Re: [PATCH v2] Documentation: dev-tools: Add Testing Overview

[Marco Elver]

On Wed, 14 Apr 2021 at 10:15, David Gow <davidgow@xxxxxxxxxx> wrote:
>
> 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>

Reviewed-by: Marco Elver <elver@xxxxxxxxxx>

Looks good to me, thanks. I think one can write whole articles (or
even books) about this topic, so it's easy to forget this is a quick
overview, and keep on adding.

> ---
> Thanks, everyone, for the comments on the doc. I've made a few of the
> suggested changes. Please let me know what you think!
>
> -- David
>
> Changes since v1:
> https://lore.kernel.org/linux-kselftest/20210410070529.4113432-1-davidgow@xxxxxxxxxx/
> - Note KUnit's speed and that one should provide selftests for syscalls
> - Mention lockdep as a Dynamic Analysis Tool
> - Refer to "Dynamic Analysis Tools" instead of "Sanitizers"
> - A number of minor formatting tweaks and rewordings for clarity.
>
> Not changed:
> - I haven't included an exhaustive list of differences, advantages, etc,
>   between KUnit and kselftest: for now, the doc continues to focus on
>   the difference between 'in-kernel' and 'userspace' testing here.
> - Similarly, I'm not linking out to docs defining and describing "Unit"
>   tests versus "End-to-end" tests. None of the existing documentation
>   elsewhere quite matches what we do in the kernel perfectly, so it
>   seems less confusing to focus on the 'in-kernel'/'userspace'
>   distinction, and leave other definitions as a passing mention for
>   those who are already familiar with the concepts.
> - I haven't linked to any talk videos here: a few of them are linked on
>   (e.g.) the KUnit webpage, but I wanted to keep the Kernel documentation
>   more self-contained for now. No objection to adding them in a follow-up
>   patch if people feel strongly about it, though.
> - The link from index.rst to this doc is unchanged. I personally think
>   that the link is prominent enough there: it's the first link, and
>   shows up a few times. One possibility if people disagreed would be to
>   merge this page with the index, but given not all dev-tools are going
>   to be testing-related, it seemed a bit arrogant. :-)
>
>  Documentation/dev-tools/index.rst            |   3 +
>  Documentation/dev-tools/testing-overview.rst | 117 +++++++++++++++++++
>  2 files changed, 120 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..ce36a8cdf6b5
> --- /dev/null
> +++ b/Documentation/dev-tools/testing-overview.rst
> @@ -0,0 +1,117 @@
> +.. 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
> +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
> +------------------------------------------
> +
> +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
> +Documentation/dev-tools/kunit/style.rst
> +
> +
> +kselftest (Documentation/dev-tools/kselftest.rst), 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 from kselftest.
> +This means that only kernel functionality which is exposed to userspace somhow
> +(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.
> +
> +: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.
> +
> +: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.
> +
> +
> +Dynamic Analysis Tools
> +======================
> +
> +The kernel also supports a number of dynamic analysis tools, 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.
> +
> +Some of these tools are listed below:
> +
> +* kmemleak detects possible memory leaks. See
> +  Documentation/dev-tools/kmemleak.rst
> +* 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
> +  Documentation/locking/lockdep-design.rst
> +* 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 a sanitizer 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.
> +
> --
> 2.31.1.295.g9ea45b61b8-goog
>