On Mon, Dec 18, 2023 at 11:10 AM Richard Fitzgerald
<rf@xxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxx>
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@xxxxxxxxxx>
-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
>
