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References: <20200415221520.2692512-1-nivedita@alum.mit.edu> <20200415221520.2692512-4-nivedita@alum.mit.edu>
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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Thu, 16 Apr 2020 09:51:30 +0200
Message-ID: <CAMj1kXFMgrj8POtc0UQ6D-dbDKU+RmuHjUH-bAkMBAqjL_p+mg@mail.gmail.com>
Subject: Re: [PATCH 3/5] efi/x86: Remove __efistub_global annotation
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     linux-efi <linux-efi@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        X86 ML <x86@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Thu, 16 Apr 2020 at 00:15, Arvind Sankar <nivedita@alum.mit.edu> wrote:
>
> Instead of using __efistub_global to force variables into the .data
> section, leave them in the .bss but pull the EFI stub's .bss section
> into .data in the linker script for the compressed kernel.
>
> Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>

With the R_386_32/R_X86_64_64 check folded in:

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>

> ---
>  arch/x86/boot/compressed/vmlinux.lds.S |  1 +
>  drivers/firmware/efi/libstub/Makefile  | 12 ++++++++++--
>  drivers/firmware/efi/libstub/efistub.h |  4 ----
>  3 files changed, 11 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S
> index 508cfa6828c5..0dc5c2b9614b 100644
> --- a/arch/x86/boot/compressed/vmlinux.lds.S
> +++ b/arch/x86/boot/compressed/vmlinux.lds.S
> @@ -52,6 +52,7 @@ SECTIONS
>                 _data = . ;
>                 *(.data)
>                 *(.data.*)
> +               *(.bss.efistub)
>                 _edata = . ;
>         }
>         . = ALIGN(L1_CACHE_BYTES);
> diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
> index e5e76677f2da..0bb2916eb12b 100644
> --- a/drivers/firmware/efi/libstub/Makefile
> +++ b/drivers/firmware/efi/libstub/Makefile
> @@ -73,8 +73,8 @@ CFLAGS_arm64-stub.o           := -DTEXT_OFFSET=$(TEXT_OFFSET)
>  # a verification pass to see if any absolute relocations exist in any of the
>  # object files.
>  #
> -extra-$(CONFIG_EFI_ARMSTUB)    := $(lib-y)
> -lib-$(CONFIG_EFI_ARMSTUB)      := $(patsubst %.o,%.stub.o,$(lib-y))
> +extra-y        := $(lib-y)
> +lib-y  := $(patsubst %.o,%.stub.o,$(lib-y))
>
>  STUBCOPY_FLAGS-$(CONFIG_ARM64) += --prefix-alloc-sections=.init \
>                                    --prefix-symbols=__efistub_
> @@ -89,6 +89,14 @@ STUBCOPY_FLAGS-$(CONFIG_ARM) += --rename-section .data=.data.efistub \
>                                    --rename-section .bss=.bss.efistub,load,alloc
>  STUBCOPY_RELOC-$(CONFIG_ARM)   := R_ARM_ABS
>
> +#
> +# For x86, bootloaders like systemd-boot or grub-efi do not zero-initialize the
> +# .bss section, so the .bss section of the EFI stub needs to be included in the
> +# .data section of the compressed kernel to ensure initialization. Rename the
> +# .bss section here so it's easy to pick out in the linker script.
> +#
> +STUBCOPY_FLAGS-$(CONFIG_X86)   += --rename-section .bss=.bss.efistub,load,alloc
> +
>  $(obj)/%.stub.o: $(obj)/%.o FORCE
>         $(call if_changed,stubcopy)
>
> diff --git a/drivers/firmware/efi/libstub/efistub.h b/drivers/firmware/efi/libstub/efistub.h
> index a92d42ffd9f7..49651e20bb9f 100644
> --- a/drivers/firmware/efi/libstub/efistub.h
> +++ b/drivers/firmware/efi/libstub/efistub.h
> @@ -25,11 +25,7 @@
>  #define EFI_ALLOC_ALIGN                EFI_PAGE_SIZE
>  #endif
>
> -#if defined(CONFIG_X86)
> -#define __efistub_global       __section(.data)
> -#else
>  #define __efistub_global
> -#endif
>
>  extern bool __pure nochunk(void);
>  extern bool __pure nokaslr(void);
> --
> 2.24.1
>