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From: Nick Desaulniers <ndesaulniers@google.com>
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>
Subject: Re: [PATCH 1/2] arch/x86: Use -fno-asynchronous-unwind-tables to suppress .eh_frame sections
Date: Mon, 24 Feb 2020 12:33:49 -0800	[thread overview]
Message-ID: <CAKwvOdniNba30cUX9QAZdVPg2MhjVETVgrvUUzwaHF70Dr3PrQ@mail.gmail.com> (raw)
In-Reply-To: <20200223193715.83729-2-nivedita@alum.mit.edu>

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

  reply	other threads:[~2020-02-24 20:34 UTC|newest]

Thread overview: 115+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-09 15:02 [PATCH 1/2] x86/boot/compressed/64: Remove .bss/.pgtable from bzImage Arvind Sankar
2020-01-09 15:02 ` [PATCH 2/2] x86/boot/compressed: Remove unnecessary sections " Arvind Sankar
2020-02-06 11:44   ` Kees Cook
2020-02-19 16:55   ` [tip: x86/boot] " tip-bot2 for Arvind Sankar
2020-02-22  5:08   ` [PATCH 2/2] " Nathan Chancellor
2020-02-22  6:55     ` Borislav Petkov
2020-02-22  7:02       ` Nathan Chancellor
2020-02-22  7:21         ` Fangrui Song
2020-02-22  7:42           ` Nathan Chancellor
2020-02-22 15:37             ` Arvind Sankar
2020-02-22 16:44               ` Arvind Sankar
2020-02-22 17:18                 ` [PATCH] x86/boot/compressed: Fix compressed kernel linking with lld Arvind Sankar
2020-02-22 18:14                   ` Nathan Chancellor
2020-02-22 18:58                     ` Fangrui Song
2020-02-22 20:17                       ` Arvind Sankar
2020-02-22 21:01                         ` Fangrui Song
2020-02-22 23:33                           ` Nick Desaulniers
2020-02-22 23:57                             ` Arvind Sankar
2020-02-23 19:37                               ` [PATCH 0/2] Stop generating .eh_frame sections Arvind Sankar
2020-02-24  4:15                                 ` Nathan Chancellor
2020-02-24 20:49                                 ` Nick Desaulniers
2020-02-24 21:53                                   ` Arvind Sankar
2020-02-24 22:01                                     ` Nick Desaulniers
2020-02-23 19:37                               ` [PATCH 1/2] arch/x86: Use -fno-asynchronous-unwind-tables to suppress " Arvind Sankar
2020-02-24 20:33                                 ` Nick Desaulniers [this message]
2020-02-24 21:05                                   ` Arvind Sankar
2020-02-24 21:12                                     ` Fangrui Song
2020-02-24 21:17                                       ` Nick Desaulniers
2020-02-24 21:22                                         ` Nick Desaulniers
2020-02-23 19:37                               ` [PATCH 2/2] arch/x86: Drop unneeded linker script discard of .eh_frame Arvind Sankar
2020-02-24 20:45                                 ` Nick Desaulniers
2020-02-24 21:33                                   ` Arvind Sankar
2020-02-24 21:58                                     ` Nick Desaulniers
2020-02-24 23:21                                       ` [PATCH v2 0/2] Stop generating .eh_frame sections Arvind Sankar
2020-02-24 23:21                                       ` [PATCH v2 1/2] arch/x86: Use -fno-asynchronous-unwind-tables to suppress " Arvind Sankar
2020-02-24 23:28                                         ` Nathan Chancellor
2020-02-24 23:30                                         ` Nick Desaulniers
2020-02-25  4:10                                         ` Kees Cook
2020-02-25 16:53                                         ` [tip: x86/boot] x86/*/Makefile: " tip-bot2 for Arvind Sankar
2020-02-24 23:21                                       ` [PATCH v2 2/2] arch/x86: Drop unneeded linker script discard of .eh_frame Arvind Sankar
2020-02-24 23:28                                         ` Nathan Chancellor
2020-02-24 23:33                                         ` Nick Desaulniers
2020-02-25  4:11                                         ` Kees Cook
2020-02-25 16:53                                         ` [tip: x86/boot] x86/vmlinux: " tip-bot2 for Arvind Sankar
2020-02-23 22:00                           ` [PATCH] x86/boot/compressed: Fix compressed kernel linking with lld Kees Cook
2020-02-24  6:06                             ` Fangrui Song
2020-02-22 17:29                 ` [PATCH 2/2] x86/boot/compressed: Remove unnecessary sections from bzImage Borislav Petkov
2020-02-22 17:53                   ` Arvind Sankar
