linux-kernel - Re: [PATCH] efi/libstub: Indicate clang the relocation mode for arm64

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Date:   Tue, 9 May 2017 13:50:36 -0700
From:   Greg Hackmann <ghackmann@...gle.com>
To:     Matthias Kaehlcke <mka@...omium.org>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
        Grant Grundler <grundler@...omium.org>,
        Michael Davidson <md@...gle.com>,
        Bernhard Rosenkränzer 
        <Bernhard.Rosenkranzer@...aro.org>
Subject: Re: [PATCH] efi/libstub: Indicate clang the relocation mode for arm64

On 05/09/2017 12:36 PM, Matthias Kaehlcke wrote:
> From: Greg Hackmann <ghackmann@...gle.com>
>
> Without any extra guidance, clang will generate libstub with either
> absolute or relative ELF relocations. Use the right combination of
> -fpic and -fno-pic on different files to avoid this.
>
> Signed-off-by: Greg Hackmann <ghackmann@...gle.com>
> Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@...aro.org>
> Signed-off-by: Matthias Kaehlcke <mka@...omium.org>
> ---
>  drivers/firmware/efi/libstub/Makefile | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
> index f7425960f6a5..ccbaaf4d8650 100644
> --- a/drivers/firmware/efi/libstub/Makefile
> +++ b/drivers/firmware/efi/libstub/Makefile
> @@ -11,6 +11,9 @@ cflags-$(CONFIG_X86)		+= -m$(BITS) -D__KERNEL__ -O2 \
>  				   -mno-mmx -mno-sse
>
>  cflags-$(CONFIG_ARM64)		:= $(subst -pg,,$(KBUILD_CFLAGS))
> +ifeq ($(cc-name),clang)
> +cflags-$(CONFIG_ARM64)		+= -fpic
> +endif
>  cflags-$(CONFIG_ARM)		:= $(subst -pg,,$(KBUILD_CFLAGS)) \
>  				   -fno-builtin -fpic -mno-single-pic-base
>
> @@ -38,6 +41,9 @@ $(obj)/lib-%.o: $(srctree)/lib/%.c FORCE
>
>  lib-$(CONFIG_EFI_ARMSTUB)	+= arm-stub.o fdt.o string.o random.o \
>  				   $(patsubst %.c,lib-%.o,$(arm-deps))
> +ifeq ($(cc-name),clang)
> +CFLAGS_arm64-stub.o            += -fno-pic
> +endif
>
>  lib-$(CONFIG_ARM)		+= arm32-stub.o
>  lib-$(CONFIG_ARM64)		+= arm64-stub.o
>

NAK.

This patch was labeled "HACK:" in our experimental tree.  There's no 
rhyme or reason to why this combination of -f[no-]pic flags generates 
code without problematic relocations.  It's inherently fragile, and was 
only intended as a temporary workaround until I (or someone more 
familiar with EFI) got a chance to revisit the problem.

Unless the gcc CFLAGS are also an artifact of "mess with -f[no-]pic 
until the compiler generates what you want", this doesn't belong upstream.