Re: [PATCH v2 2/2] arch/x86: Drop unneeded linker script discard of .eh_frame

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>,
	Andy Lutomirski <luto@kernel.org>
Subject: Re: [PATCH v2 2/2] arch/x86: Drop unneeded linker script discard of .eh_frame
Date: Mon, 24 Feb 2020 15:33:55 -0800	[thread overview]
Message-ID: <CAKwvOdm1yGkYtiqHqDvi_e+jSdbp102AbPsaXG42fR4Tyw42Aw@mail.gmail.com> (raw)
In-Reply-To: <20200224232129.597160-3-nivedita@alum.mit.edu>

On Mon, Feb 24, 2020 at 3:21 PM Arvind Sankar <nivedita@alum.mit.edu> wrote:
>
> Now that we don't generate .eh_frame sections for the files in setup.elf
> and realmode.elf, the linker scripts don't need the /DISCARD/ any more.
>
> Remove the one in the main kernel linker script as well, since there are
> no .eh_frame sections already, and fix up a comment referencing .eh_frame.
>
> Update the comment in asm/dwarf2.h referring to .eh_frame so it continues
> to make sense, as well as being more specific.
>
> Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>

Thanks for cleaning up the comments.
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

> ---
>  arch/x86/boot/compressed/vmlinux.lds.S | 5 -----
>  arch/x86/boot/setup.ld                 | 1 -
>  arch/x86/include/asm/dwarf2.h          | 4 ++--
>  arch/x86/kernel/vmlinux.lds.S          | 7 ++-----
>  arch/x86/realmode/rm/realmode.lds.S    | 1 -
>  5 files changed, 4 insertions(+), 14 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/vmlinux.lds.S b/arch/x86/boot/compressed/vmlinux.lds.S
> index 469dcf800a2c..508cfa6828c5 100644
> --- a/arch/x86/boot/compressed/vmlinux.lds.S
> +++ b/arch/x86/boot/compressed/vmlinux.lds.S
> @@ -73,9 +73,4 @@ SECTIONS
>  #endif
>         . = ALIGN(PAGE_SIZE);   /* keep ZO size page aligned */
>         _end = .;
> -
> -       /* Discard .eh_frame to save some space */
> -       /DISCARD/ : {
> -               *(.eh_frame)
> -       }
>  }
> diff --git a/arch/x86/boot/setup.ld b/arch/x86/boot/setup.ld
> index 3da1c37c6dd5..24c95522f231 100644
> --- a/arch/x86/boot/setup.ld
> +++ b/arch/x86/boot/setup.ld
> @@ -52,7 +52,6 @@ SECTIONS
>         _end = .;
>
>         /DISCARD/       : {
> -               *(.eh_frame)
>                 *(.note*)
>         }
>
> diff --git a/arch/x86/include/asm/dwarf2.h b/arch/x86/include/asm/dwarf2.h
> index ae391f609840..f71a0cce9373 100644
> --- a/arch/x86/include/asm/dwarf2.h
> +++ b/arch/x86/include/asm/dwarf2.h
> @@ -42,8 +42,8 @@
>          * Emit CFI data in .debug_frame sections, not .eh_frame sections.
>          * The latter we currently just discard since we don't do DWARF
>          * unwinding at runtime.  So only the offline DWARF information is
> -        * useful to anyone.  Note we should not use this directive if
> -        * vmlinux.lds.S gets changed so it doesn't discard .eh_frame.
> +        * useful to anyone.  Note we should not use this directive if we
> +        * ever decide to enable DWARF unwinding at runtime.
>          */
>         .cfi_sections .debug_frame
>  #else
> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> index e3296aa028fe..5cab3a29adcb 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -313,8 +313,8 @@ SECTIONS
>
>         . = ALIGN(8);
>         /*
> -        * .exit.text is discard at runtime, not link time, to deal with
> -        *  references from .altinstructions and .eh_frame
> +        * .exit.text is discarded at runtime, not link time, to deal with
> +        *  references from .altinstructions
>          */
>         .exit.text : AT(ADDR(.exit.text) - LOAD_OFFSET) {
>                 EXIT_TEXT
> @@ -412,9 +412,6 @@ SECTIONS
>         DWARF_DEBUG
>
>         DISCARDS
> -       /DISCARD/ : {
> -               *(.eh_frame)
> -       }
>  }
>
>
> diff --git a/arch/x86/realmode/rm/realmode.lds.S b/arch/x86/realmode/rm/realmode.lds.S
> index 64d135d1ee63..63aa51875ba0 100644
> --- a/arch/x86/realmode/rm/realmode.lds.S
> +++ b/arch/x86/realmode/rm/realmode.lds.S
> @@ -71,7 +71,6 @@ SECTIONS
>         /DISCARD/ : {
>                 *(.note*)
>                 *(.debug*)
> -               *(.eh_frame*)
>         }
>
>  #include "pasyms.h"
> --
> 2.24.1
>

-- 
Thanks,
~Nick Desaulniers