Linux Kernel Programming – A Comprehensive Guide to Kernel Internals, Writing Kernel Modules, and Kernel Synchronization

Linux Kernel Programming - A Comprehensive Guide to Kernel Internals, Writing Kernel Modules, and Kernel Synchronization

This book has been explicitly written with a view to helping you learn Linux kernel development in a practical, hands-on fashion, along with the necessary theoretical background to give you a well-rounded view of this vast and interesting topic area. It deliberately focuses on kernel development via the powerful Loadable Kernel Module (LKM) framework; the vast majority of kernel projects and products, which includes device driver development, are done in this manner.

The focus is kept on both working hands-on with, and understanding at a sufficiently deep level, the internals of the Linux OS. In these regards, we cover everything from building the Linux kernel from source through understanding and working with complex topics such as synchronization within the kernel.

To guide you on this exciting journey, we divide this book into three sections. The first section covers the basics – setting up a workspace required for kernel development, building the kernel from source, and writing your first kernel module.

The next section, a key one, will help you understand important and essential kernel internals – the Linux kernel architecture, the task structure, and user and kernel-mode stacks. Memory management is a key and interesting topic – we devote three whole chapters to it (covering the internals to a sufficient extent, and importantly, how exactly to allocate any free kernel memory). The working and deeper details of CPU scheduling on Linux round off this section.

The last section of the book deals with the more advanced topic of kernel synchronization – a necessity for professional design and code on the Linux kernel. We devote two whole chapters to covering key topics within this.

The book uses the, at the time of writing, latest 5.4 Long Term Support (LTS) Linux kernel. It’s a kernel that will be maintained (both bug and security fixes) from November 2019 right through December 2025! This is a key point, ensuring that this book’s content remains current and valid for years to come! We very much believe in a hands-on approach: over 20 kernel modules (besides several user apps and shell scripts) on this book’s GitHub repository make the learning come alive, making it fun, interesting, and useful.

We highly recommend you also make use of this book’s companion guide, Linux Kernel Programming (Part 2).

It’s an excellent industry-aligned beginner’s guide to writing misc character drivers, performing I/O on peripheral chip memory and handling hardware interrupts. You can get this book for free along with your copy, alternately you can also find this eBook in the GitHub repository at: https://github.com/PacktPublishing/Linux-Kernel-Programming/tree/master/Linux-Kernel-Programming-(Part-2).

We really hope you learn from and enjoy this book. Happy reading!

Who this book is for

This book is primarily for those of you beginning your journey in the vast arena of Linux kernel module development and, to some extent, Linux device driver development. It’s also very much targeted at those of you who have already been working on Linux modules and/or drivers, who wish to gain a much deeper, well-structured understanding of Linux kernel architecture, memory management, and synchronization. This level of knowledge about the underlying OS, covered in a properly structured manner, will help you no end when you face difficult-to-debug real-world situations.

What this book covers

Chapter 1, Kernel Workspace Setup, guides you on setting up a full-fledged Linux kernel development workspace (typically, as a fully virtualized guest system). You will learn how to install all required software packages on it, including a cross toolchain. You will also learn about several other open source projects that will be useful on your journey to becoming a professional kernel/driver developer. Once this chapter is done, you will be ready to build a Linux kernel as well as to start writing and testing kernel code (via the loadable kernel module framework). In our view, it’s very important for you to actually use this book in a hands-on fashion, trying out and experimenting with code. The best way to learn something is to do so empirically – not taking anyone’s word on anything at all, but by trying it out and experiencing it for yourself.

Chapter 2, Building the 5.x Linux Kernel from Source – Part 1, is the first part of explaining how to build the modern Linux kernel from scratch with source code. In this part, you will be given necessary background information – the version nomenclature, the different source trees, the layout of the kernel source – on the kernel source tree. Next, you will be shown in detail how exactly to download a stable vanilla Linux kernel source tree onto the VM. We shall then learn a little regarding the layout of the kernel source code, getting, in effect, a “10,000-foot view” of the kernel code base. The actual work of extracting and configuring the Linux kernel then follows. Creating and using a custom menu entry for kernel configuration is also shown.

Chapter 3, Building the 5.x Linux Kernel from Source – Part 2, is the second part on performing kernel builds from source code. In this part, you will continue from the previous chapter, now actually building the kernel, installing kernel modules, understanding what exactly initramfs (initrd) is and how to generate it, as well as setting up the bootloader (for x86). Also, as a valuable add-on, this chapter then explains how to cross-compile the kernel for a typical embedded ARM target (using the popular Raspberry Pi as a target device). Several tips and tricks on kernel builds, and even kernel security (hardening), are mentioned as well.

Chapter 4, Writing Your First Kernel Module – LKMs Part 1, is the first of two parts that cover a fundamental aspect of Linux kernel development – the LKM framework, and how it is to be understood and used by the “module user,” by you – the kernel module or device driver programmer. It covers the basics of the Linux kernel architecture and then, in great detail, every step involved in writing a simple “Hello, world” kernel module, compiling, inserting, checking, and removing it from the kernel space. We also cover kernel logging via the ubiquitous printk API in detail.

