Re: [PATCH] tracepoint: Do not fail unregistering a probe due to memory allocation

Mathieu Desnoyers
2020-11-17T23:13:19Z

----- On Nov 17, 2020, at 5:19 PM, rostedt rostedt@goodmis.org wrote:

> On Tue, 17 Nov 2020 13:33:42 -0800
> Kees Cook <keescook@chromium.org> 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.

That being said, the called functions have a prototype which match the
caller prototype exactly. So within the tracepoint internal data structures,
this function pointer is indeed a void pointer, but it is cast to a prototype
matching the callees to perform the calls. I suspect that as long as CFI checks
that caller/callees prototypes are compatible at runtime when the actual
calls happen, this all works fine.

Thanks,

Mathieu

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

--
Mathieu Desnoyers
EfficiOS Inc.