Adding upstream version 6.6.15.upstream/6.6.15

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 795 insertions, 0 deletions

diff --git a/Documentation/dev-tools/kunit/usage.rst b/Documentation/dev-tools/kunit/usage.rst
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+.. SPDX-License-Identifier: GPL-2.0
+
+Writing Tests
+=============
+
+Test Cases
+----------
+
+The fundamental unit in KUnit is the test case. A test case is a function with
+the signature ``void (*)(struct kunit *test)``. It calls the function under test
+and then sets *expectations* for what should happen. For example:
+
+.. code-block:: c
+
+	void example_test_success(struct kunit *test)
+	{
+	}
+
+	void example_test_failure(struct kunit *test)
+	{
+		KUNIT_FAIL(test, "This test never passes.");
+	}
+
+In the above example, ``example_test_success`` always passes because it does
+nothing; no expectations are set, and therefore all expectations pass. On the
+other hand ``example_test_failure`` always fails because it calls ``KUNIT_FAIL``,
+which is a special expectation that logs a message and causes the test case to
+fail.
+
+Expectations
+~~~~~~~~~~~~
+An *expectation* specifies that we expect a piece of code to do something in a
+test. An expectation is called like a function. A test is made by setting
+expectations about the behavior of a piece of code under test. When one or more
+expectations fail, the test case fails and information about the failure is
+logged. For example:
+
+.. code-block:: c
+
+	void add_test_basic(struct kunit *test)
+	{
+		KUNIT_EXPECT_EQ(test, 1, add(1, 0));
+		KUNIT_EXPECT_EQ(test, 2, add(1, 1));
+	}
+
+In the above example, ``add_test_basic`` makes a number of assertions about the
+behavior of a function called ``add``. The first parameter is always of type
+``struct kunit *``, which contains information about the current test context.
+The second parameter, in this case, is what the value is expected to be. The
+last value is what the value actually is. If ``add`` passes all of these
+expectations, the test case, ``add_test_basic`` will pass; if any one of these
+expectations fails, the test case will fail.
+
+A test case *fails* when any expectation is violated; however, the test will
+continue to run, and try other expectations until the test case ends or is
+otherwise terminated. This is as opposed to *assertions* which are discussed
+later.
+
+To learn about more KUnit expectations, see Documentation/dev-tools/kunit/api/test.rst.
+
+.. note::
+   A single test case should be short, easy to understand, and focused on a
+   single behavior.
+
+For example, if we want to rigorously test the ``add`` function above, create
+additional tests cases which would test each property that an ``add`` function
+should have as shown below:
+
+.. code-block:: c
+
+	void add_test_basic(struct kunit *test)
+	{
+		KUNIT_EXPECT_EQ(test, 1, add(1, 0));
+		KUNIT_EXPECT_EQ(test, 2, add(1, 1));
+	}
+
+	void add_test_negative(struct kunit *test)
+	{
+		KUNIT_EXPECT_EQ(test, 0, add(-1, 1));
+	}
+
+	void add_test_max(struct kunit *test)
+	{
+		KUNIT_EXPECT_EQ(test, INT_MAX, add(0, INT_MAX));
+		KUNIT_EXPECT_EQ(test, -1, add(INT_MAX, INT_MIN));
+	}
+
+	void add_test_overflow(struct kunit *test)
+	{
+		KUNIT_EXPECT_EQ(test, INT_MIN, add(INT_MAX, 1));
+	}
+
+Assertions
+~~~~~~~~~~
+
+An assertion is like an expectation, except that the assertion immediately
+terminates the test case if the condition is not satisfied. For example:
+
+.. code-block:: c
+
+	static void test_sort(struct kunit *test)
+	{
+		int *a, i, r = 1;
+		a = kunit_kmalloc_array(test, TEST_LEN, sizeof(*a), GFP_KERNEL);
+		KUNIT_ASSERT_NOT_ERR_OR_NULL(test, a);
+		for (i = 0; i < TEST_LEN; i++) {
+			r = (r * 725861) % 6599;
+			a[i] = r;
+		}
+		sort(a, TEST_LEN, sizeof(*a), cmpint, NULL);
+		for (i = 0; i < TEST_LEN-1; i++)
+			KUNIT_EXPECT_LE(test, a[i], a[i + 1]);
+	}
+
+In this example, we need to be able to allocate an array to test the ``sort()``
+function. So we use ``KUNIT_ASSERT_NOT_ERR_OR_NULL()`` to abort the test if
+there's an allocation error.
