Book Image

Linux Kernel Programming

By : Kaiwan N. Billimoria

Book Image

Linux Kernel Programming

By: Kaiwan N. Billimoria

Overview of this book

Linux Kernel Programming is a comprehensive introduction for those new to Linux kernel and module development. This easy-to-follow guide will have you up and running with writing kernel code in next-to-no time. This book uses the latest 5.4 Long-Term Support (LTS) Linux kernel, which will be maintained from November 2019 through to December 2025. By working with the 5.4 LTS kernel throughout the book, you can be confident that your knowledge will continue to be valid for years to come. You’ll start the journey by learning how to build the kernel from the source. Next, you’ll write your first kernel module using the powerful Loadable Kernel Module (LKM) framework. The following chapters will cover key kernel internals topics including Linux kernel architecture, memory management, and CPU scheduling. During the course of this book, you’ll delve into the fairly complex topic of concurrency within the kernel, understand the issues it can cause, and learn how they can be addressed with various locking technologies (mutexes, spinlocks, atomic, and refcount operators). You’ll also benefit from more advanced material on cache effects, a primer on lock-free techniques within the kernel, deadlock avoidance (with lockdep), and kernel lock debugging techniques. By the end of this kernel book, you’ll have a detailed understanding of the fundamentals of writing Linux kernel module code for real-world projects and products.

Preface

Who this book is for

What this book covers

To get the most out of this book

Download the example code files

Download the color images

Conventions used

Section 1: The Basics

Section 1: The Basics

Free Chapter

Kernel Workspace Setup

Kernel Workspace Setup

Technical requirements

Running Linux as a guest VM

Installing a 64-bit Linux guest

Turn on your x86 system's virtualization extension support

Allocate sufficient space to the disk

Install the Oracle VirtualBox Guest Additions

Experimenting with the Raspberry Pi

Setting up the software – distribution and packages

Installing software packages

Installing the Oracle VirtualBox guest additions

Installing required software packages

Installing a cross toolchain and QEMU

Installing a cross compiler

Important installation notes

Additional useful projects

Using the Linux man pages

The tldr variant

Locating and using the Linux kernel documentation

Generating the kernel documentation from source

Static analysis tools for the Linux kernel

Linux Trace Toolkit next generation

The procmap utility

Simple Embedded ARM Linux System FOSS project

Modern tracing and performance analysis with [e]BPF

The LDV - Linux Driver Verification - project

Further reading

Building the 5.x Linux Kernel from Source - Part 1

Building the 5.x Linux Kernel from Source - Part 1

Technical requirements

Preliminaries for the kernel build

Kernel release nomenclature

Kernel development workflow – the basics

Types of kernel source trees

Steps to build the kernel from source

Step 1 – obtaining a Linux kernel source tree

Downloading a specific kernel tree

Cloning a Git tree

Step 2 – extracting the kernel source tree

A brief tour of the kernel source tree

Step 3 – configuring the Linux kernel

Understanding the kbuild build system

Arriving at a default configuration

Obtaining a good starting point for kernel configuration

Kernel config for typical embedded Linux systems

Kernel config using distribution config as a starting point

Tuned kernel config via the localmodconfig approach

Getting started with the localmodconfig approach

Tuning our kernel configuration via the make menuconfig UI

Sample usage of the make menuconfig UI

Looking up the differences in configuration

Customizing the kernel menu – adding our own menu item

The Kconfig* files

Creating a new menu item in the Kconfig file

A few details on the Kconfig language

Further reading

Building the 5.x Linux Kernel from Source - Part 2

Building the 5.x Linux Kernel from Source - Part 2

Technical requirements

Step 4 – building the kernel image and modules

Step 5 – installing the kernel modules

Locating the kernel modules within the kernel source

Getting the kernel modules installed

Step 6 – generating the initramfs image and bootloader setup

Generating the initramfs image on Fedora 30 and above

Generating the initramfs image – under the hood

Understanding the initramfs framework

Why the initramfs framework?

