This document contains useful information how to test the Rust code in the kernel.

There are two sorts of tests:

  • The KUnit tests.

  • The #[test] tests.

The KUnit tests

These are the tests that come from the examples in the Rust documentation. They get transformed into KUnit tests.


These tests can be run via KUnit. For example via kunit_tool (kunit.py) on the command line:

./tools/testing/kunit/kunit.py run --make_options LLVM=1 --arch x86_64 --kconfig_add CONFIG_RUST=y

Alternatively, KUnit can run them as kernel built-in at boot. Refer to KUnit - Linux Kernel Unit Testing for the general KUnit documentation and KUnit Architecture for the details of kernel built-in vs. command line testing.

To use these KUnit doctests, the following must be enabled:

   Kernel hacking -> Kernel Testing and Coverage -> KUnit - Enable support for unit tests
   Kernel hacking -> Rust hacking -> Doctests for the `kernel` crate

in the kernel config system.

KUnit tests are documentation tests

These documentation tests are typically examples of usage of any item (e.g. function, struct, module...).

They are very convenient because they are just written alongside the documentation. For instance:

/// Sums two numbers.
/// ```
/// assert_eq!(mymod::f(10, 20), 30);
/// ```
pub fn f(a: i32, b: i32) -> i32 {
    a + b

In userspace, the tests are collected and run via rustdoc. Using the tool as-is would be useful already, since it allows verifying that examples compile (thus enforcing they are kept in sync with the code they document) and as well as running those that do not depend on in-kernel APIs.

For the kernel, however, these tests get transformed into KUnit test suites. This means that doctests get compiled as Rust kernel objects, allowing them to run against a built kernel.

A benefit of this KUnit integration is that Rust doctests get to reuse existing testing facilities. For instance, the kernel log would look like:

KTAP version 1
    KTAP version 1
    # Subtest: rust_doctests_kernel
    # rust_doctest_kernel_build_assert_rs_0.location: rust/kernel/build_assert.rs:13
    ok 1 rust_doctest_kernel_build_assert_rs_0
    # rust_doctest_kernel_build_assert_rs_1.location: rust/kernel/build_assert.rs:56
    ok 2 rust_doctest_kernel_build_assert_rs_1
    # rust_doctest_kernel_init_rs_0.location: rust/kernel/init.rs:122
    ok 3 rust_doctest_kernel_init_rs_0
    # rust_doctest_kernel_types_rs_2.location: rust/kernel/types.rs:150
    ok 59 rust_doctest_kernel_types_rs_2
# rust_doctests_kernel: pass:59 fail:0 skip:0 total:59
# Totals: pass:59 fail:0 skip:0 total:59
ok 1 rust_doctests_kernel

Tests using the ? operator are also supported as usual, e.g.:

/// ```
/// # use kernel::{spawn_work_item, workqueue};
/// spawn_work_item!(workqueue::system(), || pr_info!("x"))?;
/// # Ok::<(), Error>(())
/// ```

The tests are also compiled with Clippy under CLIPPY=1, just like normal code, thus also benefitting from extra linting.

In order for developers to easily see which line of doctest code caused a failure, a KTAP diagnostic line is printed to the log. This contains the location (file and line) of the original test (i.e. instead of the location in the generated Rust file):

# rust_doctest_kernel_types_rs_2.location: rust/kernel/types.rs:150

Rust tests appear to assert using the usual assert! and assert_eq! macros from the Rust standard library (core). We provide a custom version that forwards the call to KUnit instead. Importantly, these macros do not require passing context, unlike those for KUnit testing (i.e. struct kunit *). This makes them easier to use, and readers of the documentation do not need to care about which testing framework is used. In addition, it may allow us to test third-party code more easily in the future.

A current limitation is that KUnit does not support assertions in other tasks. Thus, we presently simply print an error to the kernel log if an assertion actually failed. Additionally, doctests are not run for nonpublic functions.

The #[test] tests

Additionally, there are the #[test] tests. These can be run using the rusttest Make target:

make LLVM=1 rusttest

This requires the kernel .config and downloads external repositories. It runs the #[test] tests on the host (currently) and thus is fairly limited in what these tests can test.