I presented this talk at Linaro Virtual Connect 2020. It's a follow up to my previous talk on the same subject at ELCE 2019 and I included additional material on license flags, SDKs, language package managers and SPDX document generation. Forthe discussion of language package managers, I used the Rust programming language and the associated Cargo build tool as an example.


Open Source license compliance is a concern for any company which distributes systems containing the Linux kernel and other copyleft-licensed components. The Yocto Project, a widely used Embedded Linux toolkit including the OpenEmbedded build system, contains many tools to help companies achieve license compliance. Output from a Yocto Project build can also be fed into other license compliance software such as Fossology. In this talk Paul will explore these tools, address how best to configure & connect them and discuss current best practices. Paul will explore how modern programming languages such as Go, Javascript and Rust cause license compliance headaches due to their separate package managers and how to resolve these issues. He will show how to handle common concerns such as commercially licensed media codecs and GPLv3 licensed software components. He will also bring the audience up to date with the latest developments and ongoing work in this area of the Yocto Project.

This talk will cover technical best practices and available tools, it will not give legal advice.



Slides Text

About Me


  • This is not legal advice
  • Best practices are given based on my experience as a developer and an open source community member
  • If in doubt, consult an appropriate lawyer

About This Talk

  • Introduction
  • License Compliance in Yocto Project
  • Language Package Managers
    • Case study on Rust and Cargo
  • SPDX document generation, Scancode and Fossology
  • Future Work
  • Will take questions on Slack after this talk

Previous Talk

Introduction: Why Care?

  • Selling an embedded device typically involves distribution of open source software
  • This carries the risk of legal action if not done properly
  • Doing this right gives you standing in the community
  • Need to keep sources anyway so you can rebuild old releases with minor changes
    • For debugging
    • To satisfy customer requests
  • Sources often disappear from the internet

Introduction: Yocto Project

  • Create a fully customised Embedded Linux distribution
  • Widely adopted, industry standard, welcoming community
  • Includes OpenEmbedded build system and other tools
  • Several features included to support license compliance
  • https://www.yoctoproject.org/

Recipes & Metadata

  • A recipe contains metadata & build commands for a piece of software
  • Example: hello_2.10.bb

LICENSE = "GPL-3.0-only"

LIC_FILES_CHKSUM = "file://COPYING;md5=d32239bcb673463ab874e80d47fae504"

SRC_URI = "https://ftp.gnu.org/gnu/hello/hello-${PV}.tar.gz"
SRC_URI[sha256sum] = "31e066137a962676e89f69d1b65382de95a7ef7d914b8cb956f41ea72e0f516b"

inherit gettext autotools

Providing Sources

  • Copyleft licenses typically require you to provide source code (including any modifications) along with compiled binaries.
  • Yocto Project supports this with the archiver class
  • Set INHERIT += "archiver" and choose the mode:
    • ARCHIVER_MODE = "original"
    • ARCHIVER_MODE = "patched"
    • ARCHIVER_MODE = "configured"
    • ARCHIVER_MODE = "mirror"
  • The archiver can be configured further

Providing License Text

  • Many licenses require you to provide the license text and copyright notice(s) along with compiled binaries.
  • Copy ${DEPLOY_DIR}/licenses after building an image
    • May need some pre- & post-processing
  • Include license text in images
    • Set COPY_LIC_MANIFEST = "1" & COPY_LIC_DIRS = "1"
    • Places files into /usr/share/common-licenses
  • Create license packages
    • Places license text in /usr/share/licenses
    • Provides an upgrade path for license text

Excluding Unwanted Licenses

  • The INCOMPATIBLE_LICENSE variable allows recipes to be excluded by license
    • Prevents accidental inclusion of unwanted code
  • Applies to target packages only
  • meta-gplv2 layer may be needed if excluding GPL 3.0 or later
  • Values in LICENSE and INCOMPATIBLE_LICENSE should be standardised on the SPDX License List to avoid confusion

License Flags

  • Another method of excluding recipes by license class
  • May be used to highlight non-copyright issues
    • Patented algorithms
    • Commercial license / EULA
  • Flagged recipes are excluded by default
    • Set LICENSE_FLAGS_WHITELIST to enable them

SDK Concerns

  • Yocto Project supports generation of an SDK / Extensible SDK (ESDK)
    • Allows app developers to build code outside Yocto Project
  • The archiver should capture sources for SDK components
    • This is not guaranteed for the Extensible SDK
  • Building with the SDK bypasses Yocto Project license compliance tooling
    • Be careful distributing third-party code built this way

Issues with Language Package Managers

  • Many newer languages use their own package managers
    • Go, NPM (nodejs), Cargo (Rust)
  • These present issues for Embedded development and license compliance
    • These just don’t seem to be first class concerns
  • Features we need from these package managers
    • Offline build support
    • Download source archive
      • Including license text & other collateral
    • HTTP/HTTPS proxy support
    • Source mirror support

Case Study: Rust (1)

  • Cargo is a build system and a language package manager for Rust
  • Projects usually contain a Cargo.toml file
    • Description, authors, license and other metadata
    • Dependencies
    • Configuration
  • Open Source Rust projects are typically published to crates.io
    • Provides search and download functionality
  • See https://www.rust-lang.org/ and https://crates.io/

Case Study: Rust (2)

  • Rust is supported in Yocto Project by the meta-rust layer
  • Recipes can be automatically generated by the cargo-bitbake tool
  • The cargo bbclass is used for building Rust projects
    • Performs offline builds using fetched crates
  • Integrates well with most Yocto tooling
    • Archiver, HTTP proxies, source mirrors all work
    • However, license text is not collected for dependency crates

Generating SPDX Documents

  • SPDX (https://spdx.dev/) is “An open standard for communicating software bill of material information, including components, licenses, copyrights, and security references.”
  • SPDX is supported in Yocto Project by the meta-spdxscanner layer
  • Supports scancode-toolkit for SPDX document generation

Integrating with Fossology

  • Fossology is a more fully featured system for compliance scanning and signoff
    • Runs as a service with a web interface and an API
  • Integration is also provided by the meta-spdxscanner layer
    • fossology-python or fossology-rest bbclasses may be used
    • Upload source code to a Fossology instance
  • Scanning, review and document generation is done asynchronously through the Fossology interface
    • SPDX documents are not generated directly as part of the Yocto Project build
  • See https://www.fossology.org/

Future Work

  • Better integration with language package managers
    • May require changes to NPM, Cargo, etc
  • Automatic generation of a plain text or HTML license document for an image
  • Integration with other license compliance tooling
  • License scanning & SPDX document generation for Yocto Project releases
    • Provide a feedback loop to confirm license metadata in recipes is correct
    • Non-trivial!