Linux Kernel Programming

Cover
Title Page
Copyright and Credits
Dedication
Contributors
Table of Contents
Preface
Section 1: The Basics
Chapter 1: 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
	Summary
	Questions
	Further reading
Chapter 2: 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
		More on kbuild
			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
	Summary
	Questions
	Further reading
Chapter 3: 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
	Summary
	Questions
	Further reading
Chapter 4: 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
	Exploring LKMs
		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
			Kernel headers
			Module macros
			Entry and exit points
			Return values
				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
	Summary 
	Questions
	Further reading
Chapter 5: 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
	Summary
	Questions
	Further reading
Section 2: Understanding and Working with the Kernel
Chapter 6: 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
	Summary
	Questions
	Further reading
Chapter 7: 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
		User-mode ASLR
		KASLR
		Querying/setting KASLR status with a script
	Physical memory
		Physical RAM organization
			Nodes
			Zones
		Direct-mapped RAM and address translation
	Summary
	Questions
	Further reading
Chapter 8: 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
			Graphing it
		Slab layer implementations within the kernel
	Summary
	Questions
	Further reading
Chapter 9: 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
	Summary
	Questions
	Further reading
Chapter 10: 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
	Summary
	Questions
	Further reading
Chapter 11: 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
	Summary
	Questions
	Further reading
Section 3: Delving Deeper
Chapter 12: 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
	Summary
	Questions
	Further reading
Chapter 13: 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
				Fixing it
			Example 2 – catching an AB-BA deadlock with lockdep
		lockdep – annotations and issues
			lockdep annotations
			lockdep issues
		Lock statistics
			Viewing lock stats
	Memory barriers – an introduction
		An example of using memory barriers in a device driver
	Summary
	Questions
	Further reading
About Packt
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Index