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SubjectRe: [PATCH 2/4] arm64: implement support for static call trampolines
On Mon, 25 Oct 2021 at 17:05, Peter Zijlstra <peterz@infradead.org> 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@infradead.org> 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.

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