linux-kernel - Re: [PATCH] x86/build: Move _etext to actual end of .text

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Date:   Tue, 14 May 2019 14:04:21 +0200
From:   Johannes Hirte <johannes.hirte@...enkhaos.de>
To:     Kees Cook <keescook@...omium.org>
Cc:     Borislav Petkov <bp@...e.de>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH] x86/build: Move _etext to actual end of .text

On 2019 Apr 23, Kees Cook wrote:
> When building x86 with Clang LTO and CFI, CFI jump regions are
> automatically added to the end of the .text section late in linking. As a
> result, the _etext position was being labelled before the appended jump
> regions, causing confusion about where the boundaries of the executable
> region actually are in the running kernel, and broke at least the fault
> injection code. This moves the _etext mark to outside (and immediately
> after) the .text area, as it already the case on other architectures
> (e.g. arm64, arm).
> 
> Reported-and-tested-by: Sami Tolvanen <samitolvanen@...gle.com>
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  arch/x86/kernel/vmlinux.lds.S | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
> index bad8c51fee6e..de94da2366e7 100644
> --- a/arch/x86/kernel/vmlinux.lds.S
> +++ b/arch/x86/kernel/vmlinux.lds.S
> @@ -141,11 +141,11 @@ SECTIONS
>  		*(.text.__x86.indirect_thunk)
>  		__indirect_thunk_end = .;
>  #endif
> -
> -		/* End of text section */
> -		_etext = .;
>  	} :text = 0x9090
>  
> +	/* End of text section */
> +	_etext = .;
> +
>  	NOTES :text :note
>  
>  	EXCEPTION_TABLE(16) :text = 0x9090
> -- 
> 2.17.1

This breaks the build on my system:

  RELOCS  arch/x86/boot/compressed/vmlinux.relocs
  CC      arch/x86/boot/compressed/early_serial_console.o
  CC      arch/x86/boot/compressed/kaslr.o
  AS      arch/x86/boot/compressed/mem_encrypt.o
  CC      arch/x86/boot/compressed/kaslr_64.o
Invalid absolute R_X86_64_32S relocation: _etext
make[2]: *** [arch/x86/boot/compressed/Makefile:130: arch/x86/boot/compressed/vmlinux.relocs] Error 1
make[2]: *** Deleting file 'arch/x86/boot/compressed/vmlinux.relocs'
make[2]: *** Waiting for unfinished jobs....
make[1]: *** [arch/x86/boot/Makefile:112: arch/x86/boot/compressed/vmlinux] Error 2
make: *** [arch/x86/Makefile:283: bzImage] Error 2



-- 
Regards,
  Johannes