of FreeBSD 4.11 system. I successfully copied some settings and all
user accounts from that /mnt/oldroot/home earlier today. This is what I
did before the panic:

logout::wilx:~> df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/ad0s1a           496M   57M  399M  13% /
devfs                 1.0k  1.0k     0 100% /dev
/dev/ad0s1e           989M   12k  910M   1% /tmp
/dev/ad0s1f           163G   29G  120G  20% /usr
/dev/ad0s1d           4.8G  2.0G  2.4G  45% /var
/dev/ad2s1a           492M  317M  136M  70% /mnt/oldroot
df: `/mnt/oldroot/hom': No such file or directory
/dev/ad2s1f           6.7G  5.3G 1013M  85% /mnt/oldroot/usr
/dev/ad2s1e           2.9G  2.2G  476M  83% /mnt/oldroot/var

logout::root:~> mount /mnt/oldroot/home

Notice that /mnt/oldroot/home is mounted twice here:

logout::root:~> mount
/dev/ad0s1a on / (ufs, local)
devfs on /dev (devfs, local)
/dev/ad0s1e on /tmp (ufs, local, soft-updates)
/dev/ad0s1f on /usr (ufs, local, soft-updates)
/dev/ad0s1d on /var (ufs, local, soft-updates)
/dev/ad2s1a on /mnt/oldroot (ufs, local, read-only)
/dev/ad2s1g on /mnt/oldroot/home (ufs, local, read-only)
/dev/ad2s1f on /mnt/oldroot/usr (ufs, local, read-only)
/dev/ad2s1e on /mnt/oldroot/var (ufs, local, read-only)
/dev/ad2s1g on /mnt/oldroot/home (ufs, local, read-only)

logout::root:~> umount /mnt/oldroot/home

logout::root:~> mount
/dev/ad0s1a on / (ufs, local)
devfs on /dev (devfs, local)
/dev/ad0s1e on /tmp (ufs, local, soft-updates)
/dev/ad0s1f on /usr (ufs, local, soft-updates)
/dev/ad0s1d on /var (ufs, local, soft-updates)
/dev/ad2s1a on /mnt/oldroot (ufs, local, read-only)
/dev/ad2s1g on /mnt/oldroot/home (ufs, local, read-only)
/dev/ad2s1f on /mnt/oldroot/usr (ufs, local, read-only)
/dev/ad2s1e on /mnt/oldroot/var (ufs, local, read-only)

logout::root:~> cd /mnt/oldroot/home

logout::root:/mnt/oldroot/home> ls

After the ls it panicked. The only thing I see in logs is this:

Nov 20 12:42:02 logout savecore: reboot after panic: page fault
Nov 20 12:42:02 logout savecore: writing core to vmcore.0

Unfortunatelly trying gdb on the core didn't help.  The kernel is still
the generic one from the release CD:

warning: "/var/crash/vmcore.0": no core file handler recognizes
format,
using default
Can't fetch registers from this type of core file
Can't fetch registers from this type of core file
#0  0x00000000 in ?? ()

I realise that this might have been already fixed after the release. I
will try to cvsup the sources and build a new kernel.

Vaclav Haisman
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On Sun, 20 Nov 2005, [UTF-8] V=C3=A1clav Haisman wrote:
> Hi,
> I got this panic on freshly installed FreeBSD 6. I did this df -h and
> noticed that /mnt/oldroot/home is somewhat mangled. The /mnt/oldroot is
> root of FreeBSD 4.11 system. I successfully copied some settings and all
> user accounts from that /mnt/oldroot/home earlier today. This is what I
> did before the panic:
[...]

I discovered this by accident with a CDROM the other day. In 6.0 you can
overlay read-only mounts (i.e., mount the same R/O FS on top of itself)
but unmounting it will cause GEOM to tear down the underlying device while
leaving the first mount behind. Next access to the mountpoint will panic
the system.

You can't mount a read/write mount on top of itself, or a r/o mount on a
r/w mount -- you get an error. A quick discussion with phk points to a
faulty or missing access check in GEOM. I'm not familiar with the VFS
operations required to mount a filesystem, though, so I'm not sure where
to look to put in the fix.

In the interim, be careful not to mount a read-only FS multiple times.

--=20
Doug White                    |  FreeBSD: The Power to Serve
dwhite@gumbysoft.com          |  www.FreeBSD.org