[PATCH] Documentation: dev-tools: Add Testing Overview

[David Gow]

The kernel now has a number of testing and debugging tools, and we've
seen a bit of confusion about what the differences between them are.

Add a basic documentation outlining the testing tools, when to use each,
and how they interact.

This is a pretty quick overview rather than the idealised "kernel
testing guide" that'd probably be optimal, but given the number of times
questions like "When do you use KUnit and when do you use Kselftest?"
are being asked, it seemed worth at least having something. Hopefully
this can form the basis for more detailed documentation later.

Signed-off-by: David Gow <davidgow@xxxxxxxxxx>
---
 Documentation/dev-tools/index.rst            |   3 +
 Documentation/dev-tools/testing-overview.rst | 102 +++++++++++++++++++
 2 files changed, 105 insertions(+)
 create mode 100644 Documentation/dev-tools/testing-overview.rst

diff --git a/Documentation/dev-tools/index.rst b/Documentation/dev-tools/index.rst
index 1b1cf4f5c9d9..f590e5860794 100644
--- a/Documentation/dev-tools/index.rst
+++ b/Documentation/dev-tools/index.rst
@@ -7,6 +7,8 @@ be used to work on the kernel. For now, the documents have been pulled
 together without any significant effort to integrate them into a coherent
 whole; patches welcome!
 
+A brief overview of testing-specific tools can be found in :doc:`testing-overview`.
+
 .. class:: toc-title
 
 	   Table of contents
@@ -14,6 +16,7 @@ whole; patches welcome!
 .. toctree::
    :maxdepth: 2
 
+   testing-overview
    coccinelle
    sparse
    kcov
diff --git a/Documentation/dev-tools/testing-overview.rst b/Documentation/dev-tools/testing-overview.rst
new file mode 100644
index 000000000000..8452adcb8608
--- /dev/null
+++ b/Documentation/dev-tools/testing-overview.rst
@@ -0,0 +1,102 @@
+.. 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 :doc:`kselftest
+<kselftest>` or :doc:`KUnit <kunit/index>` 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 tests.
+
+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
+------------------------------------------
+
+:doc:`KUnit <kunit/index>` 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.
+
+There is a KUnit test style guide which may give further pointers
+
+
+:doc:`kselftest <kselftest>`, on the other hand, is largely implemented in
+userspace, and tests are normal userspace scripts or programs.
+
+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 unless they're
+exposed to userspace (by a syscall, device, filesystem, etc.) Some tests to
+also provide a kernel module which is loaded by the test, though for tests
+which run mostly or entirely within the kernel, `KUnit` may be the better 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.
+
+
+Code Coverage Tools
+===================
+
+The Linux Kernel supports two different code coverage mesurement 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.
+
+:doc:`kcov` 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.
+
+:doc:`gcov` 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.
+
+
+Sanitizers
+==========
+
+The kernel also supports a number of sanitizers, which attempt to detect
+classes of issues when the occur in a running kernel. These typically
+look for undefined behaviour of some kind, such as invalid memory accesses,
+concurrency issues such as data races, or other undefined behaviour like
+integer overflows.
+
+* :doc:`kmemleak` (Kmemleak) detects possible memory leaks.
+* :doc:`kasan` detects invalid memory accesses such as out-of-bounds and
+  use-after-free errors.
+* :doc:`ubsan` detects behaviour that is undefined by the C standard, like
+  integer overflows.
+* :doc:`kcsan` detects data races.
+* :doc:`kfence` is a low-overhead detector of memory issues, which is much
+  faster than KASAN and can be used in production.
+
+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 a sanitizer enabled: you can then be sure
+that none of these errors are occurring during the test.
+
+Some of these sanitizers integrate with KUnit or kselftest and will
+automatically fail tests if an issue is detected by a sanitizer.
+
-- 
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