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