Linux Kernel Build

Build Custom Linux Kernel

I was interested to see how to do a Linux Kernel Build from the source code. I followed this guide and while it mostly made sense there were a few areas that I found less clear or had unexpected errors.

Initial Set up

I used VirtualBox rather than VMWare suggested in the guide, but don’t think didn’t encounter many differences. Some of the later steps are quite resource intensive, so set your virtual machine memory , CPU and Virtual Disk relatively high if you can spare it on your host machine.

I used a lubuntu install with 10GB virtual hard disk initially, but upped that to 20GB when I ran out of space cloning the repo.

The guide I followed is actually for submitting patches. This might be something I build up to, but realistically I didn’t think I would be submitting patches any time soon, so I skipped the email setup.

Cloning the Kernel Repo

Performance Issues

The guide recommends cloning the repository on a fast, stable, unlimited connection. They weren’t joking when they said it took over 5GB!

I didn’t follow the guidance on sensible virtual disk space and ran out of space on my first attempt.

As I was running a virtual machine, i think I may have had the issue that it goes into pause mode when it it struggling with memory. This was fixed by upping the amount of memory I gave it.

Name Resolution Error

On doing

git fetch origin

I got an error:

fatal: unable to look up git.kernel.org (port 9418) (Temporary failure in name resolution)

I checked the origin with

git remote -v

And also tried to change to https version of origin (rather than ‘git:’ in the guide):

git remote set-url origin https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging.git

This still was not working, so tried pinging git.kernel.org in the virtual machine and got the error ‘Temporary failure in name resolution’. Pinging the IP address of git.kernel.org (via this website) gave the same error.

This still didn’t work and after mucking about with pinging and checking IP addresses I found this solution to temporarily change the nameserver to 8.8.8.8.

Finally, I came across this page which mentions there had been issues with kernel.org, which could be related to the same problem

If you keep having problems, you could use the GitHub mirror if you just want to be able to build recent-ish kernel code.

Updating the build configuration

As per the guide, I found a kernel config file in /boot/and copied it to .config in the cloned folder:

git/kernels/staging/staging/

working directory with

cp /boot/config-uname -r* .config

It is also a good idea to update CONFIG_LOCALVERSION in the new config. This appends an extra label to the name of any kernel built which helps avoid name clashes.

Building The Kernel

Setting the Config

If you run ‘make’ without a .config file, it will complain that there isn’t one there.

I think the config file I used (from lubuntu 20.04) was quite out of date compared to the kernel as there were many questions not configured and appeared as ‘NEW’.

Having no idea what I was doing, I just accepted the defaults for each question until the build got started.
Lots of them seemed to be about including compatibility with various hardware.

This page actually suggests some other ways of setting up your config including automatically accepting defaults on new configuration options or only building a minimal version to speed up build time.

In my case I initially copied the configuration from an Ubuntu build, which is likely to be fairly generalised and include more than a more bespoke option might need.

(In retrospect, I may have been better starting from the same kernel version as my distro (lubuntu 20.04) so I wouldn’t see so many ‘NEW’ config options.)

Fixing Certs Issues

After building for a while, I got an error:

No rule to make target 'debian/canonical-certs.pem', needed by 'certs/x509_certificate_list'

From this link it looks like I need to update the config to set

This seemed to do the trick, although I also needed to update the config for

debian/canonical-revoked-certs.pem

Pahole is not available

This failed with an error that ‘pahole is not available’. Thankfully there was a stackoverflow question with an answer – install the ‘dwarves‘ package.

Installing dwarves did the trick and the rest of the build went ok.

Build Time

To get a feel for how long the build time might be, check out these here and here . It really depends on the computer/resources you are using and how much you are compiling.

In my case the Linux kernel build took several hours and then ran out of space on my virtual machine. I tried again with a VM with more resources (8GB RAM, 4 cpu cores) and it was done within about 30 minutes.

Modifying a Driver

As I was running on a virtual machine I followed the guide to change the e1000 driver, which worked without issues.

Reverting to your original kernel

To get back to your original kernel, you can usually hit ‘Escape’ during boot to bring up the grub menu.

If you set a CONFIG_LOCALVERSION when setting your configuration, this will have been appended to the kernel name.

Once booted into your old/original kernel you can remove the one you build.

For kernels installed with your package manager (e.g. during an upgrade) this can be done via these instructions.

However, in our case we need to do something a little different and manually remove the kernel and references to it.

The suggested method on stackoverflow is to

locate -b -e 4.4.6-my-kernel | xargs -p sudo rm -r

and enter ‘y’ to accept the deletions. To play it safe you may want to just run

locate -b -e 4.4.6-my-kernel

first to confirm what it finds.

This uses the ‘locate’ package which you may need to install as the ‘mlocate’ package.

Cleaning build folder

To clean the build folder, use

make clean

This should remove the build artifacts and leave just the source code.