+
+.. note::
+   In other test frameworks, ``ASSERT`` macros are often implemented by calling
+   ``return`` so they only work from the test function. In KUnit, we stop the
+   current kthread on failure, so you can call them from anywhere.
+
+.. note::
+   Warning: There is an exception to the above rule. You shouldn't use assertions
+   in the suite's exit() function, or in the free function for a resource. These
+   run when a test is shutting down, and an assertion here prevents further
+   cleanup code from running, potentially leading to a memory leak.
+
+Customizing error messages
+--------------------------
+
+Each of the ``KUNIT_EXPECT`` and ``KUNIT_ASSERT`` macros have a ``_MSG``
+variant.  These take a format string and arguments to provide additional
+context to the automatically generated error messages.
+
+.. code-block:: c
+
+	char some_str[41];
+	generate_sha1_hex_string(some_str);
+
+	/* Before. Not easy to tell why the test failed. */
+	KUNIT_EXPECT_EQ(test, strlen(some_str), 40);
+
+	/* After. Now we see the offending string. */
+	KUNIT_EXPECT_EQ_MSG(test, strlen(some_str), 40, "some_str='%s'", some_str);
+
+Alternatively, one can take full control over the error message by using
+``KUNIT_FAIL()``, e.g.
+
+.. code-block:: c
+
+	/* Before */
+	KUNIT_EXPECT_EQ(test, some_setup_function(), 0);
+
+	/* After: full control over the failure message. */
+	if (some_setup_function())
+		KUNIT_FAIL(test, "Failed to setup thing for testing");
+
+
+Test Suites
+~~~~~~~~~~~
+
+We need many test cases covering all the unit's behaviors. It is common to have
+many similar tests. In order to reduce duplication in these closely related
+tests, most unit testing frameworks (including KUnit) provide the concept of a
+*test suite*. A test suite is a collection of test cases for a unit of code
+with optional setup and teardown functions that run before/after the whole
+suite and/or every test case.
+
+.. note::
+   A test case will only run if it is associated with a test suite.
+
+For example:
+
+.. code-block:: c
+
+	static struct kunit_case example_test_cases[] = {
+		KUNIT_CASE(example_test_foo),
+		KUNIT_CASE(example_test_bar),
+		KUNIT_CASE(example_test_baz),
+		{}
+	};
+
+	static struct kunit_suite example_test_suite = {
+		.name = "example",
+		.init = example_test_init,
+		.exit = example_test_exit,
+		.suite_init = example_suite_init,
+		.suite_exit = example_suite_exit,
+		.test_cases = example_test_cases,
+	};
+	kunit_test_suite(example_test_suite);
+
+In the above example, the test suite ``example_test_suite`` would first run
+``example_suite_init``, then run the test cases ``example_test_foo``,
+``example_test_bar``, and ``example_test_baz``. Each would have
+``example_test_init`` called immediately before it and ``example_test_exit``
+called immediately after it. Finally, ``example_suite_exit`` would be called
+after everything else. ``kunit_test_suite(example_test_suite)`` registers the
+test suite with the KUnit test framework.
+
+.. note::
+   The ``exit`` and ``suite_exit`` functions will run even if ``init`` or
+   ``suite_init`` fail. Make sure that they can handle any inconsistent
+   state which may result from ``init`` or ``suite_init`` encountering errors
+   or exiting early.
+
+``kunit_test_suite(...)`` is a macro which tells the linker to put the
+specified test suite in a special linker section so that it can be run by KUnit
+either after ``late_init``, or when the test module is loaded (if the test was
+built as a module).
+
+For more information, see Documentation/dev-tools/kunit/api/test.rst.
+
+.. _kunit-on-non-uml:
+
+Writing Tests For Other Architectures
+-------------------------------------
+
+It is better to write tests that run on UML to tests that only run under a
+particular architecture. It is better to write tests that run under QEMU or
+another easy to obtain (and monetarily free) software environment to a specific
+piece of hardware.
