Building the Linux Kernel

Building the kernel yourself lets you enable experimental features, apply custom patches (like PREEMPT_RT), shrink the kernel for embedded systems, or simply understand how it all fits together. The process takes about 30 minutes on modern hardware.

Getting the Source

# Official source: kernel.org
# Download latest stable tarball:
wget https://cdn.kernel.org/pub/linux/kernel/v6.x/linux-6.12.tar.xz
wget https://cdn.kernel.org/pub/linux/kernel/v6.x/linux-6.12.tar.sign

# Verify GPG signature (optional but good practice):
xz -d linux-6.12.tar.xz
gpg --verify linux-6.12.tar.sign linux-6.12.tar

# Extract:
tar xf linux-6.12.tar
cd linux-6.12

# Or clone with git (full history, ~4GB):
git clone https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
cd linux

# Stable branch (production-ready point releases):
git clone https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
git checkout linux-6.12.y

Build Dependencies

# Ubuntu/Debian:
apt install build-essential libncurses-dev bison flex libssl-dev \
            libelf-dev dwarves bc make gcc pkg-config

# Fedora/RHEL:
dnf install gcc make ncurses-devel bison flex openssl-devel \
            elfutils-libelf-devel dwarves bc

# Verify gcc works:
gcc --version
make --version

# Optional: ccache speeds up repeated builds
apt install ccache
export CC="ccache gcc"

Configuration with menuconfig

Why is kernel configuration so important? The kernel has thousands of compile-time options. Each option either enables code (larger kernel, more features) or excludes it (smaller, faster to build). Your running kernel's config is a good starting point — it's already tested to work on your hardware.

# Start with your current kernel's config (safest):
cp /boot/config-$(uname -r) .config
make olddefconfig   # accept defaults for any new options

# Or: generate minimal config (just enough to boot, tiny result):
make localmodconfig  # uses only modules currently loaded on your system

# Or: start fresh (interactive, long process — thousands of questions):
make menuconfig   # terminal UI, use arrow keys + space to toggle

# menuconfig navigation:
# Arrow keys: move between options
# Space: toggle on/off/module (Y/N/M)
# Enter: enter submenu
# /: search for option
# ?: help for highlighted option
# Esc+Esc: go back

# Key config paths (in menuconfig):
# General Setup → Local version (add -custom suffix to uname -r)
# Processor type and features → Preemption Model (for RT)
# Device Drivers → (your hardware drivers)
# File systems → (filesystems you need: ext4, btrfs, etc.)
# Networking → (protocols, netfilter, etc.)

# After editing, save as .config

Building the Kernel

# Build everything (kernel + all modules):
# -j$(nproc) uses all CPU cores in parallel
make -j$(nproc)

# Build output:
# arch/x86/boot/bzImage   ← compressed kernel image
# *.ko files throughout   ← kernel modules

# Build time estimates (on modern 8-core machine):
# make localmodconfig build: ~5-10 minutes
# make olddefconfig build:   ~20-40 minutes
# Full allmodconfig build:   ~2+ hours

# Watch build with verbosity (debug build failures):
make -j$(nproc) V=1 2>&1 | tee build.log

# Build just a specific module (fast):
make -j$(nproc) drivers/net/ethernet/intel/e1000/e1000.ko

# Cross-compile for ARM64 (e.g., building for Raspberry Pi on x86):
apt install gcc-aarch64-linux-gnu
export ARCH=arm64
export CROSS_COMPILE=aarch64-linux-gnu-
make -j$(nproc)

Installing the Kernel

# Install kernel modules to /lib/modules/VERSION/:
sudo make modules_install

# Install kernel image + update initramfs + update GRUB:
sudo make install
# This runs installkernel script which:
# 1. Copies bzImage to /boot/vmlinuz-VERSION
# 2. Copies System.map to /boot/System.map-VERSION
# 3. Copies .config to /boot/config-VERSION
# 4. Generates initrd/initramfs: update-initramfs -c -k VERSION
# 5. Updates bootloader: update-grub

# Verify files are in /boot:
ls /boot/ | grep $(make kernelversion)
# config-6.12.0-custom
# initrd.img-6.12.0-custom
# System.map-6.12.0-custom
# vmlinuz-6.12.0-custom

# GRUB should now list the new kernel:
grep menuentry /boot/grub/grub.cfg | head -5

# Reboot and select new kernel from GRUB menu:
sudo reboot

# After reboot — verify:
uname -r
# 6.12.0-custom

Cleanup and Removing a Kernel

# Remove a kernel you no longer want:
# 1. Remove from /boot:
sudo rm /boot/vmlinuz-6.12.0-custom
sudo rm /boot/initrd.img-6.12.0-custom
sudo rm /boot/System.map-6.12.0-custom
sudo rm /boot/config-6.12.0-custom

# 2. Remove modules:
sudo rm -rf /lib/modules/6.12.0-custom

# 3. Update GRUB:
sudo update-grub

# Clean build artifacts (before switching configs):
make clean       # removes built files, keeps .config
make mrproper    # removes everything including .config
make distclean   # removes even editor backup files

# Check build size:
du -sh vmlinux   # uncompressed kernel (~100-300MB)
du -sh arch/x86/boot/bzImage   # compressed image (~10-15MB)