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[0/2] kunit: Function Redirection ("static stub") support

Message ID 20221208061841.2186447-1-davidgow@google.com
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Series kunit: Function Redirection ("static stub") support | expand

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David Gow Dec. 8, 2022, 6:18 a.m. UTC 
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 introduces a new "static_stub" feature add support for this
sort of redirection to KUnit tests.

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.

Note that at alternate implementation (based on ftrace) was also
proposed in an earlier RFC:
https://lore.kernel.org/linux-kselftest/20220910212804.670622-3-davidgow@google.com/

This series only implements "static" stubbing, as it is more compatible
across different architectures, and more flexible w/r/t inlined code,
but we don't rule out offering the ftrace-based solution as well if the
demand is there in the future.

This feature was presented at LPC 2022, see:
- https://lpc.events/event/16/contributions/1308/
- https://www.youtube.com/watch?v=0Nm06EdXWsE

The KUnit 'example' test suite now includes an example of static stubs
being used, and the new 'Function Redirection' API documentation
provides a step-by-step walkthrough for using the new feature.

In addition, an (in-progress) test for the atkbd driver, which provides
an example of static stubs being used, can be found here:
https://kunit-review.googlesource.com/c/linux/+/5631

Cheers,
-- David

---
David Gow (1):
  kunit: Expose 'static stub' API to redirect functions

Sadiya Kazi (1):
  Documentation: Add Function Redirection API docs

 .../kunit/api/functionredirection.rst         | 162 ++++++++++++++++++
 Documentation/dev-tools/kunit/api/index.rst   |  13 +-
 include/kunit/static_stub.h                   | 117 +++++++++++++
 lib/kunit/Makefile                            |   1 +
 lib/kunit/kunit-example-test.c                |  38 ++++
 lib/kunit/static_stub.c                       | 123 +++++++++++++
 6 files changed, 451 insertions(+), 3 deletions(-)
 create mode 100644 Documentation/dev-tools/kunit/api/functionredirection.rst
 create mode 100644 include/kunit/static_stub.h
 create mode 100644 lib/kunit/static_stub.c

Comments

Brendan Higgins Dec. 17, 2022, 5:09 a.m. UTC | #1 
On Thu, Dec 8, 2022 at 1:18 AM 'David Gow' via KUnit Development
<kunit-dev@googlegroups.com> wrote:
>
> Add a simple way of redirecting calls to functions by including a
> special prologue in the "real" function which checks to see if the
> replacement function should be called (and, if so, calls it).
>
> To redirect calls to a function, make the first (non-declaration) line
> of the function:
>
>         KUNIT_STATIC_STUB_REDIRECT(function_name, [function arguments]);
>
> (This will compile away to nothing if KUnit is not enabled, otherwise it
> will check if a redirection is active, call the replacement function,
> and return. This check is protected by a static branch, so has very
> little overhead when there are no KUnit tests running.)
>
> Calls to the real function can be redirected to a replacement using:
>
>         kunit_activate_static_stub(test, real_fn, replacement_fn);
>
> The redirection will only affect calls made from within the kthread of
> the current test, and will be automatically disabled when the test
> completes. It can also be manually disabled with
> kunit_deactivate_static_stub().
>
> The 'example' KUnit test suite has a more complete example.
>
> Co-developed-by: Daniel Latypov <dlatypov@google.com>
> Signed-off-by: Daniel Latypov <dlatypov@google.com>
> Signed-off-by: David Gow <davidgow@google.com>

Reviewed-by: Brendan Higgins <brendanhiggins@google.com>