+
+Nevertheless, there are still valid reasons to write a test that is architecture
+or hardware specific. For example, we might want to test code that really
+belongs in ``arch/some-arch/*``. Even so, try to write the test so that it does
+not depend on physical hardware. Some of our test cases may not need hardware,
+only few tests actually require the hardware to test it. When hardware is not
+available, instead of disabling tests, we can skip them.
+
+Now that we have narrowed down exactly what bits are hardware specific, the
+actual procedure for writing and running the tests is same as writing normal
+KUnit tests.
+
+.. important::
+   We may have to reset hardware state. If this is not possible, we may only
+   be able to run one test case per invocation.
+
+.. TODO(brendanhiggins@google.com): Add an actual example of an architecture-
+   dependent KUnit test.
+
+Common Patterns
+===============
+
+Isolating Behavior
+------------------
+
+Unit testing limits the amount of code under test to a single unit. It controls
+what code gets run when the unit under test calls a function. Where a function
+is exposed as part of an API such that the definition of that function can be
+changed without affecting the rest of the code base. In the kernel, this comes
+from two constructs: classes, which are structs that contain function pointers
+provided by the implementer, and architecture-specific functions, which have
+definitions selected at compile time.
+
+Classes
+~~~~~~~
+
+Classes are not a construct that is built into the C programming language;
+however, it is an easily derived concept. Accordingly, in most cases, every
+project that does not use a standardized object oriented library (like GNOME's
+GObject) has their own slightly different way of doing object oriented
+programming; the Linux kernel is no exception.
+
+The central concept in kernel object oriented programming is the class. In the
+kernel, a *class* is a struct that contains function pointers. This creates a
+contract between *implementers* and *users* since it forces them to use the
+same function signature without having to call the function directly. To be a
+class, the function pointers must specify that a pointer to the class, known as
+a *class handle*, be one of the parameters. Thus the member functions (also
+known as *methods*) have access to member variables (also known as *fields*)
+allowing the same implementation to have multiple *instances*.
+
+A class can be *overridden* by *child classes* by embedding the *parent class*
+in the child class. Then when the child class *method* is called, the child
+implementation knows that the pointer passed to it is of a parent contained
+within the child. Thus, the child can compute the pointer to itself because the
+pointer to the parent is always a fixed offset from the pointer to the child.
+This offset is the offset of the parent contained in the child struct. For
+example:
+
+.. code-block:: c
+
+	struct shape {
+		int (*area)(struct shape *this);
+	};
+
+	struct rectangle {
+		struct shape parent;
+		int length;
+		int width;
+	};
+
+	int rectangle_area(struct shape *this)
+	{
+		struct rectangle *self = container_of(this, struct rectangle, parent);
+
+		return self->length * self->width;
+	};
+
+	void rectangle_new(struct rectangle *self, int length, int width)
+	{
+		self->parent.area = rectangle_area;
+		self->length = length;
+		self->width = width;
+	}
+
+In this example, computing the pointer to the child from the pointer to the
+parent is done by ``container_of``.
+
+Faking Classes
+~~~~~~~~~~~~~~
+
+In order to unit test a piece of code that calls a method in a class, the
+behavior of the method must be controllable, otherwise the test ceases to be a
+unit test and becomes an integration test.
+
+A fake class implements a piece of code that is different than what runs in a
+production instance, but behaves identical from the standpoint of the callers.
