Re: [RFC PATCH v2 0/2] kunit: Support redirecting function calls

[Joe Fradley]

On Sat, Sep 10, 2022 at 2:28 PM David Gow <davidgow@xxxxxxxxxx> wrote:
>
> When writing tests, it'd often be very useful to be able to intercept
> calls to a function in the code being tested and replace it with a
> test-specific stub. This has always been an obviously missing piece of
> KUnit, and the solutions always involve some tradeoffs with cleanliness,
> performance, or impact on non-test code. See the folowing document for
> some of the challenges:
> https://kunit.dev/mocking.html
>
> This series consists of two prototype patches which add support for this
> sort of redirection to KUnit tests:
>
> 1: static_stub: Any function which might want to be intercepted adds a
> call to a macro which checks if a test has redirected calls to it, and
> calls the corresponding replacement.
>
> 2: ftrace_stub: Functions are intercepted using ftrace.
> This doesn't require adding a new prologue to each function being
> replaced, but does have more dependencies (which restricts it to a small
> number of architectures, not including UML), and doesn't work well with
> inline functions.
>
> The API for both implementations is very similar, so it should be easy
> to migrate from one to the other if necessary.  Both of these
> implementations restrict the redirection to the test context: it is
> automatically undone after the KUnit test completes, and does not affect
> calls in other threads. If CONFIG_KUNIT is not enabled, there should be
> no overhead in either implementation.
>
> Does either (or both) of these features sound useful, and is this
> sort-of API the right model? (Personally, I think there's a reasonable
> scope for both.) Is anything obviously missing or wrong? Do the names,
> descriptions etc. make any sense?

David,
This will be a great addition to the KUnit framework and another tool in
the toolbox for test writers. Both approaches have their merits. If all
things were equal the ftrace option would be preffered in my opinion.
The ability to add tests without having to touch the source of what
you're testing is superior. However, all things aren't equal as you've
detailed. There are a number of open items for the ftrace approach that
will limit its scope of use. Given that a solid amount of test developers
already develop on what they're testing, the static stub option sounds
like the one to go with for now, if you had to choose one.

Regarding the implementation, could there be more granualitary in the
activation of these stubs? Considering there's a small amount overhead
for these stubs when CONFIG_KUNIT is enabled, a developer's environment
could be adversely affected when they're testing a completely different
area that doesn't require mocks.

Maybe only activate these with CONFIG_KUNIT_FTRACE_STUBS and
CONFIG_KUNIT_STATIC_STUBS respectively?

Joe

>
> Note that these patches are definitely still at the "prototype" level,
> and things like error-handling, documentation, and testing are still
> pretty sparse. There is also quite a bit of room for optimisation.
> These'll all be improved for v1 if the concept seems good.
>
> We're going to be talking about this again at LPC, so it's worth having
> another look before then if you're interested and/or will be attending:
> https://lpc.events/event/16/contributions/1308/
>
> Cheers,
> -- David
>
> ---
>
> Changes since RFC v1:
> https://lore.kernel.org/lkml/20220318021314.3225240-1-davidgow@xxxxxxxxxx/
> - Fix some typos (thanks Daniel)
> - Use typecheck_fn() to fix typechecking in some cases (thanks Brendan)
> - Use ftrace_instruction_pointer_set() in place of kernel livepatch,
>   which seems to have disappeared:
>   https://lore.kernel.org/lkml/0a76550d-008d-0364-8244-4dae2981ea05@xxxxxxxxxx/T/
> - Fix a copy-paste name error in the resource finding function.
> - Rebase on top of torvalds/master, as it wasn't applying cleanly.
>
> Note that the Kernel Livepatch -> ftrace change seems to allow more
> architectures to work, but while they compile, there still seems to be
> issues. So, this will compile on (e.g.) arm64, but fails:
> $ ./tools/testing/kunit/kunit.py run 'example*' --kunitconfig lib/kunit/stubs_example.kunitconfig --arch arm64 --make_options LLVM=1
> [05:00:13] # example_ftrace_stub_test: initializing
> [05:00:13] # example_ftrace_stub_test: EXPECTATION FAILED at lib/kunit/kunit-example-test.c:179
> [05:00:13] Expected add_one(1) == 0, but
> [05:00:13] add_one(1) == 2
> [05:00:13] not ok 6 - example_ftrace_stub_test
> [05:00:13] [FAILED] example_ftrace_stub_test
>
>
>
> Daniel Latypov (1):
>   kunit: expose ftrace-based API for stubbing out functions during tests
>
> David Gow (1):
>   kunit: Expose 'static stub' API to redirect functions
>
>  include/kunit/ftrace_stub.h         |  84 +++++++++++++++++
>  include/kunit/static_stub.h         | 106 +++++++++++++++++++++
>  lib/kunit/Kconfig                   |  11 +++
>  lib/kunit/Makefile                  |   5 +
>  lib/kunit/ftrace_stub.c             | 137 ++++++++++++++++++++++++++++
>  lib/kunit/kunit-example-test.c      |  63 +++++++++++++
>  lib/kunit/static_stub.c             | 125 +++++++++++++++++++++++++
>  lib/kunit/stubs_example.kunitconfig |  10 ++
>  8 files changed, 541 insertions(+)
>  create mode 100644 include/kunit/ftrace_stub.h
>  create mode 100644 include/kunit/static_stub.h
>  create mode 100644 lib/kunit/ftrace_stub.c
>  create mode 100644 lib/kunit/static_stub.c
>  create mode 100644 lib/kunit/stubs_example.kunitconfig
>
> --
> 2.37.2.789.g6183377224-goog
>