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SubjectRe: [PATCH v2 1/2] arch/x86: Use -fno-asynchronous-unwind-tables to suppress .eh_frame sections
On Mon, Feb 24, 2020 at 3:21 PM Arvind Sankar <nivedita@alum.mit.edu> wrote:
>
> While discussing a patch to discard .eh_frame from the compressed
> vmlinux using the linker script, Fangrui Song pointed out [1] that these
> sections shouldn't exist in the first place because arch/x86/Makefile
> uses -fno-asynchronous-unwind-tables.
>
> It turns out this is because the Makefiles used to build the compressed
> kernel redefine KBUILD_CFLAGS, dropping this flag.
>
> Add the flag to the Makefile for the compressed kernel, as well as the
> EFI stub Makefile to fix this.
>
> Also add the flag to boot/Makefile and realmode/rm/Makefile so that the
> kernel's boot code (boot/setup.elf) and realmode trampoline
> (realmode/rm/realmode.elf) won't be compiled with .eh_frame sections,
> since their linker scripts also just discard it.
>
> Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
> Suggested-By: Fangrui Song <maskray@google.com>

Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

> [1] https://lore.kernel.org/lkml/20200222185806.ywnqhfqmy67akfsa@google.com/
> ---
> arch/x86/boot/Makefile | 1 +
> arch/x86/boot/compressed/Makefile | 1 +
> arch/x86/realmode/rm/Makefile | 1 +
> drivers/firmware/efi/libstub/Makefile | 3 ++-
> 4 files changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/boot/Makefile b/arch/x86/boot/Makefile
> index 012b82fc8617..24f011e0adf1 100644
> --- a/arch/x86/boot/Makefile
> +++ b/arch/x86/boot/Makefile
> @@ -68,6 +68,7 @@ clean-files += cpustr.h
> KBUILD_CFLAGS := $(REALMODE_CFLAGS) -D_SETUP
> KBUILD_AFLAGS := $(KBUILD_CFLAGS) -D__ASSEMBLY__
> KBUILD_CFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
> +KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
> GCOV_PROFILE := n
> UBSAN_SANITIZE := n
>
> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> index 26050ae0b27e..c33111341325 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 += -fno-asynchronous-unwind-tables
>
> KBUILD_AFLAGS := $(KBUILD_CFLAGS) -D__ASSEMBLY__
> GCOV_PROFILE := n
> diff --git a/arch/x86/realmode/rm/Makefile b/arch/x86/realmode/rm/Makefile
> index 99b6332ba540..b11ec5d8f8ac 100644
> --- a/arch/x86/realmode/rm/Makefile
> +++ b/arch/x86/realmode/rm/Makefile
> @@ -71,5 +71,6 @@ $(obj)/realmode.relocs: $(obj)/realmode.elf FORCE
> KBUILD_CFLAGS := $(REALMODE_CFLAGS) -D_SETUP -D_WAKEUP \
> -I$(srctree)/arch/x86/boot
> KBUILD_AFLAGS := $(KBUILD_CFLAGS) -D__ASSEMBLY__
> +KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
> GCOV_PROFILE := n
> UBSAN_SANITIZE := n
> diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
> index 98a81576213d..a1140c4ee478 100644
> --- a/drivers/firmware/efi/libstub/Makefile
> +++ b/drivers/firmware/efi/libstub/Makefile
> @@ -12,7 +12,8 @@ cflags-$(CONFIG_X86) += -m$(BITS) -D__KERNEL__ -O2 \
> -mno-mmx -mno-sse -fshort-wchar \
> -Wno-pointer-sign \
> $(call cc-disable-warning, address-of-packed-member) \
> - $(call cc-disable-warning, gnu)
> + $(call cc-disable-warning, gnu) \
> + -fno-asynchronous-unwind-tables
>
> # arm64 uses the full KBUILD_CFLAGS so it's necessary to explicitly
> # disable the stackleak plugin
> --
> 2.24.1
>


--
Thanks,
~Nick Desaulniers

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