+This is done to replace a dependency that is hard to deal with, or is slow. For
+example, implementing a fake EEPROM that stores the "contents" in an
+internal buffer. Assume we have a class that represents an EEPROM:
+
+.. code-block:: c
+
+	struct eeprom {
+		ssize_t (*read)(struct eeprom *this, size_t offset, char *buffer, size_t count);
+		ssize_t (*write)(struct eeprom *this, size_t offset, const char *buffer, size_t count);
+	};
+
+And we want to test code that buffers writes to the EEPROM:
+
+.. code-block:: c
+
+	struct eeprom_buffer {
+		ssize_t (*write)(struct eeprom_buffer *this, const char *buffer, size_t count);
+		int flush(struct eeprom_buffer *this);
+		size_t flush_count; /* Flushes when buffer exceeds flush_count. */
+	};
+
+	struct eeprom_buffer *new_eeprom_buffer(struct eeprom *eeprom);
+	void destroy_eeprom_buffer(struct eeprom *eeprom);
+
+We can test this code by *faking out* the underlying EEPROM:
+
+.. code-block:: c
+
+	struct fake_eeprom {
+		struct eeprom parent;
+		char contents[FAKE_EEPROM_CONTENTS_SIZE];
+	};
+
+	ssize_t fake_eeprom_read(struct eeprom *parent, size_t offset, char *buffer, size_t count)
+	{
+		struct fake_eeprom *this = container_of(parent, struct fake_eeprom, parent);
+
+		count = min(count, FAKE_EEPROM_CONTENTS_SIZE - offset);
+		memcpy(buffer, this->contents + offset, count);
+
+		return count;
+	}
+
+	ssize_t fake_eeprom_write(struct eeprom *parent, size_t offset, const char *buffer, size_t count)
+	{
+		struct fake_eeprom *this = container_of(parent, struct fake_eeprom, parent);
+
+		count = min(count, FAKE_EEPROM_CONTENTS_SIZE - offset);
+		memcpy(this->contents + offset, buffer, count);
+
+		return count;
+	}
+
+	void fake_eeprom_init(struct fake_eeprom *this)
+	{
+		this->parent.read = fake_eeprom_read;
+		this->parent.write = fake_eeprom_write;
+		memset(this->contents, 0, FAKE_EEPROM_CONTENTS_SIZE);
+	}
+
+We can now use it to test ``struct eeprom_buffer``:
+
+.. code-block:: c
+
+	struct eeprom_buffer_test {
+		struct fake_eeprom *fake_eeprom;
+		struct eeprom_buffer *eeprom_buffer;
+	};
+
+	static void eeprom_buffer_test_does_not_write_until_flush(struct kunit *test)
+	{
+		struct eeprom_buffer_test *ctx = test->priv;
+		struct eeprom_buffer *eeprom_buffer = ctx->eeprom_buffer;
+		struct fake_eeprom *fake_eeprom = ctx->fake_eeprom;
+		char buffer[] = {0xff};
+
+		eeprom_buffer->flush_count = SIZE_MAX;
+
+		eeprom_buffer->write(eeprom_buffer, buffer, 1);
+		KUNIT_EXPECT_EQ(test, fake_eeprom->contents[0], 0);
+
+		eeprom_buffer->write(eeprom_buffer, buffer, 1);
+		KUNIT_EXPECT_EQ(test, fake_eeprom->contents[1], 0);
+
+		eeprom_buffer->flush(eeprom_buffer);
+		KUNIT_EXPECT_EQ(test, fake_eeprom->contents[0], 0xff);
+		KUNIT_EXPECT_EQ(test, fake_eeprom->contents[1], 0xff);
+	}
+
+	static void eeprom_buffer_test_flushes_after_flush_count_met(struct kunit *test)
+	{
+		struct eeprom_buffer_test *ctx = test->priv;
+		struct eeprom_buffer *eeprom_buffer = ctx->eeprom_buffer;
+		struct fake_eeprom *fake_eeprom = ctx->fake_eeprom;
+		char buffer[] = {0xff};
+
+		eeprom_buffer->flush_count = 2;
+
+		eeprom_buffer->write(eeprom_buffer, buffer, 1);
+		KUNIT_EXPECT_EQ(test, fake_eeprom->contents[0], 0);
+
+		eeprom_buffer->write(eeprom_buffer, buffer, 1);
+		KUNIT_EXPECT_EQ(test, fake_eeprom->contents[0], 0xff);
+		KUNIT_EXPECT_EQ(test, fake_eeprom->contents[1], 0xff);
+	}
+
+	static void eeprom_buffer_test_flushes_increments_of_flush_count(struct kunit *test)
+	{
+		struct eeprom_buffer_test *ctx = test->priv;
+		struct eeprom_buffer *eeprom_buffer = ctx->eeprom_buffer;
+		struct fake_eeprom *fake_eeprom = ctx->fake_eeprom;
