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

[Daniel Latypov <dlatypov@google.com>]

On Sat, Apr 10, 2021 at 12:05 AM David Gow <davidgow@google.com> 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@google.com>
> ---
>  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
> +------------------------------------------

This section does a good job, but on a pragmatic level, there are a
few more reasons to pick one or the other.
E.g. the edit/build/test cycle will likely always be faster in KUnit.

I'd also initially drafted up a _very_ long list of reasons to prefer
kselftest as well.
But looking back at them, a lot will hopefully be mitigated soon, or
naturally get better with more usage/time, and you touched on that it
can be easier to set up state from userspace already down below.

> +
> +: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.

Nit: we have "Unit testing" here and "'system' or 'end-to-end' testing."
Perhaps: '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

Seems like this sentence got truncated?
Hmm, I'm not sure what this would be referring to however.
I'm not sure there's a doc that touches on what's amenable to being unit tested.

> +
> +
> +: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

Saying it's not possible to call kernel code before mentioning the use
of kernel modules to call kernel code directly is a bit confusing.

Perhaps instead:
  However, it's not possible to call kernel functions directly unless
you write a companion kernel module for the test. If your test is
mostly or entirely inside a kernel module, `KUnit` may be the better
tool.

> +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

*measurement

> +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.

Nit: I think gcov is the one most people reading this doc are going to
be interested in, so I'd mention it first.

> +
> +
> +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

*they occur

> +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.

Hmm, it lives elsewhere, but would also calling out lockdep here be useful?
I've also not heard anyone call it a sanitizer before, but it fits the
definition you've given.

Now that I think about it, I've never looked for documentation on it,
is this the best page?
https://www.kernel.org/doc/html/latest/locking/lockdep-design.html


> +
> +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.
> +
> --
> 2.31.1.295.g9ea45b61b8-goog
>