Making Sure the Debian Kernel is Up To Date (Shallow Thoughts)

Making Sure the Debian Kernel is Up To Date

I try to avoid Grub2 on my Linux machines, for reasons I've discussed before. Even if I run it, I usually block it from auto-updating /boot since that tends to overwrite other operating systems. But on a couple of my Debian machines, that has meant needing to notice when a system update has installed a new kernel, so I can update the relevant boot files. Inevitably, I fail to notice, and end up running an out of date kernel.

But didn't Debian use to have a /boot/vmlinuz that always linked to the latest kernel? That was such a good idea: what happened to that?

I'll get to that. But before I found out, I got sidetracked trying to find a way to check whether my kernel was up-to-date, so I could have it warn me of out-of-date kernels when I log in.

That turned out to be fairly easy using uname and a little shell pipery:

# Is the kernel running behind?
kernelvers=$(uname -a | awk '{ print $3; }')
latestvers=$(cd /boot; ls -1 vmlinuz-* | sort --version-sort | tail -1 | sed 's/vmlinuz-//')
if [[ $kernelvers != $latestvers ]]; then
    echo "======= Running kernel $kernelvers but $latestvers is available"
else
    echo "The kernel is up to date"
fi

I put that in my .login. But meanwhile I discovered that that /boot/vmlinuz link still exists -- it just isn't enabled by default for some strange reason. That, of course, is the right way to make sure you're on the latest kernel, and you can do it with the linux-update-symlinks command.

linux-update-symlinks is called automatically when you install a new kernel -- but by default it updates symlinks in the root directory, /, which isn't much help if you're trying to boot off a separate /boot partition.

But you can configure it to notice your /boot partition. Edit /etc/kernel-img.conf and change link_in_boot to yes:

link_in_boot = yes

Then linux-update-symlinks will automatically update the /boot/vmlinuz link whenever you update the kernel, and whatever bootloader you prefer can point to that image. It also updates /boot/vmlinuz.old to point to the previous kernel in case you can't boot from the new one.

Update: To get linux-update-symlinks to update symlinks to reflect the current kernel, you need to reinstall the package for the current kernel, e.g. apt-get install --reinstall linux-image-4.18.0-3-amd64. Just apt-get install --reinstall linux-image-amd64 isn't enough.