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

[Ard Biesheuvel]

On 25 February 2016 at 17:49, Ard Biesheuvel <ard.biesheuvel at linaro.org> wrote:
> On 25 February 2016 at 17:46, Will Deacon <will.deacon at 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 at 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

>> Also, can I rely on the PLT not clobbering the condition code?
>>
>
> For my particular implementation, yes.