How to compile your own kernel

Installing the Kernel

Install your new kernel with the make install command. Installing the kernel is a quick process, but you're not done yet. You have to configure GRUB to recognize your new kernel (Listing 3).

Listing 3

Configuring GRUB

# make install
sh ./arch/x86/boot/install.sh 5.12.0 arch/x86/boot/bzImage System.map "/boot"

The kernel install process copies three files to your /boot directory (Listing 4): The initramfs file is the initial filesystem that mounts your root filesystem, the system.map file is a symbol lookup table, and the vmlinuz file is your compressed kernel.

Listing 4

Files Copied to /boot Directory

-rw-------. 1 root root 96350022 Jul 12 09:58 /boot/initramfs-5.12.0.img
-rw-r--r--. 1 root root  5018232 Jul 12 09:56 /boot/System.map-5.12.0
-rw-r--r--. 1 root root  9202304 Jul 12 09:56 /boot/vmlinuz-5.12.0

Editing GRUB

You must update the GRUB configuration to refer to and use the new kernel:

# grub2-mkconfig -o /boot/grub2/grub.cfg
Generating grub configuration file ...
done

The grubby command sets your new kernel as the default, so you don't have to select it when your system next boots:

# grubby --set-default /boot/vmlinuz-5.12.0

The default is /boot/loader/entries/3f8c3788864245079651b8002d18f249-5.12.0.conf with index   and kernel /boot/vmlinuz-5.12.0.

Now the only thing left to do is to reboot and see how things go. Issue the reboot command and wait for the system to boot to a login prompt:

# reboot

When your system returns to a login prompt, login and see if the new kernel is loaded:

$ uname -a
Linux server1 5.12.0 #3 SMP Mon Jul 12 03:24:52 EDT 2021 x86_64 x86_64x86_64 GNU/Linux

Success! Your new kernel is loaded and working.

Troubleshooting

Sometimes compiling the kernel fails. In my case, it failed multiple times because I ran out of space on my system. A few other times, the process failed because there were missing certification files. When your compile fails, the messages you receive will generally point you to the correct location of the problem.

You can edit the .config file and comment the offending lines, especially for "files not found" errors that refer to certificates. Edit the file using your favorite text editor, comment the line with a leading #, save the file, and continue compiling by reissuing the make command. The compile process will pick up again where it left off.

I have never had more than three or four restarts for a kernel compile. Most often it's for those missing certificate files previously mentioned. If you encounter an error that isn't file-related and the failure doesn't designate a particular CONFIG_ entry that you can comment out, then your best option is to search online for a similar error. I know that isn't a great option. However, chances are good that someone else has encountered the same error, and you can quickly fix the problem and continue with your compile.

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