Re: violating function pointer signature

[Nick Desaulniers]

On Wed, Nov 18, 2020 at 5:23 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Tue, Nov 17, 2020 at 03:34:51PM -0500, Steven Rostedt wrote:
>
> > > > Since all tracepoints callbacks have at least one parameter (__data), we
> > > > could declare tp_stub_func as:
> > > >
> > > > static void tp_stub_func(void *data, ...)
> > > > {
> > > >   return;
> > > > }
> > > >
> > > > And now C knows that tp_stub_func() can be called with one or more
> > > > parameters, and had better be able to deal with it!
> > >
> > > AFAIU this won't work.
> > >
> > > C99 6.5.2.2 Function calls
> > >
> > > "If the function is defined with a type that is not compatible with the type (of the
> > > expression) pointed to by the expression that denotes the called function, the behavior is
> > > undefined."
> >
> > But is it really a problem in practice. I'm sure we could create an objtool
> > function to check to make sure we don't break anything at build time.
>
> I think that as long as the function is completely empty (it never
> touches any of the arguments) this should work in practise.
>
> That is:
>
>   void tp_nop_func(void) { }

or `void tp_nop_func()` if you plan to call it with different
parameter types that are all unused in the body.  If you do plan to
use them, maybe a pointer to a tagged union would be safer?

>
> can be used as an argument to any function pointer that has a void
> return. In fact, I already do that, grep for __static_call_nop().
>
> I'm not sure what the LLVM-CFI crud makes of it, but that's their
> problem.

If you have instructions on how to exercise the code in question, we
can help test it with CFI.  Better to find any potential issues before
they get committed.
-- 
Thanks,
~Nick Desaulniers