[PATCH] arm64: assembler: make adr_l work in modules under KASLR

[Ard Biesheuvel]

On 11 January 2017 at 15:25, Ard Biesheuvel <ard.biesheuvel at linaro.org> wrote:
> On 11 January 2017 at 15:18, Mark Rutland <mark.rutland at arm.com> wrote:
>> Hi Ard,
>>
>> On Wed, Jan 11, 2017 at 02:54:53PM +0000, Ard Biesheuvel wrote:
>>> When CONFIG_RANDOMIZE_MODULE_REGION_FULL=y, the offset between loaded
>>> modules and the core kernel may exceed 4 GB, putting symbols exported
>>> by the core kernel out of the reach of the ordinary adrp/add instruction
>>> pairs used to generate relative symbol references. So make the adr_l
>>> macro emit a movz/movk sequence instead when executing in module context.
>>
>> AFAICT, we only use adr_l in a few assembly files that shouldn't matter
>> to modules:
>>
>> * arch/arm64/kernel/head.S
>> * arch/arm64/kernel/sleep.S
>> * arch/arm64/kvm/hyp-init.S
>> * arch/arm64/kvm/hyp/hyp-entry.S
>>
>> ... so I don't follow why we need this.
>>
>> Have I missed something? Or do you intend to use this in module code in
>> future?
>>
>
> Yes. E.g., the scalar AES cipher that I am proposing for v4.11 reuses
> the lookup tables from crypto/aes_generic.c, which may be built into
> the core kernel, while the code itself may be built as a module.
>
> But in general, if the macro is available to modules, I would like to
> make sure that it does not result in code that builds fine but may
> fail in some cases only at runtime, especially given the fact that
> there is also a Cortex-A53 erratum regarding adrp instructions, for
> which reason we build modules with -mcmodel=large (which amounts to
> the same thing as the patch above)
>

Actually, we could test for

defined(MODULE) && defined(CONFIG_ARM64_MODULE_CMODEL_LARGE)

instead, so we revert to adrp/add pairs for modules if the erratum and
KASLR are both disabled

>> It seems somewhat surprising to me to have adr_l expand to something
>> that doesn't use adr/adrp, but that's not necessarily a problem.
>>
>
> I did realise that, but I don't think it is a problem tbh.