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

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Message-ID: <20200527143650.GA3103408@rani.riverdale.lan>
Date:   Wed, 27 May 2020 10:36:50 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
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

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/