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Date:   Thu, 29 Nov 2018 14:12:47 -0600
From:   Josh Poimboeuf <jpoimboe@redhat.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Peter Zijlstra <peterz@infradead.org>,
        Andrew Lutomirski <luto@kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, mhiramat@kernel.org,
        jbaron@akamai.com, Jiri Kosina <jkosina@suse.cz>,
        David.Laight@aculab.com, bp@alien8.de, julia@ni.com,
        jeyu@kernel.org, Peter Anvin <hpa@zytor.com>
Subject: Re: [PATCH v2 4/4] x86/static_call: Add inline static call
 implementation for x86-64
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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