linux-kernel - Re: [PATCH v2 1/2] kunit: Allow passing function pointer to kunit_activate_static

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Message-ID: <CABVgOS=kECrGMXCg49QWsO1q-CGEzJdshxZ8XAA5GLKw58nR-g@mail.gmail.com>
Date: Fri, 22 Dec 2023 16:38:53 +0800
From: David Gow <davidgow@...gle.com>
To: Richard Fitzgerald <rf@...nsource.cirrus.com>
Cc: brendan.higgins@...ux.dev, rmoar@...gle.com, 
	linux-kselftest@...r.kernel.org, kunit-dev@...glegroups.com, 
	linux-kernel@...r.kernel.org, patches@...nsource.cirrus.com
Subject: Re: [PATCH v2 1/2] kunit: Allow passing function pointer to kunit_activate_static_stub()

On Thu, 21 Dec 2023 at 18:39, Richard Fitzgerald
<rf@...nsource.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@...nsource.cirrus.com>
> Reviewed-by: Rae Moar <rmoar@...gle.com>
> ---

This makes sense to me.

I was a little worried at first that this might actually be adding a
layer of indirection (which, given we're keying on the pointer,
would've needed to be done carefully), but this is an obvious fix to
me.

Reviewed-by: David Gow <davidgow@...gle.com>

Cheers,
-- David


> No changes since V1.
> ---
>  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
>

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