Sometimes it is helpful to build a custom kernel.
Someone might want to slim down the kernel, optimize it or just building a kernel for fun - depending on ones needs of course.
Debian makes this a breeze!.
The following short article will show how to build a Linux kernel and also generating easy to install deb packages.

Getting the kernel sources

The first step is always getting the Linux kernel sources.
Debian has already packaged the kernel sources which can be easily installed via apt.

$ sudo apt install linux-source-6.9

Note: Install the kernel version that you need and want!

Another way of getting the kernel sources is downloading named sources directly from the official site, kernel.org.

These can be found here: Kernel Sources

If you’ve gone the route of leveraging apt you can list the contents of the package you have just installed via dpkg.

$ dpkg -L linux-source-6.9 
/.
/usr
/usr/share
/usr/share/doc
/usr/share/doc/linux-source-6.9
/usr/share/doc/linux-source-6.9/changelog.Debian.gz
/usr/share/doc/linux-source-6.9/copyright
/usr/src
/usr/src/linux-source-6.9.tar.xz

As you can see, the sources are stored under /usr/src/.

Preparing the build directory

I personally always create a build directory in my home folder. For example:

$ mkdir build 
$ tar xJf /usr/src/linux-source-6.9.tar.xz -C build/
$ cd build/linux-source-6.9/

Note: Where you store/build your kernel is up to you.

Installing needed dependencies

Debian makes this easy for us.

$ sudo apt install build-essential linux-source-6.9 bc kmod cpio flex libncurses5-dev libelf-dev libssl-dev dwarves bison fakeroot
$ sudo apt-get build-dep linux-source-6.9

After everything is installed we can move on to configuring the kernel.

Configuration

First things first, clean the kernel source tree.

$ make mrproper

You could from this point on go straight to configuring the kernel via menuconfig/nconfig…
But, given the fact that Debian ships the kernel config used for your kernel, you can make your life a little easier by using said config.
In the following example I will copy over the config from a installed kernel so that I already have most things taken care of.

$ cp -v /boot/config-6.9.10+bpo-amd64 .config

The next step will be to actually dive into configuring the kernel via a friendly curses menu.

$ make nconfig

Note: Make sure to save your configuration changes! The name of the resulting config will be .config (Directly in the kernel source directory)

Make the CPU go brrrrrr

All set - Time to build the kernel and generate easy to install deb packages.

$ make -j`nproc` bindeb-pkg

Note: Leveraging “nproc” will return all usable CPU cores. Be careful with this since it will use your CPU to it’s full extent!

Final chapter

The kernel was built. A big step is taken care of.
You will find the generated deb packages in the build directory we’ve created before.

$ cd ..
$ ls
total 1013080
-rw-r--r-- 1 x x   9451452 Nov  9 15:13 linux-headers-6.9.10_6.9.10-1_amd64.deb
-rw-r--r-- 1 x x 100806300 Nov  9 15:13 linux-image-6.9.10_6.9.10-1_amd64.deb
-rw-r--r-- 1 x x 925761052 Nov  9 15:14 linux-image-6.9.10-dbg_6.9.10-1_amd64.deb
-rw-r--r-- 1 x x   1356640 Nov  9 15:12 linux-libc-dev_6.9.10-1_amd64.deb
drwxr-xr-x 1 x x      2166 Nov  9 15:12 linux-source-6.9
-rw-rw-r-- 1 x x      6461 Nov  9 15:14 linux-upstream_6.9.10-1_amd64.buildinfo
-rw-rw-r-- 1 x x      2191 Nov  9 15:15 linux-upstream_6.9.10-1_amd64.changes

Q: So, what is left to do?
A: Of course, installing the packages so we can boot into our newly baked kernel.

apt:

$ sudo apt install ./*.deb

dpkg:

$ sudo dpkg -i *.deb

Select which better suits your needs.

The final countdown

All there is left to do is to reboot the system and selecting the newly compiled kernel during the Grub boot menu - if you are using Grub that is.
That is literally it - We are done.

Closing words

This is merely one way to do it… and yes, you guessed it… there is more than one way to do it.
You could also entirely leave out Debian specific parts and compile a kernel from scratch without generating any deb packages at all.

In the end: You have to chose what fits you best.

Lust but not least…

Stay Open!