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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Wed, 27 May 2020 20:30:23 +0200
Message-ID: <CAMj1kXFGzLRiioU=sqA0hhE1oJC5uxDg4GFcSr7eA1+HoqupGw@mail.gmail.com>
Subject: Re: [PATCH v2 2/3] x86/boot/compressed: force hidden visibility for
 all symbol references
To:     Brian Gerst <brgerst@gmail.com>
Cc:     Arvind Sankar <nivedita@alum.mit.edu>,
        linux-efi <linux-efi@vger.kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
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On Wed, 27 May 2020 at 20:29, Brian Gerst <brgerst@gmail.com> wrote:
>
> On Wed, May 27, 2020 at 2:08 PM Arvind Sankar <nivedita@alum.mit.edu> wrote:
> >
> > On Sat, May 23, 2020 at 02:00:20PM +0200, Ard Biesheuvel wrote:
> > > Eliminate all GOT entries in the decompressor binary, by forcing hidden
> > > visibility for all symbol references, which informs the compiler that
> > > such references will be resolved at link time without the need for
> > > allocating GOT entries.
> > >
> > > To ensure that no GOT entries will creep back in, add an assertion to
> > > the decompressor linker script that will fire if the .got section has
> > > a non-zero size.
> > >
> > > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> > > ---
> > >  arch/x86/boot/compressed/Makefile      |  1 +
> > >  arch/x86/boot/compressed/hidden.h      | 19 +++++++++++++++++++
> > >  arch/x86/boot/compressed/vmlinux.lds.S |  1 +
> > >  3 files changed, 21 insertions(+)
> > >
> > > diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> > > index 5f7c262bcc99..aa9ed814e5fa 100644
> > > --- a/arch/x86/boot/compressed/Makefile
> > > +++ b/arch/x86/boot/compressed/Makefile
> > > @@ -40,6 +40,7 @@ KBUILD_CFLAGS += $(call cc-disable-warning, gnu)
> > >  KBUILD_CFLAGS += -Wno-pointer-sign
> > >  KBUILD_CFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
> > >  KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
> > > +KBUILD_CFLAGS += -include hidden.h
> > >
> >
> > Ard, from the other thread [1] in case you missed it -- the plain
> > hidden.h fails to build in-tree. We need something like
> >         KBUILD_CFLAGS += -include $(srctree)/$(src)/hidden.h
> > instead.
> >
> > [1] https://lore.kernel.org/lkml/20200526153104.GC2190602@rani.riverdale.lan/
>
> How about using -fvisibility=hidden instead of including this header?
>

That only works for definitions, not for extern declarations.