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Date:   Tue, 19 Oct 2021 18:00:22 +0000
To:     Sami Tolvanen <samitolvanen@google.com>
From:   Alexander Lobakin <alobakin@pm.me>
Cc:     Alexander Lobakin <alobakin@pm.me>,
        Peter Zijlstra <peterz@infradead.org>,
        Borislav Petkov <bp@alien8.de>, X86 ML <x86@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        andrew.cooper3@citrix.com, LKML <linux-kernel@vger.kernel.org>,
        alexei.starovoitov@gmail.com,
        Nick Desaulniers <ndesaulniers@google.com>
Reply-To: Alexander Lobakin <alobakin@pm.me>
Subject: Re: [PATCH 4/9] x86/alternative: Implement .retpoline_sites support
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From: Sami Tolvanen <samitolvanen@google.com>
Date: Tue, 19 Oct 2021 08:37:14 -0700

> On Tue, Oct 19, 2021 at 3:17 AM Peter Zijlstra <peterz@infradead.org> wro=
te:
> >
> >
> > + Sami
> >
> > (Sami, for context:
> >
> >   https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/log/=
?h=3Dobjtool/core
> >
> > which contains the following code:
> >
> > +       void (*target)(void);
> > +       int reg, i =3D 0;
> > +
> > +       target =3D addr + insn->length + insn->immediate.value;
> > +       reg =3D (target - &__x86_indirect_thunk_rax) /
> > +             (&__x86_indirect_thunk_rcx - &__x86_indirect_thunk_rax);
> > +
> > +       if (WARN_ON_ONCE(reg & ~0xf))
> > +               return -1;
> >
> > which blows up something fierce on clang-cfi)
> >
> > On Tue, Oct 19, 2021 at 09:47:26AM +0000, Alexander Lobakin wrote:
> >
> > > Oh okay, it's because of ClangCFI:
> > >
> > > SMP alternatives: You were looking for __typeid__ZTSFvvE_global_addr+=
0x370/0x1410 at 0xffffffffa523cd60,>
> > > SMP alternatives: rax is __typeid__ZTSFvvE_global_addr+0x360/0x1410 a=
t 0xffffffffa523cd50
> > >
> > > Sorry for confusing, seems like it's a side effect of using it on
> > > Clang 12 while the original series supports only 13+. I'll double
> > > check and let know if find something.
> >
> > I'm thinking CFI will totally screw this up regardless, seeing how a
> > function pointer is taken, and the CFI magicks will turn that into one
> > of those weird trampolines instead of the actual symbol.
> >
> > The compiler could of course deduce that these addresses are never
> > called and don't escape the function, and therefore doesn't need to do
> > the CFI transformation on then, but I'm guessing it isn't quite that
> > clever.
>
> Yes, it's unfortunately not that clever.
>
> > Also doing CFI on retpoline thunks seems 'weird', they have a very
> > particular calling convention, excplicitly very much not the standard C
> > one. Can't we mark them using asmlinkage or something to tell the
> > compiler to politely 'bugger off' or somesuch ;-)
>
> I confirmed that using an opaque type for the thunk declaration fixes
> this issue with CFI. It also makes it obvious that these are not
> callable from C code.

Oh, glad we caught this then, much thanks y'all!

> Sami

Thanks,
Al