[PATCH v5sub2 1/8] arm64: add support for module PLTs

[will.deacon@arm.com (Will Deacon)]

On Thu, Feb 25, 2016 at 05:50:17PM +0100, Ard Biesheuvel wrote:
> On 25 February 2016 at 17:49, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
> > On 25 February 2016 at 17:46, Will Deacon <will.deacon@arm.com> wrote:
> >> On Thu, Feb 25, 2016 at 05:43:35PM +0100, Ard Biesheuvel wrote:
> >>> On 25 February 2016 at 17:42, Will Deacon <will.deacon@arm.com> wrote:
> >>> > On Thu, Feb 25, 2016 at 05:33:25PM +0100, Ard Biesheuvel wrote:
> >>> >> AFAIK, gcc never uses x18 (the platform register) so we may be able to
> >>> >> use that instead. We'd need confirmation from the toolchain guys,
> >>> >> though ...
> >>> >
> >>> > In fact, a better thing to do is probably for the atomic code to
> >>> > save/restore those register explicitly and then remove them from the
> >>> > cflags above.
> >>> >
> >>> > I'll try hacking something together...
> >>> >
> >>>
> >>> That would be more correct, indeed. Also, the PLT can only use br
> >>> <reg> so it will always enter the callee with the preserved value
> >>> stacked, and so the callee needs to be modified in any case.
> >>
> >> Wait -- why does the callee need to be modified here? It has no idea
> >> about whether or not it was invoked via a PLT.
> >>
> >
> > No, it doesn't, and that is exactly the problem: the PLT must end in a
> > br <reg> instruction because that is the only branch instruction with
> > unlimited range. That means you will always enter the callee with its
> > address in some register rather than the value that should have been
> > preserved. I don't see a way to manage that from the callee
> >
> 
> *caller! dammit

The caller can do:

save x16, 17
bl plt
restore x16, x17

the plt will do:

get_addr_of_callee_into_x16_and_clobber_x17_or_something
br callee

then the callee will be compiled with all those weird options, but *not*
the ones specifying x16 and x17. That means it can happily use those guys
as scratch, because the caller will take care of them.

Will