linux-kernel - Re: "arm64: vdso: Substitute gettimeofday() with C implementation" breaks clang build

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Message-ID: <20190626093836.y2lofo54rhxw3xsm@willie-the-truck>
Date:   Wed, 26 Jun 2019 10:38:37 +0100
From:   Will Deacon <will@...nel.org>
To:     Vincenzo Frascino <vincenzo.frascino@....com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Qian Cai <cai@....pw>, Thomas Gleixner <tglx@...utronix.de>,
        Nathan Chancellor <natechancellor@...il.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: "arm64: vdso: Substitute gettimeofday() with C implementation"
 breaks clang build

On Tue, Jun 25, 2019 at 06:00:27PM +0100, Vincenzo Frascino wrote:
> On 25/06/2019 17:26, Nick Desaulniers wrote:
> > On Tue, Jun 25, 2019 at 7:54 AM Vincenzo Frascino
> > <vincenzo.frascino@....com> wrote:
> >>> but clang 7.0 is still use in many distros by default, so maybe this commit can
> >>> be fixed by adding a conditional check to use "small" if clang version < 8.0.
> >>>
> >>
> >> Could you please verify that the patch below works for you?
> > 
> > Should it be checking against CONFIG_CLANG_VERSION, or better yet be
> > using cc-option macro?
> > 
> 
> This is what I did in my proposed patch, but I was surprised that clang-7
> generates relocations that clang-8 does not.
> 
>   LD      arch/arm64/kernel/vdso/vdso.so.dbg
>   VDSOCHK arch/arm64/kernel/vdso/vdso.so.dbg
> 00000000000009d0 R_AARCH64_JUMP_SLOT  _mcount

Hmm. It would be nice to understand where the reference to _mcount is coming
from, since that sounds like ftrace is getting involved where it shouldn't
be.

Will