linux-kernel - Re: [PATCH v2 2/3] x86/boot/compressed: force hidden visibility for all symbol references

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Date:   Wed, 27 May 2020 14:29:01 -0400
From:   Brian Gerst <brgerst@...il.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Ard Biesheuvel <ardb@...nel.org>, linux-efi@...r.kernel.org,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 2/3] x86/boot/compressed: force hidden visibility for
 all symbol references

On Wed, May 27, 2020 at 2:08 PM Arvind Sankar <nivedita@...m.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@...nel.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?

--
Brian Gerst