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🔨 Chapter 25: Kernel Compilation & Custom Modules

Expert Chapter 25

📑 Table of Contents

Why Compile Your Own Kernel?

Reason Example
Enable specific features Real-time patches, new filesystems
Remove bloat Smaller kernel for embedded/VMs
Hardware support Custom drivers not in mainline
Security Remove unused features = smaller attack surface
Learning Understand Linux internals deeply

Compiling the Linux Kernel

Step 1: Get the Source

# Install dependencies
sudo apt install build-essential libncurses-dev bison flex libssl-dev \
  libelf-dev dwarves bc rsync

# Download kernel source
cd /usr/src
sudo wget https://cdn.kernel.org/pub/linux/kernel/v6.x/linux-6.8.tar.xz
sudo tar -xJf linux-6.8.tar.xz
cd linux-6.8

Step 2: Configure

# Start from current config
cp /boot/config-$(uname -r) .config
make olddefconfig                   # Accept defaults for new options

# Or interactive configuration
make menuconfig                     # TUI (recommended)
make xconfig                        # Qt GUI
make nconfig                        # Newer TUI

Step 3: Compile

# Compile (use all CPU cores)
make -j$(nproc)

# Compile modules
make modules -j$(nproc)

# Install modules
sudo make modules_install

# Install kernel
sudo make install

# Update bootloader
sudo update-grub                    # Debian/Ubuntu
sudo grub2-mkconfig -o /boot/grub2/grub.cfg  # RHEL/Fedora

# Reboot to new kernel
sudo reboot
uname -r                           # Verify

Kernel Configuration

Key Options in menuconfig

General setup →
  Local version: -custom            # Append to kernel version
  Default hostname: myhost

Processor type and features →
  Processor family: (select yours)
  Preemption Model: (desktop vs server)

Enable block layer →
  IO Schedulers: (select needed ones)

File systems →
  Btrfs, XFS, ext4, etc.

Networking support →
  Networking options →
    TCP/IP networking
    Network packet filtering (Netfilter)

Device Drivers →
  (Enable/disable hardware support)

Security options →
  SELinux, AppArmor, etc.

Quick Configuration Tips

# Enable only modules currently in use
make localmodconfig              # Uses lsmod to determine needed modules

# Minimal config
make tinyconfig                  # Absolute minimal kernel

# Compare configs
scripts/diffconfig .config.old .config

Writing a Kernel Module

Hello World Module

// hello.c
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Sovon");
MODULE_DESCRIPTION("Hello World Kernel Module");
MODULE_VERSION("1.0");

static int __init hello_init(void) {
    printk(KERN_INFO "Hello from kernel module!\n");
    return 0;
}

static void __exit hello_exit(void) {
    printk(KERN_INFO "Goodbye from kernel module!\n");
}

module_init(hello_init);
module_exit(hello_exit);

Makefile

obj-m += hello.o

KDIR := /lib/modules/$(shell uname -r)/build
PWD := $(shell pwd)

all:
	make -C $(KDIR) M=$(PWD) modules

clean:
	make -C $(KDIR) M=$(PWD) clean

Build & Load

make                              # Compile
sudo insmod hello.ko              # Load module
dmesg | tail -1                   # "Hello from kernel module!"
lsmod | grep hello                # Verify loaded
sudo rmmod hello                  # Unload
dmesg | tail -1                   # "Goodbye from kernel module!"

Module with Parameters

#include <linux/init.h>
#include <linux/module.h>
#include <linux/moduleparam.h>

static int count = 1;
static char *name = "world";

module_param(count, int, 0644);
MODULE_PARM_DESC(count, "Number of greetings");
module_param(name, charp, 0644);
MODULE_PARM_DESC(name, "Name to greet");

static int __init hello_init(void) {
    int i;
    for (i = 0; i < count; i++)
        printk(KERN_INFO "Hello, %s!\n", name);
    return 0;
}

// ... exit function and module macros
sudo insmod hello.ko count=3 name="Linux"

DKMS — Dynamic Kernel Module Support

DKMS automatically rebuilds modules when the kernel is upgraded.

sudo apt install dkms

# Create DKMS structure
sudo mkdir /usr/src/hello-1.0/
sudo cp hello.c Makefile /usr/src/hello-1.0/

# Create dkms.conf
sudo tee /usr/src/hello-1.0/dkms.conf << 'EOF'
PACKAGE_NAME="hello"
PACKAGE_VERSION="1.0"
BUILT_MODULE_NAME[0]="hello"
DEST_MODULE_LOCATION[0]="/updates/dkms"
AUTOINSTALL="yes"
MAKE[0]="make -C /lib/modules/${kernelver}/build M=${dkms_tree}/${PACKAGE_NAME}/${PACKAGE_VERSION}/build modules"
CLEAN="make -C /lib/modules/${kernelver}/build M=${dkms_tree}/${PACKAGE_NAME}/${PACKAGE_VERSION}/build clean"
EOF

# Register and build
sudo dkms add -m hello -v 1.0
sudo dkms build -m hello -v 1.0
sudo dkms install -m hello -v 1.0

# Status
dkms status

🏋️ Practice Exercises

  1. Config: Copy your current kernel config and explore make menuconfig
  2. (VM) Compile a custom kernel with a different LOCALVERSION string
  3. Module: Build and load the hello world kernel module
  4. Params: Modify the module to accept parameters
  5. dmesg: Watch kernel messages while loading/unloading modules
  6. DKMS: Package your module with DKMS

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