[1/2] kunit: Allow passing function pointer to kunit_activate_static_stub()

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

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
>