kunit: Add a macro to wrap a deferred action function

KUnit's deferred action API accepts a void(*)(void *) function pointer
which is called when the test is exited. However, we very frequently
want to use existing functions which accept a single pointer, but which
may not be of type void*. While this is probably dodgy enough to be on
the wrong side of the C standard, it's been often used for similar
callbacks, and gcc's -Wcast-function-type seems to ignore cases where
the only difference is the type of the argument, assuming it's
compatible (i.e., they're both pointers to data).

However, clang 16 has introduced -Wcast-function-type-strict, which no
longer permits any deviation in function pointer type. This seems to be
because it'd break CFI, which validates the type of function calls.

This rather ruins our attempts to cast functions to defer them, and
leaves us with a few options. The one we've chosen is to implement a
macro which will generate a wrapper function which accepts a void*, and
casts the argument to the appropriate type.

For example, if you were trying to wrap:
void foo_close(struct foo *handle);
you could use:
KUNIT_DEFINE_ACTION_WRAPPER(kunit_action_foo_close,
			    foo_close,
			    struct foo *);

This would create a new kunit_action_foo_close() function, of type
kunit_action_t, which could be passed into kunit_add_action() and
similar functions.

In addition to defining this macro, update KUnit and its tests to use
it.

Link: https://github.com/ClangBuiltLinux/linux/issues/1750
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

Documentation/dev-tools/kunit/usage.rst

@@ -651,12 +651,16 @@ For example:
 	}
 
 Note that, for functions like device_unregister which only accept a single
-pointer-sized argument, it's possible to directly cast that function to
-a ``kunit_action_t`` rather than writing a wrapper function, for example:
+pointer-sized argument, it's possible to automatically generate a wrapper
+with the ``KUNIT_DEFINE_ACTION_WRAPPER()`` macro, for example:
 
 .. code-block:: C
 
-	kunit_add_action(test, (kunit_action_t *)&device_unregister, &dev);
+	KUNIT_DEFINE_ACTION_WRAPPER(device_unregister, device_unregister_wrapper, struct device *);
+	kunit_add_action(test, &device_unregister_wrapper, &dev);
+
+You should do this in preference to manually casting to the ``kunit_action_t`` type,
+as casting function pointers will break Control Flow Integrity (CFI).
 
 ``kunit_add_action`` can fail if, for example, the system is out of memory.
 You can use ``kunit_add_action_or_reset`` instead which runs the action