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From:   Brendan Higgins <brendanhiggins@google.com>
Date:   Mon, 4 Apr 2022 16:14:19 -0400
Message-ID: <CAFd5g47p-1OAM8fgUdTCUTJ+5Tj0+wPfTgiNVQ_VccbLjUYyWg@mail.gmail.com>
Subject: Re: [RFC PATCH 0/2] kunit: Support redirecting function calls
To:     David Gow <davidgow@google.com>
Cc:     Daniel Latypov <dlatypov@google.com>,
        Kees Cook <keescook@chromium.org>,
        Shuah Khan <skhan@linuxfoundation.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        kunit-dev@googlegroups.com, linux-kselftest@vger.kernel.org,
        linux-kernel@vger.kernel.org, Steve Muckle <smuckle@google.com>
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+Steve Muckle - since I think this might affect things he is working on.

On Thu, Mar 17, 2022 at 10:13 PM David Gow <davidgow@google.com> 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 and livepatch.
> 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?
>
> 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.
>
> Cheers,
> -- David
>
> 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             | 138 ++++++++++++++++++++++++++++
>  lib/kunit/kunit-example-test.c      |  64 +++++++++++++
>  lib/kunit/static_stub.c             | 125 +++++++++++++++++++++++++
>  lib/kunit/stubs_example.kunitconfig |  11 +++
>  8 files changed, 544 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
>
> --
> 2.35.1.894.gb6a874cedc-goog
>