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Embedded Linux Essentials

A collection of topics related to (Embedded) Linux that a professional in this field should be familiar with.

The general chapters and main topics associated to the individual subchapters are meant to represent the broad knowledge that can be expected from any Embedded Linux professional. For some topics, detailed knowledge could be expected from more advanced professionals - the corresponding sections can be expanded with a click.

Beyond this point, there is only a rough guidance on what level of detail could be expected in relation to a professional's experience grade, represented by the level of indentation.

Contents

Some helpful learning resources are attached to the individual topics in the respective details section.

1 Linux Fundamentals

General concepts & properties of any Linux based system (including desktop or server distributions). A solid foundation here allows to reuse as many OS mechanisms as possible and to avoid building conceptually broken software.

Resources:

1.1 Principles & Concepts

The Linux Kernel

How the kernel operates and interacts with the applications on the system.

Subtopics:

  • scheduling
  • signals
Kernel- & Userspace

How the two separate runtime contexts differ and interact.

Subtopics:

  • system calls
Everything is a File

How the system's entities are represented to the user.

Subtopics:

  • file descriptors
  • symlinks, hardlinks
Processes & Threads

How application logic is executed.
User & Access Management

How access to and ownership of data and execution logic is handled.

Subtopics:

  • Access Control Lists (ACL)
  • Passwords & keys
  • Real & effective IDs
  • Users (id, whoami, sudo, su)
  • Ownership (chown, chmod)
System Startup & Shutdown

How the system enters & quits operation.

From 1st stage to 2nd stage bootloader, kernel (+ initramfs) and finally the system manager (systemd) and userspace.

1.2 Filesystem

System Root

How the filesystem is structured, visible and protected against manipulation.

Subtopics:

  • rootfs
  • chroot
Block Device Filesystems

How to choose and work with the filesystems on disc.

Subtopics:

  • types (ext4, squashfs, ubifs)
  • utilities
    • mkfs
    • fsck
  • mounting (mount, umount)
Special Filesystems

How to use the filesystems exposing kernel features or virtually extending the system.

Subtopics:

  • procfs
  • sysfs
  • tmpfs
  • overlayfs

1.3 Partitioning

Partition Table

How to define and interpret the system's partition table.

Subtopics:

  • formats (msdos, gpt)
  • utilities (parted)
Mounting

How to add filesystems to the rootfs.

Subtopics:

  • formats (msdos, gpt)
  • utilities (parted)

1.4 Processing & Communication

Event Handling

How to detect and react on (file) events.

Subtopics:

  • ppoll, select
Interprocess Communication (IPC)

How to use built-in features to communicate between processes.

Subtopics:

  • sockets
  • pipes
  • shared memory API (shm_open, mmap, ...)
  • message queues
  • signals

1.5 System Management

Orchestration

How to manage dependencies, resources and the lifetime of applications on the system.

Subtopics:

  • systemd
    • units
    • overrides
    • utilities (systemctl, journalctl, systemd-run, systemd-analyze, ...)
  • cgroups
Monitoring

How to monitor applications on the system.

Subtopics:

  • Software watchdog (systemd_notify)

1.6 External Devices

Device Rules

How to integrate external devices.

Subtopics:

  • udev

1.7 Networking

Networking & Protocols

How to interconnect multiple devices in a network.

Subtopics:

  • ISO/OSI protocol layers and common protocols (IPv4, IPv6, TCP, UDP, IEEE 802.11)
  • Configuring network interfaces in Linux (netctl, NetworkManager, ConnMan, iwd, wpa_supplicant, systemd-networkd, ...)
  • Troubleshooting (ping, traceroute, ssh, netstat, nmap, dig, Wireshark)

2 Embedded Specifics

Topics mostly relating to embedded devices and their specific product use case.

General resources:

2.1 Board Support

Bootloader

How to interact with and/or extend the behavior of the bootloader.

Subtopics:

  • U-Boot
    • Repository structure
    • Configuring / patching u-boot
    • u-boot environments
    • fw_*env tools
  • barebox
  • grub
    • grub.env
  • (efi)
Device Trees

How to enable hardware features in the operating system.

Resources:

Drivers

How to use (or implement) drivers for extended system features.

Resources:

Peripherals

How to interact with peripherals on the hardware board.

Subtopics:

  • HSM / TPM
  • Flash memory
    • eMMC
    • NAND/NOR

2.2 Libraries & Tools

Constrained Devices Support

How to use the reduced feature set of embedded-specific tools.

Subtopics:

  • busybox
Cryptography

How to implement security features with existing libraries.

Subtopics:

  • openssl
  • libsodium
  • wolfssl
  • Mbed TLS
Binaries

How binaries are assembled in Linux.

Subtopics:

  • static & dynamic linking
  • RPATH, LD_PRELOAD and other environment variables
  • cross-compilation, canadian cross concept
  • troubleshooting (ldd, readelf, objdump, file)
Flashing tools

How to deploy software on the actual hardware.
System testing

What frameworks to use when testing with hardware-in-the-loop.

3 Working with Linux

Knowledge helpful when working with Linux-based systems.

File operations

How to use the built-in tools.

Subtopics:

  • ls, cat, head, tail, less, echo
  • grep, awk
Editors

How to view and edit files on the device.

Subtopics:

  • vi / vim
  • nano
Scripting

How to implement complex logic using the native command line.

Subtopics:

  • sh / bash
  • zsh, hsh, ...
  • interoperability (e.g. avoiding bashisms)
Process Management

How to use the built-in process related tools.

Subtopics:

  • fuser, lsof
  • kill
Logging

How to leverage built-in logging features.

Subtopics:

  • dmesg
  • syslog (logger)
  • journald
Application Analytics

How to get performance metrics, profiles and call graphs for applications.

Subtopics:

  • perf, gprof, hotspot
  • valgrind, kcachegrind, ...
System Inspection

How to gather information about the operating system and applications.

Subtopics:

  • Disk usage (df, du)
  • Runtime stats (ps, top)
Debugging

How to analyse the system in case of unexpected behavior.

Subtopics:

  • coredump
  • kdump

4 Building Blocks

Partial solutions or technical approaches for use cases commonly seen in embedded Linux products.

4.1 Build Systems

Yocto

How the package-based image build system works and when to choose it.

Resources:

Buildroot

How the distribution build system works and when to choose it.

4.2 Application Management

Package Managers

How to manage static application versioning and dependencies on the device.

Subtopics:

  • dpkg / apt
  • ipkg / opkg
  • dnf / rpm

4.3 Update Frameworks

Update Clients

How to handle updates on the device.

Subtopics:

  • rauc
  • swupdate

Resources:

Incremental Update Support

How to reduce update transmission bandwidth.

Subtopics:

  • casync
  • OSTree

4.4 Robustness

Read-only Rootfs

How to reduce the number of writes to the rootfs and protect it against undesired manipulation.

Subtopics:

  • systemd requirements
  • pre-init & overlays

Resources:

4.5 Security

TODO: Extend security section.