linux-kernel - Re: [PATCH] x86/vdso: drop implicit common-page-size linker flag

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Message-ID: <CAKwvOd=75MRjaDbEjHRaMTm1MSC1wkLxPKhH_S3KXreP8yU9Uw@mail.gmail.com>
Date:   Fri, 7 Dec 2018 09:45:42 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     bp@...en8.de
Cc:     luto@...nel.org, Fangrui Song <maskray@...gle.com>,
        Andi Kleen <ak@...ux.intel.com>, andi@...stfloor.org,
        Dmitry Golovin <dima@...ovin.in>,
        Bill Wendling <morbo@...gle.com>, Rui Ueyama <ruiu@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>, mingo@...hat.com,
        hpa@...or.com, x86@...nel.org, LKML <linux-kernel@...r.kernel.org>,
        rafael@...indo.la, Stephen Hines <srhines@...gle.com>,
        Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] x86/vdso: drop implicit common-page-size linker flag

On Fri, Dec 7, 2018 at 2:18 AM Borislav Petkov <bp@...en8.de> wrote:
>
> On Thu, Dec 06, 2018 at 11:12:31AM -0800, ndesaulniers@...gle.com wrote:
> > These are implied by the target architecture and for x86_64 match the
> > max-page-size. The default for non-NaCl x86_64 is 0x1000 (4096).
> >
> > In bfd the common page size is defined as 0x1000 (4096) for non-NaCl
>
> Sodium Chloride?

Google's Native Client, a technology for running native code in a web
browser.  It's since been superseded by Portable Native Client (pNaCl)
and to an extent Web Assembly.  It seems that BFD still contains code
for NaCl.

>
> > x86_64 targets:
> >
> > bfd/elf64-x86-64.c:
> > 4998:#define ELF_COMMONPAGESIZE             0x1000
> >
> > For gold, the common page size is defined as 0x1000 (4096) for non-NaCl
> > x86_64 targets:
> >
> > gold/x86_64.cc:
> > 1413:  0x1000, // common_pagesize (overridable by -z common-page-size)
> > 1442:  0x1000, // common_pagesize (overridable by -z common-page-size)
> >
> > (ELF_COMMONPAGESIZE also defaults to ELF_MAXPAGESIZE when not set
> > explicitly for a target architecture in bfd/elfxx-target.h, but that's
> > not relevant for x86_64).
> >
> > Because it's implied by the target architecture, it's of questionable
> > use to implement in LLD.  This patch resolves one of the issues towards
> > using LLD to link an x86_64 kernel.
>
> LLD?

LLD is LLVM's linker.
https://lld.llvm.org/

>
> I can only guess what this commit message is about and have to look at
> the patch itself and then look at the LD(1) man page and rhyme up what
> it is aiming to do.
>
> How about rewriting it for mere mortals?

How does this sound:
TL;DR
-z common-page-size's default value is based on the target
architecture.  arch/x86/entry/vdso/Makefile sets it to the
architecture default, which is implicit and redundant.  Drop it.

-- 
Thanks,
~Nick Desaulniers