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

Josh Poimboeuf
2018-11-29T20:12:54Z

On Thu, Nov 29, 2018 at 10:58:40AM -0800, Linus Torvalds wrote:
> On Thu, Nov 29, 2018 at 10:47 AM Steven Rostedt <rostedt@goodmis.org> 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);
>
> 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.

I think objtool could warn about many such issues, including function
pointer references to trampolines and short tail call jumps.

--
Josh