Re: [PATCH v2 4/4] x86/static_call: Add inline static call implementation for x86-64

[Steven Rostedt]

On Thu, 29 Nov 2018 10:58:40 -0800
Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> On Thu, Nov 29, 2018 at 10:47 AM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> >
> > Note, we do have a bit of control at what is getting called. The patch
> > set requires that the callers are wrapped in macros. We should not
> > allow just any random callers (like from asm).  
> 
> Actually, I'd argue that asm is often more controlled than C code.
> 
> Right now you can do odd things if you really want to, and have the
> compiler generate indirect calls to those wrapper functions.
> 
> For example, I can easily imagine a pre-retpoline compiler turning
> 
>      if (cond)
>         fn1(a,b)
>      else
>        fn2(a,b);
> 
> into a function pointer conditional
> 
>     (cond ? fn1 : fn2)(a,b);

If we are worried about such a construct, wouldn't a compiler barrier
before and after the static_call solve that?

	barrier();
	static_call(func...);
	barrier();

It should also stop tail calls too.

> 
> and honestly, the way "static_call()" works now, can you guarantee
> that the call-site doesn't end up doing that, and calling the
> trampoline function for two different static calls from one indirect
> call?
> 
> See what I'm talking about? Saying "callers are wrapped in macros"
> doesn't actually protect you from the compiler doing things like that.
> 
> In contrast, if the call was wrapped in an inline asm, we'd *know* the
> compiler couldn't turn a "call wrapper(%rip)" into anything else.

But then we need to implement all numbers of parameters.

-- Steve