Understanding the basics of the boot process on the x86

More on the initramfs framework

Step 7 – customizing the GRUB bootloader

Customizing GRUB – the basics

Selecting the default kernel to boot into

Booting our VM via the GNU GRUB bootloader

Experimenting with the GRUB prompt

Verifying our new kernel's configuration

Kernel build for the Raspberry Pi

Step 1 – cloning the kernel source tree

Step 2 – installing a cross-toolchain

First method – package install via apt

Second method – installation via the source repo

Step 3 – configuring and building the kernel

Miscellaneous tips on the kernel build

Minimum version requirements

Building a kernel for another site

Watching the kernel build run

A shortcut shell syntax to the build procedure

Dealing with compiler switch issues

Dealing with missing OpenSSL development headers

Further reading

Writing Your First Kernel Module - LKMs Part 1

Writing Your First Kernel Module - LKMs Part 1

Technical requirements

Understanding kernel architecture – part 1

User space and kernel space

Library and system call APIs

Kernel space components

The LKM framework

Kernel modules within the kernel source tree

Writing our very first kernel module

Introducing our Hello, world LKM C code

Breaking it down

Entry and exit points

The 0/-E return convention

The ERR_PTR and PTR_ERR macros

The __init and __exit keywords

Common operations on kernel modules

Building the kernel module

Running the kernel module

A quick first look at the kernel printk()

Listing the live kernel modules

Unloading the module from kernel memory

Our lkm convenience script

Understanding kernel logging and printk

Using the kernel memory ring buffer

Kernel logging and systemd's journalctl

Using printk log levels

The pr_ convenience macros

Wiring to the console

Writing output to the Raspberry Pi console

Enabling the pr_debug() kernel messages

Rate limiting the printk instances

Generating kernel messages from the user space

Standardizing printk output via the pr_fmt macro

Portability and the printk format specifiers

Understanding the basics of a kernel module Makefile

Further reading

Writing Your First Kernel Module - LKMs Part 2

Writing Your First Kernel Module - LKMs Part 2

Technical requirements

A "better" Makefile template for your kernel modules

Configuring a "debug" kernel

Cross-compiling a kernel module

Setting up the system for cross-compilation

Attempt 1 – setting the "special" environment variables

Attempt 2 – pointing the Makefile to the correct kernel source tree for the target

