Adds a variant of example_static_stub_test() that shows use of a pointer-to-function with kunit_activate_static_stub().

A const pointer to the add_one() function is declared. This pointer-to-function is passed to kunit_activate_static_stub() and kunit_deactivate_static_stub() instead of passing add_one directly.

Signed-off-by: Richard Fitzgerald rf@opensource.cirrus.com --- lib/kunit/kunit-example-test.c | 27 +++++++++++++++++++++++++++ 1 file changed, 27 insertions(+)

diff --git a/lib/kunit/kunit-example-test.c b/lib/kunit/kunit-example-test.c index d2f7a3c62c18..9e57f341dc37 100644 --- a/lib/kunit/kunit-example-test.c +++ b/lib/kunit/kunit-example-test.c @@ -168,6 +168,8 @@ static int subtract_one(int i) return i - 1; }

+static int (* const add_one_fn_ptr)(int i) = add_one; + /* * This test shows the use of static stubs. */ @@ -187,6 +189,30 @@ static void example_static_stub_test(struct kunit *test) KUNIT_EXPECT_EQ(test, add_one(1), 2); }

+/* + * This test shows the use of static stubs when the function being + * replaced is provided as a pointer-to-function instead of the + * actual function. This is useful for providing access to static + * functions in a module by exporting a pointer to that function + * instead of having to change the static function to a non-static + * exported function. + */ +static void example_static_stub_using_fn_ptr_test(struct kunit *test) +{ + /* By default, function is not stubbed. */ + KUNIT_EXPECT_EQ(test, add_one(1), 2); + + /* Replace add_one() with subtract_one(). */ + kunit_activate_static_stub(test, add_one_fn_ptr, subtract_one); + + /* add_one() is now replaced. */ + KUNIT_EXPECT_EQ(test, add_one(1), 0); + + /* Return add_one() to normal. */ + kunit_deactivate_static_stub(test, add_one_fn_ptr); + KUNIT_EXPECT_EQ(test, add_one(1), 2); +} + static const struct example_param { int value; } example_params_array[] = { @@ -245,6 +271,7 @@ static struct kunit_case example_test_cases[] = { KUNIT_CASE(example_mark_skipped_test), KUNIT_CASE(example_all_expect_macros_test), KUNIT_CASE(example_static_stub_test), + KUNIT_CASE(example_static_stub_using_fn_ptr_test), KUNIT_CASE_PARAM(example_params_test, example_gen_params), KUNIT_CASE_SLOW(example_slow_test), {}

On Mon, Dec 18, 2023 at 11:10 AM Richard Fitzgerald rf@opensource.cirrus.com wrote:

Swap the arguments to typecheck_fn() in kunit_activate_static_stub() so that real_fn_addr can be either the function itself or a pointer to that function.

This is useful to simplify redirecting static functions in a module. Having to pass the actual function meant that it must be exported from the module. Either making the 'static' and EXPORT_SYMBOL*() conditional (which makes the code messy), or change it to always exported (which increases the export namespace and prevents the compiler inlining a trivial stub function in non-test builds).

With the original definition of kunit_activate_static_stub() the address of real_fn_addr was passed to typecheck_fn() as the type to be passed. This meant that if real_fn_addr was a pointer-to-function it would resolve to a ** instead of a *, giving an error like this:

error: initialization of ‘int (**)(int)’ from incompatible pointer type ‘int (*)(int)’ [-Werror=incompatible-pointer-types] kunit_activate_static_stub(test, add_one_fn_ptr, subtract_one); | ^~~~~~~~~~~~ ./include/linux/typecheck.h:21:25: note: in definition of macro ‘typecheck_fn’ 21 | ({ typeof(type) __tmp = function; \

Swapping the arguments to typecheck_fn makes it take the type of a pointer to the replacement function. Either a function or a pointer to function can be assigned to that. For example:

static int some_function(int x) { /* whatever */ }

int (* some_function_ptr)(int) = some_function;

static int replacement(int x) { /* whatever */ }

Then: kunit_activate_static_stub(test, some_function, replacement); yields: typecheck_fn(typeof(&replacement), some_function);

and: kunit_activate_static_stub(test, some_function_ptr, replacement); yields: typecheck_fn(typeof(&replacement), some_function_ptr);

The two typecheck_fn() then resolve to:

int (*__tmp)(int) = some_function; and int (*__tmp)(int) = some_function_ptr;

Both of these are valid. In the first case the compiler inserts an implicit '&' to take the address of the supplied function, and in the second case the RHS is already a pointer to the same type.

Signed-off-by: Richard Fitzgerald rf@opensource.cirrus.com

Hello!

This seems fine to me. I have tested it and the reasoning behind this seems sensible. However, let's see what David thinks when he returns to office as he is the expert on static stubbing.

Reviewed-by: Rae Moar rmoar@google.com

-Rae

---

include/kunit/static_stub.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/kunit/static_stub.h b/include/kunit/static_stub.h index 85315c80b303..bf940322dfc0 100644 --- a/include/kunit/static_stub.h +++ b/include/kunit/static_stub.h @@ -93,7 +93,7 @@ void __kunit_activate_static_stub(struct kunit *test,

• The redirection can be disabled again with kunit_deactivate_static_stub().

*/ #define kunit_activate_static_stub(test, real_fn_addr, replacement_addr) do { \

•   typecheck_fn(typeof(&real_fn_addr), replacement_addr);                  \
  

•   typecheck_fn(typeof(&replacement_addr), real_fn_addr);                  \
    __kunit_activate_static_stub(test, real_fn_addr, replacement_addr);     \
  

} while (0)

-- 2.30.2