Building the Linux Kernel (Part 2 of 2)

In Part 1 I looked at obtaining the kernel source code and setting up an initial configuration file. This file keeps track of any alterations or additions that I might want to add. Small examples of such alterations might include renaming the kernel version or disabling unneeded features that are on by default.

The next step entails building a kernel image based off of this config file, along with any supporting drive modules, that will be loaded into memory at boot time.

Understanding Kernel Build Artifacts

When you build the Linux kernel, you’ll encounter several key artifacts, each serving a unique purpose:

vmlinux: The raw, uncompressed kernel image
- Contains full debug symbols, making it crucial for kernel developers
- Much larger than the final kernel image (often hundreds of megabytes)
- Used primarily for debugging and development, not for actual booting
- Includes detailed symbol information that helps diagnose kernel-level issues[^1]
bzImage: The bootable, compressed kernel image
- “bz” stands for “big zipped” (not related to bzip2)
- Compressed to reduce memory footprint during system startup
- Designed to be efficiently loaded into memory by bootloaders
- Contains only essential information needed for system boot
Modules: Dynamically loadable kernel components
- Marked with “M” in kernel configuration
- Allow extending kernel functionality without recompiling
- Can be loaded/unloaded at runtime
- Provide flexibility for hardware support and optional features[^2]

Build the Kernel

Thanks to the work of many developers, actually “building” the kernel is often as simple as running the make command. When run without arguments, it executes the first target in the Makefile, typically “all”:

# These commands are equivalent
make all
make

Kernel Build Process

Compile kernel core components
Build loadable modules
Create compressed boot image
Prepare installation artifacts

Solving Common Build Errors

Building the kernel from source doesn’t always go smoothly, especially for beginners. Here are common challenges and solutions:

Missing Dependencies
Install necessary packages with:

sudo apt-get install build-essential libncurses-dev \
                     bison flex libssl-dev libelf-dev

Certificate Issues on Ubuntu Some Ubuntu versions require additional steps to handle kernel module signing:

# Generate a local signing key
openssl req -new -x509 -newkey rsa:2048 \
            -keyout signing_key.pem \
            -outform DER -out signing_key.x509 \
            -days 36500 -subj "/CN=Local Kernel Module Signing/"

Compilation Warnings
- Don’t panic if you see numerous warnings
- Most are informational and don’t prevent compiling successfully
- Use make -j$(nproc) to utilize all CPU cores and speed up build

Installation and Testing

After successful compilation, install the new kernel:

# Install modules
sudo make modules_install

# Install kernel
sudo make install

Initramfs preparation

Initramfs stands for “initial RAM filesystem,” which is exactly what it sounds like: When the system boots up and has no idea whether disks are present and how to talk to them, it relies on initramfs to already be present in main memory and ready to provide the kernel with essential drivers and other necessities for booting the system and finally getting access to the disk(s).

To prepare initramfs, enter this command in the root source directory: sudo make install

Assuming this worked for you, then congratulations, you should be ready to boot into your new kernel. All that remains is to enter the Grub boot-loader menu and choose the correct entry!

It’s ok if this doesn’t work for you right away

Linux is endlessly fascinating, but that can sometimes make newcomers feel like they are way out of their depth when things don’t go to plan.

I chose brevity over exhaustive detail in this guide. Like the rest of this site, it exists primarily as a document of my own learning. If you see anything wrong or ill-advised herein, please reach out.

Thank you for reading!