+		char buffer[] = {0xff, 0xff};
+
+		eeprom_buffer->flush_count = 2;
+
+		eeprom_buffer->write(eeprom_buffer, buffer, 1);
+		KUNIT_EXPECT_EQ(test, fake_eeprom->contents[0], 0);
+
+		eeprom_buffer->write(eeprom_buffer, buffer, 2);
+		KUNIT_EXPECT_EQ(test, fake_eeprom->contents[0], 0xff);
+		KUNIT_EXPECT_EQ(test, fake_eeprom->contents[1], 0xff);
+		/* Should have only flushed the first two bytes. */
+		KUNIT_EXPECT_EQ(test, fake_eeprom->contents[2], 0);
+	}
+
+	static int eeprom_buffer_test_init(struct kunit *test)
+	{
+		struct eeprom_buffer_test *ctx;
+
+		ctx = kunit_kzalloc(test, sizeof(*ctx), GFP_KERNEL);
+		KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ctx);
+
+		ctx->fake_eeprom = kunit_kzalloc(test, sizeof(*ctx->fake_eeprom), GFP_KERNEL);
+		KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ctx->fake_eeprom);
+		fake_eeprom_init(ctx->fake_eeprom);
+
+		ctx->eeprom_buffer = new_eeprom_buffer(&ctx->fake_eeprom->parent);
+		KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ctx->eeprom_buffer);
+
+		test->priv = ctx;
+
+		return 0;
+	}
+
+	static void eeprom_buffer_test_exit(struct kunit *test)
+	{
+		struct eeprom_buffer_test *ctx = test->priv;
+
+		destroy_eeprom_buffer(ctx->eeprom_buffer);
+	}
+
+Testing Against Multiple Inputs
+-------------------------------
+
+Testing just a few inputs is not enough to ensure that the code works correctly,
+for example: testing a hash function.
+
+We can write a helper macro or function. The function is called for each input.
+For example, to test ``sha1sum(1)``, we can write:
+
+.. code-block:: c
+
+	#define TEST_SHA1(in, want) \
+		sha1sum(in, out); \
+		KUNIT_EXPECT_STREQ_MSG(test, out, want, "sha1sum(%s)", in);
+
+	char out[40];
+	TEST_SHA1("hello world",  "2aae6c35c94fcfb415dbe95f408b9ce91ee846ed");
+	TEST_SHA1("hello world!", "430ce34d020724ed75a196dfc2ad67c77772d169");
+
+Note the use of the ``_MSG`` version of ``KUNIT_EXPECT_STREQ`` to print a more
+detailed error and make the assertions clearer within the helper macros.
+
+The ``_MSG`` variants are useful when the same expectation is called multiple
+times (in a loop or helper function) and thus the line number is not enough to
+identify what failed, as shown below.
+
+In complicated cases, we recommend using a *table-driven test* compared to the
+helper macro variation, for example:
+
+.. code-block:: c
+
+	int i;
+	char out[40];
+
+	struct sha1_test_case {
+		const char *str;
+		const char *sha1;
+	};
+
+	struct sha1_test_case cases[] = {
+		{
+			.str = "hello world",
+			.sha1 = "2aae6c35c94fcfb415dbe95f408b9ce91ee846ed",
+		},
+		{
+			.str = "hello world!",
+			.sha1 = "430ce34d020724ed75a196dfc2ad67c77772d169",
+		},
+	};
+	for (i = 0; i < ARRAY_SIZE(cases); ++i) {
+		sha1sum(cases[i].str, out);
+		KUNIT_EXPECT_STREQ_MSG(test, out, cases[i].sha1,
+		                      "sha1sum(%s)", cases[i].str);
+	}
+
+
+There is more boilerplate code involved, but it can:
+
+* be more readable when there are multiple inputs/outputs (due to field names).
+
+  * For example, see ``fs/ext4/inode-test.c``.
+
+* reduce duplication if test cases are shared across multiple tests.
+
+  * For example: if we want to test ``sha256sum``, we could add a ``sha256``
+    field and reuse ``cases``.
+
+* be converted to a "parameterized test".
+
+Parameterized Testing
+~~~~~~~~~~~~~~~~~~~~~
+
+The table-driven testing pattern is common enough that KUnit has special
+support for it.
+
+By reusing the same ``cases`` array from above, we can write the test as a
+"parameterized test" with the following.
+
+.. code-block:: c
+
+	// This is copy-pasted from above.
