Re: [PATCH 2/4] arm64: implement support for static call trampolines

[Ard Biesheuvel]

On Mon, 25 Oct 2021 at 17:05, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Mon, Oct 25, 2021 at 04:55:17PM +0200, Ard Biesheuvel wrote:
> > On Mon, 25 Oct 2021 at 16:47, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> > > Perhaps a little something like so.. Shaves 2 instructions off each
> > > trampoline.
> > >
> > > --- a/arch/arm64/include/asm/static_call.h
> > > +++ b/arch/arm64/include/asm/static_call.h
> > > @@ -11,9 +11,7 @@
> > >             "   hint    34      /* BTI C */                             \n" \
> > >                 insn "                                                  \n" \
> > >             "   ldr     x16, 0b                                         \n" \
> > > -           "   cbz     x16, 1f                                         \n" \
> > >             "   br      x16                                             \n" \
> > > -           "1: ret                                                     \n" \
> > >             "   .popsection                                             \n")
> > >
> > >  #define ARCH_DEFINE_STATIC_CALL_TRAMP(name, func)                      \
> > > --- a/arch/arm64/kernel/patching.c
> > > +++ b/arch/arm64/kernel/patching.c
> > > @@ -90,6 +90,11 @@ int __kprobes aarch64_insn_write(void *a
> > >         return __aarch64_insn_write(addr, &i, AARCH64_INSN_SIZE);
> > >  }
> > >
> > > +asm("__static_call_ret:                \n"
> > > +    "  ret                     \n")
> > > +
> >
> > This breaks BTI as it lacks the landing pad, and it will be called indirectly.
>
> Argh!
>
> > > +extern void __static_call_ret(void);
> > > +
> >
> > Better to have an ordinary C function here (with consistent linkage),
> > but we need to take the address in a way that works with Clang CFI.
>
> There is that.
>
> > As the two additional instructions are on an ice cold path anyway, I'm
> > not sure this is an obvious improvement tbh.
>
> For me it's both simpler -- by virtue of being more consistent, and
> smaller. So double win :-)
>
> That is; you're already relying on the literal being unconditionally
> updated for the normal B foo -> NOP path, and having the RET -> NOP path
> be handled differently is just confusing.
>
> At least, that's how I'm seeing it today...

Fair enough. I don't have a strong opinion either way, so I'll let
some other arm64 folks chime in as well.