2020-02-22  7:42           ` Borislav Petkov
2020-02-22 16:22             ` Arvind Sankar
2020-02-22 23:20               ` Nick Desaulniers
2020-02-24 13:28                 ` Michael Matz
2020-02-24 20:51                   ` Nick Desaulniers
2020-02-24 21:28                     ` Fangrui Song
2020-02-24 21:48                       ` Arvind Sankar
2020-02-24 22:17                         ` Fangrui Song
2020-02-24 22:43                           ` Arvind Sankar
2020-02-24 22:50                             ` Fangrui Song
2020-02-24 23:08                               ` Arvind Sankar
2020-02-25  5:35                 ` --orphan-handling=warn (was Re: [PATCH 2/2] x86/boot/compressed: Remove unnecessary sections) " Kees Cook
2020-02-25 16:42                   ` --orphan-handling=warn Arvind Sankar
2020-02-25 18:29                   ` --orphan-handling=warn Arvind Sankar
2020-02-25 19:42                     ` --orphan-handling=warn Kees Cook
2020-02-25 20:37                       ` --orphan-handling=warn Nick Desaulniers
2020-02-25 22:02                         ` --orphan-handling=warn Kees Cook
2020-02-26  1:56                           ` --orphan-handling=warn Fangrui Song
2020-02-26  5:35                             ` --orphan-handling=warn Kees Cook
2020-02-26 19:11                               ` --orphan-handling=warn Kristen Carlson Accardi
2020-02-26 19:26                                 ` --orphan-handling=warn Nick Desaulniers
2020-02-24 11:37   ` [tip: x86/boot] x86/boot/compressed: Remove .eh_frame section from bzImage tip-bot2 for Arvind Sankar
2020-02-24 16:41     ` Arvind Sankar
2020-02-24 17:16       ` Borislav Petkov
2020-02-24 17:28         ` Arvind Sankar
2020-02-05 16:29 ` [PATCH 1/2] x86/boot/compressed/64: Remove .bss/.pgtable " Arvind Sankar
2020-02-18 18:03   ` Arvind Sankar
2020-02-19 12:09     ` Borislav Petkov
2020-02-19 17:57       ` Arvind Sankar
2020-02-19 18:22         ` Borislav Petkov
2020-02-19 19:06           ` Arvind Sankar
2020-02-06 11:18 ` Kees Cook
2020-02-19 16:55 ` [tip: x86/boot] " tip-bot2 for Arvind Sankar
2020-04-05 15:42 ` [PATCH 1/2] " Sergey Shatunov
2020-04-05 23:18   ` Arvind Sankar
2020-04-06  0:00     ` Sergey Shatunov
2020-04-06  3:51       ` Arvind Sankar
2020-04-06  7:32         ` Ard Biesheuvel
2020-04-06  8:47           ` Borislav Petkov
2020-04-06  9:11             ` Ard Biesheuvel
2020-04-06 11:20               ` Borislav Petkov
2020-04-06 13:22                 ` Arvind Sankar
2020-04-06 13:29                   ` Ard Biesheuvel
2020-04-06 16:01                     ` Arvind Sankar
2020-04-06 16:22                       ` Ard Biesheuvel
2020-04-06 16:52                         ` Arvind Sankar
2020-04-06 16:59                           ` Ard Biesheuvel
2020-04-06 18:06                             ` [PATCH 1/2] efi/x86: Move efi stub globals from .bss to .data Arvind Sankar
2020-04-06 18:06                               ` [PATCH 2/2] efi/x86: Always relocate the kernel for EFI handover entry Arvind Sankar
2020-04-14  8:20                                 ` [tip: efi/urgent] " tip-bot2 for Arvind Sankar
2020-04-06 18:29                               ` [PATCH 1/2] efi/x86: Move efi stub globals from .bss to .data Ard Biesheuvel
2020-04-08  7:43                               ` Dave Young
2020-04-08  7:49                                 ` Ard Biesheuvel
2020-04-09 14:39                                   ` Arvind Sankar
2020-04-09 14:47                                     ` Ard Biesheuvel
2020-04-09 16:35                                       ` Arvind Sankar
2020-04-10 14:47                                         ` Arvind Sankar
2020-04-10 15:26                                           ` Ard Biesheuvel
2020-04-14 14:57                                             ` Daniel Kiper
2020-04-10 11:26                               ` Thomas Meyer
2020-04-10 14:38                                 ` Arvind Sankar
2020-04-11  8:50                                   ` Thomas Meyer
2020-04-14  8:20                               ` [tip: efi/urgent] " tip-bot2 for Arvind Sankar
2020-04-06 17:21                     ` [PATCH 1/2] x86/boot/compressed/64: Remove .bss/.pgtable from bzImage Borislav Petkov
2020-04-06  8:44     ` Ard Biesheuvel
2020-04-06 12:36       ` Sergey Shatunov
2020-04-06 13:20         ` Ard Biesheuvel

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