[PATCH v2 1/2] kunit: Allow passing function pointer to kunit_activate_static_stub()

[PATCH v2 1/2] kunit: Allow passing function pointer to kunit_activate_static_stub()

[Richard Fitzgerald]

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>
Reviewed-by: Rae Moar <rmoar@google.com>
---
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

[PATCH v2 2/2] kunit: Add example of kunit_activate_static_stub() with pointer-to-function

[Richard Fitzgerald]

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>
---
V2: Added comment for add_one_fn_ptr.
---
 lib/kunit/kunit-example-test.c | 35 ++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/lib/kunit/kunit-example-test.c b/lib/kunit/kunit-example-test.c
index 359dbee10201..798924f7cc86 100644
--- a/lib/kunit/kunit-example-test.c
+++ b/lib/kunit/kunit-example-test.c
@@ -168,6 +168,16 @@ static int subtract_one(int i)
 	return i - 1;
 }
 
+/*
+ * If the function to be replaced is static within a module it is
+ * useful to export a pointer to that function instead of having
+ * to change the static function to a non-static exported function.
+ *
+ * This pointer simulates a module exporting a pointer to a static
+ * function.
+ */
+static int (* const add_one_fn_ptr)(int i) = add_one;
+
 /*
  * This test shows the use of static stubs.
  */
@@ -187,6 +197,30 @@ 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[] = {
@@ -259,6 +293,7 @@ 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(example_priv_test),
 	KUNIT_CASE_PARAM(example_params_test, example_gen_params),
 	KUNIT_CASE_SLOW(example_slow_test),
-- 
2.30.2

Re: [PATCH v2 1/2] kunit: Allow passing function pointer to kunit_activate_static_stub()

[David Gow]

[-- Attachment #1: Type: text/plain, Size: 3684 bytes --]

On Thu, 21 Dec 2023 at 18:39, 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>
> Reviewed-by: Rae Moar <rmoar@google.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@google.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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Re: [PATCH v2 2/2] kunit: Add example of kunit_activate_static_stub() with pointer-to-function

[David Gow]

[-- Attachment #1: Type: text/plain, Size: 546 bytes --]

On Thu, 21 Dec 2023 at 18:39, Richard Fitzgerald
<rf@opensource.cirrus.com> wrote:
>
> 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>
> ---

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

Thanks,
-- David

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