Downloading the Kernel Source

As the final step before getting started on compiling the kernel, you need to download the kernel source from kernel.org. To streamline the different steps of downloading, verifying, and unpacking the kernel source, I used a script supplied by kernel.org [1]. Copy the contents of the script into your favorite text editor, save it, and add the executable permission to it. Prior to running the script, create the Downloads directory in the root user's home directory. Proceed from here on as the root user. You must supply the kernel version number you wish to download as an argument to the script. For this example, I used 5.12:

# mkdir Downloads
# ./get-verified-tarball.sh 5.12

Listing 2 shows the script's output.

Using TMPDIR=/root/Downloads/linux-tarball-verify.yceAuEZym.untrusted
Making sure we have all the necessary keys
gpg: WARNING: unacceptable HTTP redirect from server was cleaned up
gpg: WARNING: unacceptable HTTP redirect from server was cleaned up
pub   rsa4096 2013-01-24 [SC]
      B8868C80BA62A1FFFAF5FDA9632D3A06589DA6B1
uid           [ unknown] Kernel.org checksum autosigner <autosigner@kernel.org>
pub   rsa4096 2011-09-23 [SC]
      647F28654894E3BD457199BE38DBBDC86092693E
uid           [ unknown] Greg Kroah-Hartman <gregkh@kernel.org>
sub   rsa4096 2011-09-23 [E]
pub   rsa2048 2011-09-20 [SC]
      ABAF11C65A2970B130ABE3C479BE3E4300411886
uid           [ unknown] Linus Torvalds <torvalds@kernel.org>
sub   rsa2048 2011-09-20 [E]
Downloading the checksums file for linux-5.12
Verifying the checksums file
gpgv: Signature made Sun 11 Jul 2021 07:25:09 AM EDT
gpgv:                using RSA key 632D3A06589DA6B1
gpgv: Good signature from "Kernel.org checksum autosigner <autosigner@kernel.org>"
Downloading the signature file for linux-5.12
Downloading the XZ tarball for linux-5.12
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  112M  100  112M    0     0  2445k      0  0:00:47  0:00:47 --:--:--  354k
Verifying checksum on linux-5.12.tar.xz
linux-5.12.tar.xz: OK
Verifying developer signature on the tarball
gpgv: Signature made Mon 26 Apr 2021 12:49:05 AM EDT
gpgv:                using RSA key 647F28654894E3BD457199BE38DBBDC86092693E
gpgv: Good signature from "Greg Kroah-Hartman <gregkh@kernel.org>"
Successfully downloaded and verified /root/Downloads/linux-5.12.tar.xz

You can copy the kernel source to /usr/src/kernels, or you can leave it where it is (/root/Downloads) and extract the kernel source tar file:

# cd Downloads
# unxz -v linux-5.12.tar.xz
# tar xvf linux-5.12.tar
# cd linux-5.12

Now you're ready to begin compiling the kernel.

Compiling the Kernel

This first step, which is completely optional, is my favorite part of compiling a new kernel or adding and subtracting features from an existing one. It is the menuconfig command, which results in the Linux Kernel Configuration menu (Figure 1):

# make menuconfig

Figure 1: Linux Kernel Configuration menu.

Using this graphical interface, you can enable and disable features for your new kernel. Once you've made the changes you require, exit the menu. This procedure saves your choices to the hidden configuration file .config and backs up any existing .config file to .config.old. You're now ready to compile the kernel. Start the compile process by running the make command:

# make

Compiling the kernel can take a very long time depending on your system's CPU and memory capacity. For some small virtual machines, the process can take hours. For example, mine took roughly 10 hours to complete after performing a bit of troubleshooting along the way. For details, see the "Troubleshooting" section.

Once complete, you will find your new kernel listed as an executable file named vmlinux. You're now almost ready to install the new kernel onto your system. But first, you need to install the new kernel modules.

Kernel Modules

Install the Linux kernel modules created during the compile process with:

# make modules_install

This process only takes a few minutes and occurs without intervention.