linux-kernel - Re: [PATCH] arm64/efi: Mark __efistub_stext_offset as an absolute symbol explicitly

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Date:   Wed, 26 Jun 2019 09:06:36 +0200
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Fangrui Song <maskray@...gle.com>,
        Peter Smith <peter.smith@...aro.org>
Subject: Re: [PATCH] arm64/efi: Mark __efistub_stext_offset as an absolute
 symbol explicitly

On Wed, 26 Jun 2019 at 06:20, Nathan Chancellor
<natechancellor@...il.com> wrote:
>
> After r363059 and r363928 in LLVM, a build using ld.lld as the linker
> with CONFIG_RANDOMIZE_BASE enabled fails like so:
>
> ld.lld: error: relocation R_AARCH64_ABS32 cannot be used against symbol
> __efistub_stext_offset; recompile with -fPIC
>
> Fangrui and Peter figured out that ld.lld is incorrectly considering
> __efistub_stext_offset as a relative symbol because of the order in
> which symbols are evaluated. _text is treated as an absolute symbol
> and stext is a relative symbol, making __efistub_stext_offset a
> relative symbol.
>
> Adding ABSOLUTE will force ld.lld to evalute this expression in the
> right context and does not change ld.bfd's behavior. ld.lld will
> need to be fixed but the developers do not see a quick or simple fix
> without some research (see the linked issue for further explanation).
> Add this simple workaround so that ld.lld can continue to link kernels.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/561
> Link: https://github.com/llvm/llvm-project/commit/025a815d75d2356f2944136269aa5874721ec236
> Link: https://github.com/llvm/llvm-project/commit/249fde85832c33f8b06c6b4ac65d1c4b96d23b83
> Debugged-by: Fangrui Song <maskray@...gle.com>
> Debugged-by: Peter Smith <peter.smith@...aro.org>
> Suggested-by: Fangrui Song <maskray@...gle.com>
> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> ---
>  arch/arm64/kernel/image.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/arm64/kernel/image.h b/arch/arm64/kernel/image.h
> index 04ca08086d35..9a2d2227907c 100644
> --- a/arch/arm64/kernel/image.h
> +++ b/arch/arm64/kernel/image.h
> @@ -67,7 +67,7 @@
>
>  #ifdef CONFIG_EFI
>
> -__efistub_stext_offset = stext - _text;
> +__efistub_stext_offset = ABSOLUTE(stext - _text);
>
>  /*
>   * The EFI stub has its own symbol namespace prefixed by __efistub_, to
> --
> 2.22.0
>