linux-kernel - Re: [PATCH] um: clang: Strip out -mno-global-merge from USER

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Date:   Fri, 4 Mar 2022 12:42:54 +0100
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        David Gow <davidgow@...gle.com>, Jeff Dike <jdike@...toit.com>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        linux-um@...ts.infradead.org, linux-kbuild@...r.kernel.org,
        linux-kselftest@...r.kernel.org, kunit-dev@...glegroups.com,
        llvm@...ts.linux.dev, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] um: clang: Strip out -mno-global-merge from USER_CFLAGS

On Thu, Mar 3, 2022 at 9:43 PM Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>
> On Thu, Mar 3, 2022 at 10:26 AM Nathan Chancellor <nathan@...nel.org> wrote:
> >
> > On Thu, Mar 03, 2022 at 10:04:28AM -0800, Kees Cook wrote:
> > > How does -mno-global-merge get KBUILD_CFLAGS in the first place? If it's
> > > arm/arm64 only, shouldn't that get relocated to those architectures?
> > >
> > > *time travel* found it:
> > >
> > > 61163efae020 ("kbuild: LLVMLinux: Add Kbuild support for building kernel with Clang")
> > >
> > > So I think this may have been universally true long ago, and now only
> > > arm/arm64 need it?
>
> Looks like that's the case from LLVM sources.
>
> <snip>
>
> > > diff --git a/arch/arm/Makefile b/arch/arm/Makefile
> > > index a2391b8de5a5..dcab28c44c26 100644
> > > --- a/arch/arm/Makefile
> > > +++ b/arch/arm/Makefile
> > > @@ -48,6 +48,13 @@ CHECKFLAGS += -D__ARMEL__
> > >  KBUILD_LDFLAGS       += -EL
> > >  endif
> > >
> > > +ifdef CONFIG_CC_IS_CLANG
> > > +# 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.
>
> I think there's a typo in the original comment.
> s/on of/one of/ ?
>
> Also, I'm not sure what's meant by _MergedGlobals. Perhaps this is an
> opportunity to make this clearer?
>
> "Clang's "global-merge" pass (implemented only for arm and aarch64)
> may break modpost Pattern 2 if symbols are renamed and thus don't
> appear on modpost's allowlist.
>
> > > +# See modpost pattern 2
> > > +KBUILD_CFLAGS        += -mno-global-merge
> > > +endif
> > > +
>

I can remember on x86-64 I was able to build and boot by dropping it entirely.

- Sedat -

>
>
> --
> Thanks,
> ~Nick Desaulniers