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 other “pure functions”) this is trivial, for others (like device drivers, filesystems, core subsystems) the code is heavily coupled with other parts of the kernel.

This coupling is often due to global state in some way: be it a global list of devices, the filesystem, or some hardware state. Tests need to either carefully manage, isolate, and restore state, or they can avoid it altogether by replacing access to and mutation of this state with a “fake” or “mock” variant.

By refactoring access to such state, such as 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 and replace 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:

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:
```
void send_data_to_hardware(const char *str)
{
        KUNIT_STATIC_STUB_REDIRECT(send_data_to_hardware, str);
        /* real implementation */
}
```
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:
```
void fake_send_data_to_hardware(const char *str)
{
        struct kunit *test = kunit_get_current_test();
        KUNIT_EXPECT_STREQ(test, str, "Hello World!");
}
```
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
```
kunit_activate_static_stub(test,
                           send_data_to_hardware,
                           fake_send_data_to_hardware);
```
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:
```
send_data_to_hardware("Hello World!"); /* Succeeds */
send_data_to_hardware("Something else"); /* Fails the test. */
```
(Optionally) disable the stub.

When you no longer need it, disable the redirection (and hence resume the original behaviour of the ‘real’ function) using kunit_deactivate_static_stub(). Otherwise, it will be automatically disabled when the test exits.

For example:
```
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:

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)
{
        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¶

KUNIT_STATIC_STUB_REDIRECT¶

KUNIT_STATIC_STUB_REDIRECT (real_fn_name, args...)

call a replacement ‘static stub’ if one exists

Parameters

real_fn_name: The name of this function (as an identifier, not a string)
args...: All of the arguments passed to this function

Description

This is a function prologue which is used to allow calls to the current function to be redirected by a KUnit test. KUnit tests can call kunit_activate_static_stub() to pass a replacement function in. The replacement function will be called by KUNIT_STATIC_STUB_REDIRECT(), which will then return from the function. If the caller is not in a KUnit context, the function will continue execution as normal.

int real_func(int n)
{
        KUNIT_STATIC_STUB_REDIRECT(real_func, n);
        return 0;
}

int replacement_func(int n)
{
        return 42;
}

void example_test(struct kunit *test)
{
        kunit_activate_static_stub(test, real_func, replacement_func);
        KUNIT_EXPECT_EQ(test, real_func(1), 42);
}

Example

kunit_activate_static_stub¶

kunit_activate_static_stub (test, real_fn_addr, replacement_addr)

replace a function using static stubs.

Parameters

test: A pointer to the ‘struct kunit’ test context for the current test.
real_fn_addr: The address of the function to replace.
replacement_addr: The address of the function to replace it with.

Description

When activated, calls to real_fn_addr from within this test (even if called indirectly) will instead call replacement_addr. The function pointed to by real_fn_addr must begin with the static stub prologue in KUNIT_STATIC_STUB_REDIRECT() for this to work. real_fn_addr and replacement_addr must have the same type.

The redirection can be disabled again with kunit_deactivate_static_stub().

void kunit_deactivate_static_stub(struct kunit *test, void *real_fn_addr)¶: disable a function redirection

Parameters

struct kunit *test: A pointer to the ‘struct kunit’ test context for the current test.
void *real_fn_addr: The address of the function to no-longer redirect

Description

Deactivates a redirection configured with kunit_activate_static_stub(). After this function returns, calls to real_fn_addr() will execute the original real_fn, not any previously-configured replacement.

The Linux Kernel

Contents

This Page

Function Redirection API¶

Overview¶

Static Stubs¶

API Reference¶