Attempt 3 – cross-compiling our kernel module

Attempt 4 – cross-compiling our kernel module

Gathering minimal system information

Being a bit more security-aware

Licensing kernel modules

Emulating "library-like" features for kernel modules

Performing library emulation via multiple source files

Understanding function and variable scope in a kernel module

Understanding module stacking

Trying out module stacking

Passing parameters to a kernel module

Declaring and using module parameters

Getting/setting module parameters after insertion

Module parameter data types and validation

Validating kernel module parameters

Overriding the module parameter's name

Hardware-related kernel parameters

Floating point not allowed in the kernel

Auto-loading modules on system boot

Module auto-loading – additional details

Kernel modules and security – an overview

Proc filesystem tunables affecting the system log

The cryptographic signing of kernel modules

Disabling kernel modules altogether

Coding style guidelines for kernel developers

Contributing to the mainline kernel

Getting started with contributing to the kernel

Further reading

Section 2: Understanding and Working with the Kernel

Section 2: Understanding and Working with the Kernel

Kernel Internals Essentials - Processes and Threads

Kernel Internals Essentials - Processes and Threads

Technical requirements

Understanding process and interrupt contexts

Understanding the basics of the process VAS

Organizing processes, threads, and their stacks – user and kernel space

User space organization

Kernel space organization

Summarizing the current situation

Viewing the user and kernel stacks

Traditional approach to viewing the stacks

Viewing the kernel space stack of a given thread or process

Viewing the user space stack of a given thread or process

[e]BPF – the modern approach to viewing both stacks

The 10,000-foot view of the process VAS

Understanding and accessing the kernel task structure

Looking into the task structure

Accessing the task structure with current

Determining the context

Working with the task structure via current

Built-in kernel helper methods and optimizations

Trying out the kernel module to print process context info

Seeing that the Linux OS is monolithic

Coding for security with printk

Iterating over the kernel's task lists

Iterating over the task list I – displaying all processes

Iterating over the task list II – displaying all threads

Differentiating between the process and thread – the TGID and the PID

Iterating over the task list III – the code

Further reading

Memory Management Internals - Essentials

Memory Management Internals - Essentials

Technical requirements

Understanding the VM split

Looking under the hood – the Hello, world C program

Going beyond the printf() API

VM split on 64-bit Linux systems

Virtual addressing and address translation

The process VAS – the full view

Examining the process VAS

Examining the user VAS in detail

Directly viewing the process memory map using procfs

Interpreting the /proc/PID/maps output

The vsyscall page

Frontends to view the process memory map

The procmap process VAS visualization utility

Understanding VMA basics

Examining the kernel segment

High memory on 32-bit systems

Writing a kernel module to show information about the kernel segment

Viewing the kernel segment on a Raspberry Pi via dmesg

Macros and variables describing the kernel segment layout

Trying it out – viewing kernel segment details

The kernel VAS via procmap

Trying it out – the user segment

The null trap page

Viewing kernel documentation on the memory layout

Randomizing the memory layout – KASLR

Querying/setting KASLR status with a script

Physical memory

Physical RAM organization

Direct-mapped RAM and address translation

Further reading

Kernel Memory Allocation for Module Authors - Part 1

Kernel Memory Allocation for Module Authors - Part 1

Technical requirements

Introducing kernel memory allocators

Understanding and using the kernel page allocator (or BSA)

The fundamental workings of the page allocator

Freelist organization

The workings of the page allocator

Working through a few scenarios

The simplest case

A more complex case

The downfall case

Page allocator internals – a few more details

Learning how to use the page allocator APIs

Dealing with the GFP flags

Freeing pages with the page allocator

Writing a kernel module to demo using the page allocator APIs

Deploying our lowlevel_mem_lkm kernel module

The page allocator and internal fragmentation

The exact page allocator APIs

The GFP flags – digging deeper

Never sleep in interrupt or atomic contexts

Understanding and using the kernel slab allocator

The object caching idea

Learning how to use the slab allocator APIs

Allocating slab memory

Freeing slab memory

Data structures – a few design tips

The actual slab caches in use for kmalloc

Writing a kernel module to use the basic slab APIs

Size limitations of the kmalloc API

Testing the limits – memory allocation with a single call

Checking via the /proc/buddyinfo pseudo-file

Slab allocator – a few additional details

Using the kernel's resource-managed memory allocation APIs

Additional slab helper APIs

Control groups and memory

Caveats when using the slab allocator

Background details and conclusions

Testing slab allocation with ksize() – case 1

Testing slab allocation with ksize() – case 2

Interpreting the output from case 2

Slab layer implementations within the kernel

Further reading

Kernel Memory Allocation for Module Authors - Part 2

Kernel Memory Allocation for Module Authors - Part 2

Technical requirements

Creating a custom slab cache

Creating and using a custom slab cache within a kernel module

Creating a custom slab cache

Using the new slab cache's memory

Destroying the custom cache

Custom slab – a demo kernel module

Understanding slab shrinkers

The slab allocator – pros and cons – a summation

Debugging at the slab layer

Debugging through slab poisoning

Trying it out – triggering a UAF bug

SLUB debug options at boot and runtime

Understanding and using the kernel vmalloc() API

Learning to use the vmalloc family of APIs

A brief note on memory allocations and demand paging

Friends of vmalloc()

Specifying the memory protections

Testing it – a quick Proof of Concept

Why make memory read-only?

The kmalloc() and vmalloc() APIs – a quick comparison

Memory allocation in the kernel – which APIs to use when

Visualizing the kernel memory allocation API set

Selecting an appropriate API for kernel memory allocation

A word on DMA and CMA

Stayin' alive – the OOM killer

Reclaiming memory – a kernel housekeeping task and OOM

Deliberately invoking the OOM killer

Invoking the OOM killer via Magic SysRq

Invoking the OOM killer with a crazy allocator program

Understanding the rationale behind the OOM killer

Case 1 – vm.overcommit set to 2, overcommit turned off

Case 2 – vm.overcommit set to 0, overcommit on, the default

Demand paging and OOM

Understanding the OOM score

Further reading

The CPU Scheduler - Part 1

The CPU Scheduler - Part 1

Technical requirements

Learning about the CPU scheduling internals – part 1 – essential background

What is the KSE on Linux?

The POSIX scheduling policies

Visualizing the flow

Using perf to visualize the flow

Visualizing the flow via alternate (CLI) approaches

Learning about the CPU scheduling internals – part 2

Understanding modular scheduling classes

Asking the scheduling class

A word on CFS and the vruntime value

Threads – which scheduling policy and priority

Learning about the CPU scheduling internals – part 3

Who runs the scheduler code?

When does the scheduler run?

