linux-kernel - Re: [PATCH] tracepoint: Fix CFI failures with tp_stub

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Date:   Thu, 23 Mar 2023 12:26:50 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-kernel@...r.kernel.org, mhiramat@...nel.org,
        "Jose E. Marchesi" <jose.marchesi@...cle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH] tracepoint: Fix CFI failures with tp_stub_func

On Thu, 23 Mar 2023 08:53:21 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> > --- a/include/linux/tracepoint.h
> > +++ b/include/linux/tracepoint.h
> > @@ -303,6 +303,7 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
> >  	__section("__tracepoints_strings") = #_name;			\
> >  	extern struct static_call_key STATIC_CALL_KEY(tp_func_##_name);	\
> >  	int __traceiter_##_name(void *__data, proto);			\
> > +	void __tracestub_##_name(void *, proto);			\
> >  	struct tracepoint __tracepoint_##_name	__used			\
> >  	__section("__tracepoints") = {					\
> >  		.name = __tpstrtab_##_name,				\
> > @@ -310,6 +311,7 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
> >  		.static_call_key = &STATIC_CALL_KEY(tp_func_##_name),	\
> >  		.static_call_tramp = STATIC_CALL_TRAMP_ADDR(tp_func_##_name), \
> >  		.iterator = &__traceiter_##_name,			\
> > +		.stub = &__tracestub_##_name,				\
> >  		.regfunc = _reg,					\
> >  		.unregfunc = _unreg,					\
> >  		.funcs = NULL };					\
> > @@ -330,6 +332,9 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
> >  		}							\
> >  		return 0;						\
> >  	}								\
> > +	void __tracestub_##_name(void *__data, proto)			\
> > +	{								\
> > +	}								\  
> 
> I purposely did not do this because this adds over a thousand stub
> functions! It adds one for *every* tracepoint (and that is a superset of
> trace events).

And the commit that added this code:

  befe6d946551 ("tracepoint: Do not fail unregistering a probe due to memory failure")

Has this in the change log:

    [ Note, this version does use undefined compiler behavior (assuming that
      a stub function with no parameters or return, can be called by a location
      that thinks it has parameters but still no return value. Static calls
      do the same thing, so this trick is not without precedent.

      There's another solution that uses RCU tricks and is more complex, but
      can be an alternative if this solution becomes an issue.

      Link: https://lore.kernel.org/lkml/20210127170721.58bce7cc@gandalf.local.home/
    ]

-- Steve