Chapter 5, Writing Your First Kernel Module – LKMs Part 2, is the second part that covers the LKM framework. Here, we begin with something critical – learning how to use a “better” Makefile, which will help you generate more robust code (having several code-checking, correction, static analysis targets, and so on). We then show in detail the steps to successfully cross-compile a kernel module for an alternate architecture, how to emulate “library-like” code in the kernel (via both the “linking” and the module-stacking approaches), defining and using passing parameters to your kernel module. Additional topics include the auto-loading of modules at boot, important security guidelines, and some information on the kernel documentation and how to access it. Several example kernel modules make the learning more interesting.

Chapter 6, Kernel Internals Essentials – Processes and Threads, delves into some essential kernel internals topics. We begin with what is meant by execution in process and interrupt contexts, and minimal but required coverage of the process user virtual address space (VAS) layout. This sets the stage for you; you’ll then learn about Linux kernel architecture in more depth, focusing on the organization of process/thread task structures and their corresponding stacks – user- and kernel-mode. We then show you more on the kernel task structure (a “root” data structure), how to practically glean information from it, and even iterate over various (task) lists. Several kernel modules make the topic come alive.

Chapter 7, Memory Management Internals – Essentials, a key chapter, delves into essential internals of the Linux memory management subsystem, to the level of detail required for the typical module author or driver developer. This coverage is thus necessarily more theoretical in nature; nevertheless, the knowledge gained here is crucial to you, the kernel developer, both for deep understanding and usage of appropriate kernel memory APIs as well as for performing meaningful debugging at the level of the kernel. We cover the VM split (and how it is on various actual architectures), gaining deep insight into the user VAS (our procmap utility will be an eye-opener), as well as the kernel segment (or kernel VAS). We then briefly delve into the security technique of memory layout randomization ([K]ASLR), and end this chapter with a discussion on physical memory organization within Linux.

Chapter 8, Kernel Memory Allocation for Module Authors Part 1, gets our hands dirty with the kernel memory allocation (and obviously, deallocation) APIs. You will first learn about the two allocation “layers” within Linux – the slab allocator that’s layered above the kernel memory allocation “engine,” and the page allocator (or BSA). We shall briefly learn about the underpinnings of the page allocator algorithm and its “freelist” data structure; this information is valuable when deciding which layer to use. Next, we dive straight into the hands-on work of learning about the usage of these key APIs. The ideas behind the slab allocator (or cache) and the primary kernel allocator APIs – the kzalloc/kfree – are covered. Importantly, the size limitations, downsides, and caveats when using these common APIs are covered in detail as well. Also, especially useful for driver authors, we cover the kernel’s modern resource-managed memory allocation APIs (the devm_*() routines).

Chapter 9, Kernel Memory Allocation for Module Authors Part 2, goes further, in a logical fashion, from the previous chapter. Here, you will learn how to create custom slab caches (useful for high-frequency (de)allocations for, say, a custom driver), along with some help regarding debugging memory allocations at the slab layer. Next, you’ll understand and use the vmalloc() API (and friends). Very importantly, having covered many APIs for kernel memory (de)allocation, you will now learn how to pick and choose an appropriate API given the real-world situation you find yourself in. This chapter is rounded off with important coverage of the kernel’s Out Of Memory (OOM) “killer” framework. Understanding it will also lead to a much deeper understanding of how user space memory allocation really works, via the demand paging technique.

Chapter 10, The CPU Scheduler – Part 1, the first part of two chapters, covers a useful mix of theory and practice regarding CPU scheduling on the Linux OS. The minimal necessary theoretical background on the thread as the KSE and available kernel scheduling policies are topics initially covered. Next, sufficient kernel internal details on CPU scheduling are covered to have you understand how scheduling on the modern Linux OS works. Along the way, you will learn how to “visualize” PU scheduling with powerful tools such as perf; thread scheduling attributes (policy and real-time priority) are delved into as well.

Chapter 11, The CPU Scheduler – Part 2, the second part on CPU scheduling, continues to cover the topic in more depth. Here, we cover further visualization tools for CPU scheduling (leveraging powerful software such as LTTng and the trace-cmd utility). Next, the CPU affinity mask and how to query/set it, controlling scheduling policy and priority on a per-thread basis – such a powerful feature! – are delved into. An overview of the meaning and importance of control groups (cgroups), along with an interesting example on CPU bandwidth allocation via cgroups v2 is seen. Can you run Linux as an RTOS? Indeed you can! The details on actually doing so are then shown. We round off this chapter with a discussion on (scheduling) latencies and how to measure them.

Chapter 12, Kernel Synchronization – Part 1, first covers the key concepts regarding critical sections, atomicity, what a lock conceptually achieves and, very importantly, the why of all this. We then cover concurrency concerns when working within the Linux kernel; this moves us naturally on to important locking guidelines, what deadlock means, and key approaches to preventing deadlock. Two of the most popular kernel locking technologies – the mutex lock and the spinlock – are then discussed in depth along with several (driver) code examples.

Chapter 13, Kernel Synchronization – Part 2, continues the journey on kernel synchronization. Here, you’ll learn about key locking optimizations – using lightweight atomic and (the more recent) refcount operators to safely operate on integers, RMW bit operators to safely perform bit ops, and the usage of the reader-writer spinlock over the regular one. Inherent risks, such as cache “false sharing” are discussed as well. An overview of lock-free programming techniques (with an emphasis on per-CPU variables and their usage, along with examples) is then covered. A critical topic – lock debugging techniques, including the usage of the kernel’s powerful “lockdep” lock validator, is then covered. The chapter is rounded off with a brief look at memory barriers (along with an example).

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