The timer interrupt part

The process context part

Preemptible kernel

CPU scheduler entry points

The context switch

Further reading

The CPU Scheduler - Part 2

The CPU Scheduler - Part 2

Technical requirements

Visualizing the flow with LTTng and trace-cmd

Visualization with LTTng and Trace Compass

Recording a kernel tracing session with LTTng

Reporting with a GUI – Trace Compass

Visualizing with trace-cmd

Recording a sample session with trace-cmd record

Reporting and interpretation with trace-cmd report (CLI)

Reporting and interpretation with a GUI frontend

Understanding, querying, and setting the CPU affinity mask

Querying and setting a thread's CPU affinity mask

Using taskset(1) to perform CPU affinity

Setting the CPU affinity mask on a kernel thread

Querying and setting a thread’s scheduling policy and priority

Within the kernel – on a kernel thread

CPU bandwidth control with cgroups

Looking up cgroups v2 on a Linux system

Trying it out – a cgroups v2 CPU controller

Converting mainline Linux into an RTOS

Building RTL for the mainline 5.x kernel (on x86_64)

Obtaining the RTL patches

Applying the RTL patch

Configuring and building the RTL kernel

Mainline and RTL – technical differences summarized

Latency and its measurement

Measuring scheduling latency with cyclictest

Getting and applying the RTL patchset

Installing cyclictest (and other required packages) on the device

Running the test cases

Viewing the results

Measuring scheduler latency via modern BPF tools

Further reading

Section 3: Delving Deeper

Section 3: Delving Deeper

Kernel Synchronization - Part 1

Kernel Synchronization - Part 1

Critical sections, exclusive execution, and atomicity

What is a critical section?

A classic case – the global i ++

Concepts – the lock

A summary of key points

Concurrency concerns within the Linux kernel

Multicore SMP systems and data races

Preemptible kernels, blocking I/O, and data races

Hardware interrupts and data races

Locking guidelines and deadlocks

Mutex or spinlock? Which to use when

Determining which lock to use – in theory

Determining which lock to use – in practice

Using the mutex lock

Initializing the mutex lock

Correctly using the mutex lock

Mutex lock and unlock APIs and their usage

Mutex lock – via [un]interruptible sleep?

Mutex locking – an example driver

The mutex lock – a few remaining points

Mutex lock API variants

The mutex trylock variant

The mutex interruptible and killable variants

The mutex io variant

The semaphore and the mutex

Priority inversion and the RT-mutex

Internal design

Using the spinlock

Spinlock – simple usage

Spinlock – an example driver

Test – sleep in an atomic context

Testing on a 5.4 debug kernel

Testing on a 5.4 non-debug distro kernel

Locking and interrupts

Using spinlocks – a quick summary

Further reading

Kernel Synchronization - Part 2

Kernel Synchronization - Part 2

Using the atomic_t and refcount_t interfaces

The newer refcount_t versus older atomic_t interfaces

The simpler atomic_t and refcount_t interfaces

Examples of using refcount_t within the kernel code base

64-bit atomic integer operators

Using the RMW atomic operators

RMW atomic operations – operating on device registers

Using the RMW bitwise operators

Using bitwise atomic operators – an example

Efficiently searching a bitmask

Using the reader-writer spinlock

Reader-writer spinlock interfaces

A word of caution

The reader-writer semaphore

Cache effects and false sharing

Lock-free programming with per-CPU variables

Per-CPU variables

Working with per-CPU

Allocating, initialization, and freeing per-CPU variables

Performing I/O (reads and writes) on per-CPU variables

Per-CPU – an example kernel module

Per-CPU usage within the kernel

Lock debugging within the kernel

Configuring a debug kernel for lock debugging

The lock validator lockdep – catching locking issues early

Examples – catching deadlock bugs with lockdep

Example 1 – catching a self deadlock bug with lockdep

Example 2 – catching an AB-BA deadlock with lockdep

lockdep – annotations and issues

lockdep annotations

Lock statistics

Viewing lock stats

Memory barriers – an introduction

An example of using memory barriers in a device driver

Further reading

About Packt

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Creating and using a custom slab cache within a kernel module

In this section, we're about to create, use, and subsequently destroy a custom slab cache. At a broad level, we'll be performing the following steps:

Creating a custom slab cache of a given size with the kmem_cache_create() API. This is often done as part of the init code path of the kernel module (or within the probe method when in a driver).
Using the slab cache. Here we will do the following:
1. Issue the kmem_cache_alloc() API to allocate a single instance of the custom object(s) within your slab cache.
2. Use the object.
3. Free it back to the cache with the kmem_cache_free() API.

Destroying the custom slab cache when done with kmem_cache_destroy(). This is often done as part of the cleanup code path of the kernel module (or within the remove/detach/disconnect method when in a driver).

Let's explore each of these APIs in a bit of detail. We start...