linux-kernel - Re: [PATCH 0/2] Stop generating .eh

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Message-ID: <20200224041542.GA55909@ubuntu-m2-xlarge-x86>
Date:   Sun, 23 Feb 2020 21:15:42 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        Michael Matz <matz@...e.de>, Fangrui Song <maskray@...gle.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH 0/2] Stop generating .eh_frame sections

On Sun, Feb 23, 2020 at 02:37:13PM -0500, Arvind Sankar wrote:
> In three places in the x86 kernel we are currently generating .eh_frame
> sections only to discard them later via linker script. This is in the
> boot code (setup.elf), the realmode trampoline (realmode.elf) and the
> compressed kernel.
> 
> Implement Fangrui and Nick's suggestion [1] to fix KBUILD_CFLAGS by
> adding -fno-asynchronous-unwind-tables to avoid generating .eh_frame
> sections in the first place, rather than discarding it in the linker
> script.
> 
> Arvind Sankar (2):
>   arch/x86: Use -fno-asynchronous-unwind-tables to suppress .eh_frame sections
>   arch/x86: Drop unneeded linker script discard of .eh_frame
> 
>  arch/x86/boot/Makefile                | 1 +
>  arch/x86/boot/compressed/Makefile     | 1 +
>  arch/x86/boot/setup.ld                | 1 -
>  arch/x86/kernel/vmlinux.lds.S         | 3 ---
>  arch/x86/realmode/rm/Makefile         | 1 +
>  arch/x86/realmode/rm/realmode.lds.S   | 1 -
>  drivers/firmware/efi/libstub/Makefile | 3 ++-
>  7 files changed, 5 insertions(+), 6 deletions(-)
> 
> -- 
> 2.24.1
> 

With both of these patches applied on top of next-20200221 and a revert
of commit e11831d0ada3 ("x86/boot/compressed: Remove unnecessary
sections from bzImage"), I do not see any .eh_frame sections in the
following files:

$ readelf -S arch/x86/boot/setup.elf \
arch/x86/realmode/rm/realmode.elf \
arch/x86/boot/compressed/vmlinux \
vmlinux |& grep eh_frame

Additionally, I can see -fno-asynchronous-unwind-tables get added to
files that didn't previously have it.

Reviewed-by: Nathan Chancellor <natechancellor@...il.com>
Tested-by: Nathan Chancellor <natechancellor@...il.com>