linux-kernel - Re: [PATCH v2 08/18] arm64: don't disable ADR_PREL_PG_HI21* with ARM64_ERRATUM

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Date:   Thu, 16 Nov 2017 22:25:46 +0000
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kees Cook <keescook@...omium.org>,
        linux-kbuild@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Greg Hackmann <ghackmann@...gle.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Yury Norov <ynorov@...iumnetworks.com>,
        Alex Matveev <alxmtvv@...il.com>,
        Matthias Kaehlcke <mka@...omium.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Maxim Kuvyrkov <maxim.kuvyrkov@...aro.org>
Subject: Re: [PATCH v2 08/18] arm64: don't disable ADR_PREL_PG_HI21* with ARM64_ERRATUM_843419

On 16 November 2017 at 22:14, Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:
> On 16 November 2017 at 21:37, Sami Tolvanen <samitolvanen@...gle.com> wrote:
>> On Thu, Nov 16, 2017 at 04:34:03PM +0000, Ard Biesheuvel wrote:
>>> You still have not explained to us how GOLD avoids the erratum.
>>
>> Sorry, I didn't realize you were asking that. If gold spots erratum
>> sequences, looks like it creates stubs to break them up:
>>
>>   https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=gold/aarch64.cc#l8396
>>
>> It also attempts to optimize the code by replacing adrps in these
>> sequences with adr where possible, but otherwise doesn't appear to
>> touch them:
>>
>>   https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=gold/aarch64.cc#l2053
>>
>
> OK, so my concern here is that this code probably only operates on
> fully linked binaries, and not partially linked object files like
> kernel modules. The same applies to ld.bfd, which is why we need to
> use the large module

*model* not module

> instead of a code model that may emit adrp
> instructions..
>
> What is preventing us from using the large model with clang? I know it
> uses movk/movz pairs rather than literals, but this shouldn't matter
> for modules, given that we support static ELF relocations.