Introduction to Linux Kernel Internals

What is an Operating System Kernel?

The kernel is the core component of any operating system. It manages:

• Hardware resources (CPU, ,memory, I/O devices)
• Process scheduling
• System calls
• Security
• Inter-process communication

In short, it acts as a bride between software applications and the physical hardware.

When you write an application like a web browser or a text editor, it cannot directly access hardware. Instead, it makes system calls to the kernel, which then interacts with the hardware of behalf of the application.

Kernel Space vs User Space

Kernel Space

• Privileged mode (ring 0)
• Full access to hardware and memory
• Where the kernel and drivers run

User Space

• Non-privileged mode (ring 3)
• Applications run here
• System calls are used to access kernel services

Context Switching happens when the CPU switches from user mode to kernel mode (e.g., during a system call).

Analogy: Think of kernel space as the engine room of a ship, while user space is where passengers relax. Only engineers (the kernel) can operate the machinery.

Monolithic vs Microkernel

Monolithic Kernel (like Linux):

• All core services (device drivers, file systems, memory management) run in kernel space.
• Pros: High performance, low overhead.
• Cons: Bugs in any module can crash the system.

Microkernel (e.g., MINIX, QNS)

• Only essential services run in kernel space; others run in user space.
• Pros: Better fault isolation
• Cons: More context switching → performance overhead

Linux is a monolithic kernel, but supports modular components (loadable modules) to improve flexibility.

Linux Kernel Structure and Subsystems

The Linux kernel consists of several core subsystems:

Kernel Modules vs Built-in Kernel Code

Built-in Code

• Compiled directly into the kernel binary (vmlinuz)
• Always loaded at boot
• Cannot be removed without recompiling the kernel

Loadable Kernel Modules (LKM)

• Compiled separately
• Can be loaded/unloaded dynamically at runtime
• Useful for drivers, filesystems, or experimental features

Module Lifecycle:

1. Load with insmod module.ko
2. Use (by kernel or user apps)
3. Unload with rmmod module
4. Check info with modinfo, lsmod

Tools and Commands Preview

Here are some tools you will use throughout driver development: