Messages in this thread | | | Date | Thu, 29 Nov 2018 14:16:48 -0500 | From | Steven Rostedt <> | Subject | Re: [PATCH v2 4/4] x86/static_call: Add inline static call implementation for x86-64 |
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On Thu, 29 Nov 2018 10:58:40 -0800 Linus Torvalds <torvalds@linux-foundation.org> 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);
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
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