+	struct sha1_test_case {
+		const char *str;
+		const char *sha1;
+	};
+	const struct sha1_test_case cases[] = {
+		{
+			.str = "hello world",
+			.sha1 = "2aae6c35c94fcfb415dbe95f408b9ce91ee846ed",
+		},
+		{
+			.str = "hello world!",
+			.sha1 = "430ce34d020724ed75a196dfc2ad67c77772d169",
+		},
+	};
+
+	// Need a helper function to generate a name for each test case.
+	static void case_to_desc(const struct sha1_test_case *t, char *desc)
+	{
+		strcpy(desc, t->str);
+	}
+	// Creates `sha1_gen_params()` to iterate over `cases`.
+	KUNIT_ARRAY_PARAM(sha1, cases, case_to_desc);
+
+	// Looks no different from a normal test.
+	static void sha1_test(struct kunit *test)
+	{
+		// This function can just contain the body of the for-loop.
+		// The former `cases[i]` is accessible under test->param_value.
+		char out[40];
+		struct sha1_test_case *test_param = (struct sha1_test_case *)(test->param_value);
+
+		sha1sum(test_param->str, out);
+		KUNIT_EXPECT_STREQ_MSG(test, out, test_param->sha1,
+				      "sha1sum(%s)", test_param->str);
+	}
+
+	// Instead of KUNIT_CASE, we use KUNIT_CASE_PARAM and pass in the
+	// function declared by KUNIT_ARRAY_PARAM.
+	static struct kunit_case sha1_test_cases[] = {
+		KUNIT_CASE_PARAM(sha1_test, sha1_gen_params),
+		{}
+	};
+
+Allocating Memory
+-----------------
+
+Where you might use ``kzalloc``, you can instead use ``kunit_kzalloc`` as KUnit
+will then ensure that the memory is freed once the test completes.
+
+This is useful because it lets us use the ``KUNIT_ASSERT_EQ`` macros to exit
+early from a test without having to worry about remembering to call ``kfree``.
+For example:
+
+.. code-block:: c
+
+	void example_test_allocation(struct kunit *test)
+	{
+		char *buffer = kunit_kzalloc(test, 16, GFP_KERNEL);
+		/* Ensure allocation succeeded. */
+		KUNIT_ASSERT_NOT_ERR_OR_NULL(test, buffer);
+
+		KUNIT_ASSERT_STREQ(test, buffer, "");
+	}
+
+Registering Cleanup Actions
+---------------------------
+
+If you need to perform some cleanup beyond simple use of ``kunit_kzalloc``,
+you can register a custom "deferred action", which is a cleanup function
+run when the test exits (whether cleanly, or via a failed assertion).
+
+Actions are simple functions with no return value, and a single ``void*``
+context argument, and fulfill the same role as "cleanup" functions in Python
+and Go tests, "defer" statements in languages which support them, and
+(in some cases) destructors in RAII languages.
+
+These are very useful for unregistering things from global lists, closing
+files or other resources, or freeing resources.
+
+For example:
+
+.. code-block:: C
+
+	static void cleanup_device(void *ctx)
+	{
+		struct device *dev = (struct device *)ctx;
+
+		device_unregister(dev);
+	}
+
+	void example_device_test(struct kunit *test)
+	{
+		struct my_device dev;
+
+		device_register(&dev);
+
+		kunit_add_action(test, &cleanup_device, &dev);
+	}
+
+Note that, for functions like device_unregister which only accept a single
+pointer-sized argument, it's possible to directly cast that function to
+a ``kunit_action_t`` rather than writing a wrapper function, for example:
+
+.. code-block:: C
+
+	kunit_add_action(test, (kunit_action_t *)&device_unregister, &dev);
+
+``kunit_add_action`` can fail if, for example, the system is out of memory.
+You can use ``kunit_add_action_or_reset`` instead which runs the action
+immediately if it cannot be deferred.
+
+If you need more control over when the cleanup function is called, you
+can trigger it early using ``kunit_release_action``, or cancel it entirely
+with ``kunit_remove_action``.
