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Date:   Sun, 24 May 2020 17:12:14 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     linux-efi@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Arvind Sankar <nivedita@...m.mit.edu>
Subject: Re: [PATCH v2 2/3] x86/boot/compressed: force hidden visibility for
 all symbol references


* Ard Biesheuvel <ardb@...nel.org> 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

> + * When building position independent code with GCC using the -fPIC option,
> + * (or even the -fPIE one on older versions), it will assume that we are
> + * building a dynamic object (either a shared library or an executable) that
> + * may have symbol references that can only be resolved at load time. For a
> + * variety of reasons (ELF symbol preemption, the CoW footprint of the section
> + * that is modified by the loader), this results in all references to symbols
> + * with external linkage to go via entries in the Global Offset Table (GOT),
> + * which carries absolute addresses which need to be fixed up when the
> + * executable image is loaded at an offset which is different from its link
> + * time offset.
> + *
> + * Fortunately, there is a way to inform the compiler that such symbol
> + * references will be satisfied at link time rather than at load time, by
> + * giving them 'hidden' visibility.
> + */
> +
> +#pragma GCC visibility push(hidden)

BTW., how many such GOT entries did we have before this change, on a typical kernel?

> +ASSERT (SIZEOF(.got) == 0, "Unexpected GOT entries detected!")

s/ASSERT (
 /ASSERT(

Thanks,

	Ingo

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