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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Thu, 3 Mar 2022 10:39:58 -0800
Message-ID: <CAKwvOdkA1c3U6+-6hB3Nj9m2Z8eYzsFWm5X4se2eq_cF3NB2jA@mail.gmail.com>
Subject: Re: [PATCH] um: clang: Strip out -mno-global-merge from USER_CFLAGS
To:     Nathan Chancellor <nathan@kernel.org>
Cc:     Kees Cook <keescook@chromium.org>, David Gow <davidgow@google.com>,
        Jeff Dike <jdike@addtoit.com>,
        Richard Weinberger <richard@nod.at>,
        Anton Ivanov <anton.ivanov@cambridgegreys.com>,
        Masahiro Yamada <masahiroy@kernel.org>,
        linux-um@lists.infradead.org, linux-kbuild@vger.kernel.org,
        linux-kselftest@vger.kernel.org, kunit-dev@googlegroups.com,
        llvm@lists.linux.dev, linux-kernel@vger.kernel.org,
        linux-hardening@vger.kernel.org
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On Thu, Mar 3, 2022 at 10:26 AM Nathan Chancellor <nathan@kernel.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
> > +


-- 
Thanks,
~Nick Desaulniers