On Wed, Nov 18, 2020 at 02:59:29PM +0100, Florian Weimer wrote:

* Peter Zijlstra:

quoted

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) { }

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().

You can pass it as a function parameter, but in general, you cannot
call the function with a different prototype.  Even trivial
differences such as variadic vs non-variadic prototypes matter.

I don't think any tracepoint uses variadic argument.

The default Linux calling conventions are all of the cdecl family,
where the caller pops the argument off the stack.  You didn't quote
enough to context to tell whether other calling conventions matter in
your case.

This is strictly in-kernel, and I think we're all cdecl, of which the
important part is caller-cleanup. The function compiles to:

	RET

so whatever the arguments are is irrelevant.

quoted

I'm not sure what the LLVM-CFI crud makes of it, but that's their
problem.

LTO can cause problems as well, particularly with whole-program
optimization.

I don't think LTO can de-virtualize a dynamic array of function
pointers, so there's very little risk. That said, the __static_call_nop
case, where everything is inlined, is compiled sub-optimally for both
LLVM and GCC.