On Tue, 17 Nov 2020 13:33:42 -0800
Kees Cook [off-list ref] wrote:

As I think got discussed in the thread, what you had here wouldn't work
in a CFI build if the function prototype of the call site and the
function don't match. (Though I can't tell if .func() is ever called?)

i.e. .func's prototype must match tp_stub_func()'s.

Hmm, I wonder how you handle tracepoints? This is called here:

include/linux/tracepoint.h:


#define DEFINE_TRACE_FN(_name, _reg, _unreg, proto, args)		\
	static const char __tpstrtab_##_name[]				\
	__section("__tracepoints_strings") = #_name;			\
	extern struct static_call_key STATIC_CALL_KEY(tp_func_##_name);	\
	int __traceiter_##_name(void *__data, proto);			\
	struct tracepoint __tracepoint_##_name	__used			\
	__section("__tracepoints") = {					\
		.name = __tpstrtab_##_name,				\
		.key = STATIC_KEY_INIT_FALSE,				\
		.static_call_key = &STATIC_CALL_KEY(tp_func_##_name),	\
		.static_call_tramp = STATIC_CALL_TRAMP_ADDR(tp_func_##_name), \
		.iterator = &__traceiter_##_name,			\
		.regfunc = _reg,					\
		.unregfunc = _unreg,					\
		.funcs = NULL };					\
	__TRACEPOINT_ENTRY(_name);					\
	int __traceiter_##_name(void *__data, proto)			\
	{								\
		struct tracepoint_func *it_func_ptr;			\
		void *it_func;						\
									\
		it_func_ptr =						\
			rcu_dereference_raw((&__tracepoint_##_name)->funcs); \
		do {							\
			it_func = (it_func_ptr)->func;			\
			__data = (it_func_ptr)->data;			\

			((void(*)(void *, proto))(it_func))(__data, args); \

			^^^^ called above ^^^^

Where args is unique for every tracepoint, but func is simply a void
pointer.

-- Steve


		} while ((++it_func_ptr)->func);			\
		return 0;						\
	}								\