linux-kernel - Re: [PATCH v2 2/2] arch/x86: Drop unneeded linker script discard of .eh

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Message-ID: <202002242011.541C9A57@keescook>
Date:   Mon, 24 Feb 2020 20:11:07 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Borislav Petkov <bp@...en8.de>,
        Nathan Chancellor <natechancellor@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Michael Matz <matz@...e.de>, Fangrui Song <maskray@...gle.com>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH v2 2/2] arch/x86: Drop unneeded linker script discard of
 .eh_frame

On Mon, Feb 24, 2020 at 06:21:29PM -0500, Arvind Sankar 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@...m.mit.edu>

Reviewed-by: Kees Cook <keescook@...omium.org>

-Kees

> ---
>  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
> 

-- 
Kees Cook