linux-kernel - Re: [PATCH 1/2] kbuild: Remove '-mno-global-merge'

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Message-ID: <YkXKnRwvbMdvOtlJ@thelio-3990X>
Date:   Thu, 31 Mar 2022 08:37:01 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Sedat Dilek <sedat.dilek@...il.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>,
        Kees Cook <keescook@...omium.org>,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-um@...ts.infradead.org, llvm@...ts.linux.dev,
        patches@...ts.linux.dev
Subject: Re: [PATCH 1/2] kbuild: Remove '-mno-global-merge'

On Thu, Mar 31, 2022 at 09:11:12AM +0200, Sedat Dilek wrote:
> On Thu, Mar 31, 2022 at 5:27 AM Nathan Chancellor <nathan@...nel.org> wrote:
> >
> > This flag is specific to clang, where it is only used by the 32-bit and
> > 64-bit ARM backends. In certain situations, the presence of this flag
> > will cause a warning, as shown by commit 6580c5c18fb3 ("um: clang: Strip
> > out -mno-global-merge from USER_CFLAGS").
> >
> > Since commit 61163efae020 ("kbuild: LLVMLinux: Add Kbuild support for
> > building kernel with Clang") that added this flag back in 2014, there
> > have been quite a few changes to the GlobalMerge pass in LLVM. Building
> > several different ARCH=arm and ARCH=arm64 configurations with LLVM 11
> > (minimum) and 15 (current main version) with this flag removed (i.e.,
> > with the default of '-mglobal-merge') reveals no modpost warnings, so it
> > is likely that the issue noted in the comment is no longer relevant due
> > to changes in LLVM or modpost, meaning this flag can be removed.
> >
> > If any new warnings show up that are a result of the removal of this
> > flag, it can be added back under arch/arm{,64}/Makefile to avoid
> > warnings on other architectures.
> >
> > Signed-off-by: Nathan Chancellor <nathan@...nel.org>
> > ---
> >  Makefile | 4 ----
> >  1 file changed, 4 deletions(-)
> >
> > diff --git a/Makefile b/Makefile
> > index daeb5c88b50b..f2723d9bfca4 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -784,10 +784,6 @@ ifdef CONFIG_CC_IS_CLANG
> >  KBUILD_CPPFLAGS += -Qunused-arguments
> >  # The kernel builds with '-std=gnu89' so use of GNU extensions is acceptable.
> >  KBUILD_CFLAGS += -Wno-gnu
> > -# CLANG uses a _MergedGlobals as optimization, but this breaks modpost, as the
> > -# source of a reference will be _MergedGlobals and not on of the whitelisted names.
> > -# See modpost pattern 2
> > -KBUILD_CFLAGS += -mno-global-merge
> >  else
> >
> >  # gcc inanely warns about local variables called 'main'
> > --
> > 2.35.1
> >
> 
> I have tested this several times and was able to boot into bar metal -
> no problems with building and/or booting my kernel-modules.
> 
> Tested-by: Sedat Dilek <sedat.dilek@...il.com>
> Reviewed-by: Sedat Dilek <sedat.dilek@...il.com>

I would be very concerned if you did see any impact, given this flag is
ARM specific :) thanks as always for verifying!

> Just as a side-note:
> As with Linux v5.18-rc1 and -std=gnu11 we change the above comment ...?
> 
> # The kernel builds with '-std=gnu89' so use of GNU extensions is acceptable.
> KBUILD_CFLAGS += -Wno-gnu

It was updated as part of the shift to '-std=gnu11':

https://git.kernel.org/linus/e8c07082a810fbb9db303a2b66b66b8d7e588b53

The UML tree is based on 5.17-rc6, which does not have that change.
There should not be a merge conflict though.

Cheers,
Nathan