+
+
+Testing Static Functions
+------------------------
+
+If we do not want to expose functions or variables for testing, one option is to
+conditionally ``#include`` the test file at the end of your .c file. For
+example:
+
+.. code-block:: c
+
+	/* In my_file.c */
+
+	static int do_interesting_thing();
+
+	#ifdef CONFIG_MY_KUNIT_TEST
+	#include "my_kunit_test.c"
+	#endif
+
+Injecting Test-Only Code
+------------------------
+
+Similar to as shown above, we can add test-specific logic. For example:
+
+.. code-block:: c
+
+	/* In my_file.h */
+
+	#ifdef CONFIG_MY_KUNIT_TEST
+	/* Defined in my_kunit_test.c */
+	void test_only_hook(void);
+	#else
+	void test_only_hook(void) { }
+	#endif
+
+This test-only code can be made more useful by accessing the current ``kunit_test``
+as shown in next section: *Accessing The Current Test*.
+
+Accessing The Current Test
+--------------------------
+
+In some cases, we need to call test-only code from outside the test file.  This
+is helpful, for example, when providing a fake implementation of a function, or
+to fail any current test from within an error handler.
+We can do this via the ``kunit_test`` field in ``task_struct``, which we can
+access using the ``kunit_get_current_test()`` function in ``kunit/test-bug.h``.
+
+``kunit_get_current_test()`` is safe to call even if KUnit is not enabled. If
+KUnit is not enabled, or if no test is running in the current task, it will
+return ``NULL``. This compiles down to either a no-op or a static key check,
+so will have a negligible performance impact when no test is running.
+
+The example below uses this to implement a "mock" implementation of a function, ``foo``:
+
+.. code-block:: c
+
+	#include <kunit/test-bug.h> /* for kunit_get_current_test */
+
+	struct test_data {
+		int foo_result;
+		int want_foo_called_with;
+	};
+
+	static int fake_foo(int arg)
+	{
+		struct kunit *test = kunit_get_current_test();
+		struct test_data *test_data = test->priv;
+
+		KUNIT_EXPECT_EQ(test, test_data->want_foo_called_with, arg);
+		return test_data->foo_result;
+	}
+
+	static void example_simple_test(struct kunit *test)
+	{
+		/* Assume priv (private, a member used to pass test data from
+		 * the init function) is allocated in the suite's .init */
+		struct test_data *test_data = test->priv;
+
+		test_data->foo_result = 42;
+		test_data->want_foo_called_with = 1;
+
+		/* In a real test, we'd probably pass a pointer to fake_foo somewhere
+		 * like an ops struct, etc. instead of calling it directly. */
+		KUNIT_EXPECT_EQ(test, fake_foo(1), 42);
+	}
+
+In this example, we are using the ``priv`` member of ``struct kunit`` as a way
+of passing data to the test from the init function. In general ``priv`` is
+pointer that can be used for any user data. This is preferred over static
+variables, as it avoids concurrency issues.
+
+Had we wanted something more flexible, we could have used a named ``kunit_resource``.
+Each test can have multiple resources which have string names providing the same
+flexibility as a ``priv`` member, but also, for example, allowing helper
+functions to create resources without conflicting with each other. It is also
+possible to define a clean up function for each resource, making it easy to
+avoid resource leaks. For more information, see Documentation/dev-tools/kunit/api/resource.rst.
+
+Failing The Current Test
+------------------------
+
+If we want to fail the current test, we can use ``kunit_fail_current_test(fmt, args...)``
+which is defined in ``<kunit/test-bug.h>`` and does not require pulling in ``<kunit/test.h>``.
+For example, we have an option to enable some extra debug checks on some data
+structures as shown below:
+
+.. code-block:: c
+
+	#include <kunit/test-bug.h>
+
+	#ifdef CONFIG_EXTRA_DEBUG_CHECKS
+	static void validate_my_data(struct data *data)
+	{
+		if (is_valid(data))
+			return;
+
+		kunit_fail_current_test("data %p is invalid", data);
+
+		/* Normal, non-KUnit, error reporting code here. */
+	}
+	#else
+	static void my_debug_function(void) { }
+	#endif
+
+``kunit_fail_current_test()`` is safe to call even if KUnit is not enabled. If
+KUnit is not enabled, or if no test is running in the current task, it will do
+nothing. This compiles down to either a no-op or a static key check, so will
+have a negligible performance impact when no test is running.