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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Mon, 24 Feb 2020 12:33:49 -0800
Message-ID: <CAKwvOdniNba30cUX9QAZdVPg2MhjVETVgrvUUzwaHF70Dr3PrQ@mail.gmail.com>
Subject: Re: [PATCH 1/2] arch/x86: Use -fno-asynchronous-unwind-tables to
 suppress .eh_frame sections
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     Borislav Petkov <bp@alien8.de>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Michael Matz <matz@suse.de>, Fangrui Song <maskray@google.com>,
        Kees Cook <keescook@chromium.org>
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Sender: linux-kernel-owner@vger.kernel.org
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On Sun, Feb 23, 2020 at 11:37 AM 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.

Another benefit is that -fno-asynchronous-unwind-tables may help
reduce the size of .text!
https://stackoverflow.com/a/26302715/1027966

>
> 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>
> [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

I think we want to add this flag a little lower, line 27 has:

KBUILD_CFLAGS     := $(cflags-y) -DDISABLE_BRANCH_PROFILING \

so the `cflags-y` variable you modify in this hunk will only set
-fno-asynchronous-unwind-tables for CONFIG_X86, which I don't think is
intentional.  Though when I run

$ llvm-readelf -S drivers/firmware/efi/libstub/lib.a | grep eh_frame

after doing an x86_64 defconfig, I don't get any hits. Do you observe
.eh_frame sections on any of these objects in this dir? (I'm fine
adding it to be safe, but I'm curious why I'm not seeing any
.eh_frame)

>
>  # arm64 uses the full KBUILD_CFLAGS so it's necessary to explicitly
>  # disable the stackleak plugin
> --
> 2.24.1
>


-- 
Thanks,
~Nick Desaulniers