KUnit

Contents:

What is KUnit?

KUnit is a lightweight unit testing framework for the Linux kernel. These tests are able to be run locally on a developer’s workstation without a VM or special hardware.

KUnit is heavily inspired by JUnit, Python’s unittest.mock, and Googletest/Googlemock for C++. KUnit provides facilities for defining unit test cases, grouping related test cases into test suites, providing common infrastructure for running tests, and more.

Check out the Getting Started guide.

Who is it for?

If you work on the Linux kernel, then KUnit is for you.

Why unit test the kernel?

The first question is, “why write unit tests in the first place?”

A unit test is a test focused on testing a small “unit” of code. This is usually on the level of a few functions at most. Unit tests try to avoid external dependencies like hardware and/or stub them out where possible.

This ideally makes unit tests more stable, faster, and easier to debug when they fail.

In-kernel testing

Note

In our case, “unit test” implies in-kernel test code. It’s not really possible to be a “unit test” if you’re relying on a kernel working enough to have a functional userspace on top of it.

For internal libraries (e.g. include/linux/list.h), there’s no good way to test from userspace. It’s far easier to write in-kernel code to test these. That are some tests that make this work, but if you’re starting from scratch, just use KUnit.

For code with a user facing component, you’ll want some tests in userspace. But it can be useful to have unit tests as well. For example, it can be hard to trigger certain code paths from userspace, particularly error paths. Furthermore, it’s probably much faster and more hermetic to build and run the tests under KUnit instead of booting a machine and waiting for all the needed userspace processes to come up.

Why KUnit?

KUnit provides a library to help making writing tests easy: including making assertions, setting up and cleaning up test resources, and more. It also provides a python script to make building and running tests easy.

kselftest has a way of writing unit tests as kernel modules via the helpers in tools/testing/selftests/kselftest_module.h. But this requires more boilerplate scattered across several files and potentially several directories.

Check out the Kernel Testing Guide for an overview of when to use KUnit.

kunit.py and UML

KUnit provides a simple python script, kunit.py which can automate configuring, building, and running kernels with KUnit tests. It also parses test results in KTAP format, providing a nice summary.

To get started, run: ./tools/testing/kunit/kunit.py run

KUnit is able to run tests without needing a virtual machine or actual hardware with User Mode Linux. User Mode Linux is a Linux architecture, like ARM or x86; however, unlike other architectures it compiles to a standalone program that can be run like any other program directly inside of a host operating system; to be clear, it does not require any virtualization support; it is just a regular program. User Mode Linux is fast: on my desktop it boots to init process in under a second.

The provided tooling (tools/testing/kunit/kunit.py) uses UML by default, but also integrates with QEMU to support other architectures.

How do I use it?

All of these can also be found in the Documentation/ directory in the kernel source tree.

Where do I get it?

  • Upstream: https://www.kernel.org/ (version 5.5 or later)

  • KUnit can be found in the lib/kunit directory.

  • kunit.py lives in the tools/testing/kunit directory.

Connect with us

Contributing

If you want to contribute to KUnit in the Linux kernel (which is just the same as contributing to the Linux kernel, please see the Linux kernel’s guide on contributing.

For other KUnit repositories (CI/CD, vim plugin, etc), please see CONTRIBUTING.md.

In all cases, you will also want to take a look at Development.