sbz's blog

Technical writings and recipes on UNIX, software and systems

ZFS Kernel panic

Posted at — May 20, 2021

Recovering personal data from ZFS after FreeBSD kernel panic

Faulty kernel upgrade

After a kernel upgrade that went very bad, the boot loader couldn’t load the default kernel /boot/kernel.

What about kernel.old?

Of course, when something like that happens, we could rely on /boot/kernel.old to load the previous kernel from the boot loader, but in this case loading the previous kernel allow to boot but directly ends up with the following Kernel panic.

Why a crash?

Somehow, ZFS seems wrongly assume 3.5TB (35184375549952B) of range tree space according the panic message. cf. VERIFY3U macro and the rt->rt_space size test inside range_tree_remove_impl().

Unfortunately from here nothing is possible to do except reboot. I didn’t investigate further the root cause.

Here comes the Live CD!

FreeSBD releases memstick.img images contains a running kernel and could be used to access the ZFS data from the Live CD shell.

Let’s prepare a USB bootable key with the image and boot from it.

$ fetch
$ dd if=FreeBSD-12.2-RELEASE-amd64-memstick.img of=/dev/da0 bs=1m conv=sync

Prepare USB boot

Boot from the USB as Multi user (Press enter or 1), at the FreeBSD installer prompt select Live CD and login as as root

# uname -a
FreeBSD 12.2-RELEASE FreeBSD 12.2-RELEASE r366954 GENERIC amd64

Where is my data?

We are looking for the partition freebsd-zfs

# gpart list ada0
# gpart show /dev/ada0 | grep zfs

In my case it showed as ada0p3.

ZFS import

# kldlstat | grep -q zfs || kldload zfs
# zpool list
no pools available
# zfs list
no datasets available

The Live CD has a tmpfs partition mounted under /tmp that we could use to write in as mountpoint and mount our zfs datasets as readonly .

# mkdir -p /tmp/zroot
# zpool import  -f -o readonly=on -R /tmp/zroot zroot    

Now our pool zroot is online, we could explore our datasets

# zpool list
# zfs list -ro mounted,canmount,mountpoint,name

Our Personal data should be inside the user home directory referenced with the name zroot/usr/home and mounted in the folder /tmp/zroot/usr/home/sbz for me

# cd /tmp/zroot/user/home/sbz/
# ls .*src
.bashrc .cshrc .shrc .xinitrc .mailrc .vimrc 
# dh -sh .
5.7G    .

Great looks like all the previous files are accessible and I could start thinking about how to retrieve them and do my backup before re-installing from scratch.

Setup Wi-Fi on the Live CD

I want to use my Wi-Fi home network in order to be able to access and copy data either via SSH. For that, I need to configure the Wi-Fi and wpa_supplicant(8) to connect on my Access point.

# kldload iwn
# ifconfig wlan0 create wlandev iwn0
# ifconfig wlan0 up # load firmware
# ifconfig wlan0 list scan |grep sbzAP
sbzAP                            04:f1:69:86:25:cb   11   54M  -67:-95   100 EPS  RSN WME BSSLOAD HTCAP VHTCAP VHTOPMODE
# mkdir -p /tmp/etc/
# cat > /tmp/etc/wpa_supplicant.conf
# wpa_supplicant -i wlan0 -c /tmp/etc/wpa_supplicant.conf -D bsd -B
# dhclient wlan0
DHCPDISCOVER on wlan0 to port 67 interval 7
DHCPREQUEST on wlan0 port 67
bound to -- renewal in 18000 seconds
# ping -c 1

I’m now connected on my access point sbzAP and I have network and can have access to Internet.

Access the Data

via SSH

In order to access via SSH, we need to configure manually the daemon sshd(8) on the Live CD to use SSH to backup the data.

# mkdir -p /tmp/etc/ssh
# /usr/sbin/ssh-keygen -q -t rsa -f /tmp/etc/ssh/ssh_host_rsa_key -N ''

We start the SSH daemon to be able to accept connections with root user and passwordless on the local network. We don’t really need security for this case.

# /usr/sbin/sshd -h /tmp/etc/ssh/ssh_host_rsa_key \
-o PermitEmptyPasswords=yes -o PermitRootLogin=yes \
-o PasswordAuthentication=yes -o UsePAM=no -D -d

Then with ssh(1) ssh [email protected].168.43.211 to connect and scp(1) to transfer files we could recover our data.

n.b.: I could have use zfs send but I wanted to use something simple and straight forward

Finally, Data recovery was done successfully

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