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

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Date:   Wed, 8 Jan 2020 16:47:51 +0100
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Ard Biesheuvel <ardb@...nel.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Andy Lutomirski <luto@...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: [RFC PATCH 2/3] x86/boot/compressed: force hidden visibility for
 all symbol references

On Wed, 8 Jan 2020 at 16:40, Arvind Sankar <nivedita@...m.mit.edu> wrote:
>
> On Wed, Jan 08, 2020 at 11:23:03AM +0100, 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 56aa5fa0a66b..361df91b2288 100644
> > --- a/arch/x86/boot/compressed/Makefile
> > +++ b/arch/x86/boot/compressed/Makefile
> > @@ -39,6 +39,7 @@ KBUILD_CFLAGS += $(call cc-disable-warning, address-of-packed-member)
> >  KBUILD_CFLAGS += $(call cc-disable-warning, gnu)
> >  KBUILD_CFLAGS += -Wno-pointer-sign
> >  KBUILD_CFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
> > +KBUILD_CFLAGS += -include hidden.h
> >
>
> This should be added to drivers/firmware/efi/libstub as well in case
> future code changes bring in global references there?

The EFI stub already sets the hidden visibility attribute for the few
external symbol references that it contains, so it is not needed in
the context of this series.

In the future, we can revisit this if we want to get rid of the
various __pure getter functions, but that requires thorough testing on
other architectures and toolchains, so I'd prefer to leave that for
later.