|.. SPDX-License-Identifier: GPL-2.0
|Kernel Testing Guide
|There are a number of different tools for testing the Linux kernel, so knowing
|when to use each of them can be a challenge. This document provides a rough
|overview of their differences, and how they fit together.
|Writing and Running Tests
|The bulk of kernel tests are written using either the kselftest or KUnit
|frameworks. These both provide infrastructure to help make running tests and
|groups of tests easier, as well as providing helpers to aid in writing new
|If you're looking to verify the behaviour of the Kernel — particularly specific
|parts of the kernel — then you'll want to use KUnit or kselftest.
|The Difference Between KUnit and kselftest
|KUnit (Documentation/dev-tools/kunit/index.rst) is an entirely in-kernel system
|for "white box" testing: because test code is part of the kernel, it can access
|internal structures and functions which aren't exposed to userspace.
|KUnit tests therefore are best written against small, self-contained parts
|of the kernel, which can be tested in isolation. This aligns well with the
|concept of 'unit' testing.
|For example, a KUnit test might test an individual kernel function (or even a
|single codepath through a function, such as an error handling case), rather
|than a feature as a whole.
|This also makes KUnit tests very fast to build and run, allowing them to be
|run frequently as part of the development process.
|There is a KUnit test style guide which may give further pointers in
|kselftest (Documentation/dev-tools/kselftest.rst), on the other hand, is
|largely implemented in userspace, and tests are normal userspace scripts or
|This makes it easier to write more complicated tests, or tests which need to
|manipulate the overall system state more (e.g., spawning processes, etc.).
|However, it's not possible to call kernel functions directly from kselftest.
|This means that only kernel functionality which is exposed to userspace somehow
|(e.g. by a syscall, device, filesystem, etc.) can be tested with kselftest. To
|work around this, some tests include a companion kernel module which exposes
|more information or functionality. If a test runs mostly or entirely within the
|kernel, however, KUnit may be the more appropriate tool.
|kselftest is therefore suited well to tests of whole features, as these will
|expose an interface to userspace, which can be tested, but not implementation
|details. This aligns well with 'system' or 'end-to-end' testing.
|For example, all new system calls should be accompanied by kselftest tests.
|Code Coverage Tools
|The Linux Kernel supports two different code coverage measurement tools. These
|can be used to verify that a test is executing particular functions or lines
|of code. This is useful for determining how much of the kernel is being tested,
|and for finding corner-cases which are not covered by the appropriate test.
|Documentation/dev-tools/gcov.rst is GCC's coverage testing tool, which can be
|used with the kernel to get global or per-module coverage. Unlike KCOV, it
|does not record per-task coverage. Coverage data can be read from debugfs,
|and interpreted using the usual gcov tooling.
|Documentation/dev-tools/kcov.rst is a feature which can be built in to the
|kernel to allow capturing coverage on a per-task level. It's therefore useful
|for fuzzing and other situations where information about code executed during,
|for example, a single syscall is useful.
|Dynamic Analysis Tools
|The kernel also supports a number of dynamic analysis tools, which attempt to
|detect classes of issues when they occur in a running kernel. These typically
|each look for a different class of bugs, such as invalid memory accesses,
|concurrency issues such as data races, or other undefined behaviour like
|Some of these tools are listed below:
|* kmemleak detects possible memory leaks. See
|* KASAN detects invalid memory accesses such as out-of-bounds and
| use-after-free errors. See Documentation/dev-tools/kasan.rst
|* UBSAN detects behaviour that is undefined by the C standard, like integer
| overflows. See Documentation/dev-tools/ubsan.rst
|* KCSAN detects data races. See Documentation/dev-tools/kcsan.rst
|* KFENCE is a low-overhead detector of memory issues, which is much faster than
| KASAN and can be used in production. See Documentation/dev-tools/kfence.rst
|* lockdep is a locking correctness validator. See
|* There are several other pieces of debug instrumentation in the kernel, many
| of which can be found in lib/Kconfig.debug
|These tools tend to test the kernel as a whole, and do not "pass" like
|kselftest or KUnit tests. They can be combined with KUnit or kselftest by
|running tests on a kernel with these tools enabled: you can then be sure
|that none of these errors are occurring during the test.
|Some of these tools integrate with KUnit or kselftest and will
|automatically fail tests if an issue is detected.