On Wed, Apr 14, 2021 at 1:15 AM 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: Daniel Latypov <dlatypov@xxxxxxxxxx> Looks good to me. Some minor typos and nits about wording here and there. > --- > 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 *s/somhow/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. I like this slightly tweaked wording better, thanks. Still might be a bit confusing for a reader to see "it's not possible" => "it's possible if you have a companion module," but I'm happy enough with it as-is. > + > +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 *s/the occur/they occur > +look for undefined behaviour of some kind, such as invalid memory accesses, nit: "look for undefined behaviour of some kind" Given that I think most readers will interpret UB in the sense that C uses it, this might be a bit misleading. E.g. lockdep errors aren't UB in that sense. Perhaps we can reword this to "look for invalid behaviour" or even just "look for bugs" > +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 nit: we refer to "sanitizers" again, I assume this needs to be updated as well? > +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 >
