From: Sadiya Kazi sadiyakazi@google.com
Added a new page (functionredirection.rst) that describes the Function Redirection (static stubbing) API. This page will be expanded if we add, for example, ftrace-based stubbing.
In addition, 1. Updated the api/index.rst page to create an entry for function redirection api 2. Updated the toctree to be hidden, reducing redundancy on the generated page.
Signed-off-by: Sadiya Kazi sadiyakazi@google.com Co-developed-by: David Gow davidgow@google.com Signed-off-by: David Gow davidgow@google.com ---
Note that this patch is new to v1 of the series, and wasn't included in the previous RFCs.
--- .../kunit/api/functionredirection.rst | 162 ++++++++++++++++++ Documentation/dev-tools/kunit/api/index.rst | 13 +- 2 files changed, 172 insertions(+), 3 deletions(-) create mode 100644 Documentation/dev-tools/kunit/api/functionredirection.rst
diff --git a/Documentation/dev-tools/kunit/api/functionredirection.rst b/Documentation/dev-tools/kunit/api/functionredirection.rst new file mode 100644 index 000000000000..fc7644dfea65 --- /dev/null +++ b/Documentation/dev-tools/kunit/api/functionredirection.rst @@ -0,0 +1,162 @@ +.. SPDX-License-Identifier: GPL-2.0 + +======================== +Function Redirection API +======================== + +Overview +======== + +When writing unit tests, it's important to be able to isolate the code being +tested from other parts of the kernel. This ensures the reliability of the test +(it won't be affected by external factors), reduces dependencies on specific +hardware or config options (making the test easier to run), and protects the +stability of the rest of the system (making it less likely for test-specific +state to interfere with the rest of the system). + +While for some code (typically generic data structures, helpers, and toher +"pure function") this is trivial, for others (like device drivers, filesystems, +core subsystems) the code is heavily coupled with other parts of the kernel. + +This often involves global state in some way: be it global lists of devices, +the filesystem, or hardware state, this needs to be either carefully managed, +isolated, and restored, or avoided altogether by replacing access to and +mutation of this state with a "fake" or "mock" variant. + +This can be done by refactoring the code to abstract out access to such state, +by introducing a layer of indirection which can use or emulate a separate set of +test state. However, such refactoring comes with its own costs (and undertaking +significant refactoring before being able to write tests is suboptimal). + +A simpler way to intercept some of the function calls is to use function +redirection via static stubs. + + +Static Stubs +============ + +Static stubs are a way of redirecting calls to one function (the "real" +function) to another function (the "replacement" function). + +It works by adding a macro to the "real" function which checks to see if a test +is running, and if a replacement function is available. If so, that function is +called in place of the original. + +Using static stubs is pretty straightforward: + +1. Add the KUNIT_STATIC_STUB_REDIRECT() macro to the start of the "real" + function. + + This should be the first statement in the function, after any variable + declarations. KUNIT_STATIC_STUB_REDIRECT() takes the name of the + function, followed by all of the arguments passed to the real function. + + For example: + + .. code-block:: c + + void send_data_to_hardware(const char *str) + { + KUNIT_STATIC_STUB_REDIRECT(send_data_to_hardware, str); + /* real implementation */ + } + +2. Write one or more replacement functions. + + These functions should have the same function signature as the real function. + In the event they need to access or modify test-specific state, they can use + kunit_get_current_test() to get a struct kunit pointer. This can then + be passed to the expectation/assertion macros, or used to look up KUnit + resources. + + For example: + + .. code-block:: c + + void fake_send_data_to_hardware(const char *str) + { + struct kunit *test = kunit_get_current_test(); + KUNIT_EXPECT_STREQ(test, str, "Hello World!"); + } + +3. Activate the static stub from your test. + + From within a test, the redirection can be enabled with + kunit_activate_static_stub(), which accepts a struct kunit pointer, + the real function, and the replacement function. You can call this several + times with different replacement functions to swap out implementations of the + function. + + In our example, this would be + + .. code-block:: c + + kunit_activate_static_stub(test, + send_data_to_hardware, + fake_send_data_to_hardware); + +4. Call (perhaps indirectly) the real function. + + Once the redirection is activated, any call to the real function will call + the replacement function instead. Such calls may be buried deep in the + implementation of another function, but must occur from the test's kthread. + + For example: + + .. code-block:: c + + send_data_to_hardware("Hello World!"); /* Succeeds */ + send_data_to_hardware("Something else"); /* Fails the test. */ + +5. (Optionally) disable the stub. + + When you no longer need it, the redirection can be disabled (and hence the + original behaviour of the 'real' function resumed) using + kunit_deactivate_static_stub(). If the stub is not manually deactivated, it + will nevertheless be disabled when the test finishes. + + For example: + + .. code-block:: c + + kunit_deactivate_static_stub(test, send_data_to_hardware); + + +It's also possible to use these replacement functions to test to see if a +function is called at all, for example: + +.. code-block:: c + + void send_data_to_hardware(const char *str) + { + KUNIT_STATIC_STUB_REDIRECT(send_data_to_hardware, str); + /* real implementation */ + } + + /* In test file */ + int times_called = 0; + void fake_send_data_to_hardware(const char *str) + { + /* fake implementation */ + times_called++; + } + ... + /* In the test case, redirect calls for the duration of the test */ + kunit_activate_static_stub(test, send_data_to_hardware, fake_send_data_to_hardware); + + send_data_to_hardware("hello"); + KUNIT_EXPECT_EQ(test, times_called, 1); + + /* Can also deactivate the stub early, if wanted */ + kunit_deactivate_static_stub(test, send_data_to_hardware); + + send_data_to_hardware("hello again"); + KUNIT_EXPECT_EQ(test, times_called, 1); + + + +API Reference +============= + +.. kernel-doc:: include/kunit/static_stub.h + :internal: diff --git a/Documentation/dev-tools/kunit/api/index.rst b/Documentation/dev-tools/kunit/api/index.rst index 45ce04823f9f..2d8f756aab56 100644 --- a/Documentation/dev-tools/kunit/api/index.rst +++ b/Documentation/dev-tools/kunit/api/index.rst @@ -4,17 +4,24 @@ API Reference ============= .. toctree:: + :hidden:
test resource + functionredirection
-This section documents the KUnit kernel testing API. It is divided into the + +This page documents the KUnit kernel testing API. It is divided into the following sections:
Documentation/dev-tools/kunit/api/test.rst
- - documents all of the standard testing API + - Documents all of the standard testing API
Documentation/dev-tools/kunit/api/resource.rst
- - documents the KUnit resource API + - Documents the KUnit resource API + +Documentation/dev-tools/kunit/api/functionredirection.rst + + - Documents the KUnit Function Redirection API