Recovery from a failed kernel upgrade on Lamobo R1

Yesterday I have updated my Lamobo R1 router board running Debian Stretch the usual way with apt update;apt upgrade. This time many updates were involved including a kernel update from 4.9.130-2 to 4.9.144-3. After rebooting the device it did not come up again. The same happens with the 4.9.144-3 kernel and the Bananapi board which is nearly identical to the Lamobo R1.

After attaching a screen to the HDMI output I was able to see the boot messages. The last thing I noticed was that uboot was struck at “Starting kernel”. Unfortunately there has been no more useful information. Maybe one could get some more information using the serial console. Obviously the new kernel is broken for the Lamobo and the Bananapi. The corresponding bug report can be monitored on bugs.debian.org. The natural reaction is to downgrade the kernel to the previous state, which is not as simple as on Intel based devices. Since it is quite probable that others experience the same problem with the kernel upgrade I will show in the following how to recover from the upgrade.

First of all I inserted the sd card containing the operating system into a notebook and mounted the boot partition. Luckily there still has been a older backup kernel remaining from system setup. In my case this has been a 4.9.0-7 kernel. So I extracted the boot script from the boot.scr file.

dd if=boot.scr of=boot.script bs=72 skip=1

In the resulting boot.script I replaced the line

setenv fk_kvers ‘4.9.0-8-armmp-lpae’

with

setenv fk_kvers ‘4.9.0-7-armmp-lpae’

This might be different if the backup kernel available has a different version. If there is no working backup kernel, one might try to get a kernel image from a different source, in example extracting from one of the older Debian packages, from the boot partition of a second device or if one has been paranoid, from a backup.

Afterwards the boot.scr file has to be regenerated. For this the u-boot-tools package has to be present on the system used for the recovery.

mkimage -C none -A arm -T script -d boot.script boot.scr

After reinserting the card into the Lamobo R1 eventually I have been able to boot the system with the old kernel version. Afterwards it has been possible to restore the previous kernel version (4.9.130-2) with:

dpkg -i linux-image-4.9.0-8-armmp-lpae_4.9.130-2_armhf.deb

It might be a good idea to prevent further kernel upgrades until the mentioned bug has been solved. This can easily be done by:

apt-mark hold linux-image-4.9.0-8-armmp-lpae

Hopefully this helps others facing the same problem after this or any other kernel update on these or similar devices.

Jürgen

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