linux-kernel - Re: [PATCH v2 1/1] x86/Makefile: make -stack-alignment conditional on LLD

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Date:   Thu, 10 Jun 2021 14:07:30 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Tor Vic <torvic9@...lbox.org>
Cc:     clang-built-linux@...glegroups.com, x86@...nel.org,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Sami Tolvanen <samitolvanen@...gle.com>
Subject: Re: [PATCH v2 1/1] x86/Makefile: make -stack-alignment conditional on
 LLD < 13.0.0

On 6/10/2021 1:58 PM, Tor Vic wrote:
> Since LLVM commit 3787ee4, the '-stack-alignment' flag has been dropped
> [1], leading to the following error message when building a LTO kernel
> with Clang-13 and LLD-13:
> 
>      ld.lld: error: -plugin-opt=-: ld.lld: Unknown command line argument
>      '-stack-alignment=8'.  Try 'ld.lld --help'
>      ld.lld: Did you mean '--stackrealign=8'?
> 
> It also appears that the '-code-model' flag is not necessary anymore
> starting with LLVM-9 [2].
> 
> Drop '-code-model' and make '-stack-alignment' conditional on LLD < 13.0.0.
> 
> These flags were necessary because these flags were not encoded in the
> IR properly, so the link would restart optimizations without them. Now
> there are properly encoded in the IR, and these flags exposing
> implementation details are no longer necessary.
> 
> Changes from v1:
> - based on mainline
> - provide more information about the flags (Nick)
> - use correct tags

Small note for the future, probably not worth resending unless the x86 
folks feel it is necessary but these comments belong between the '---' 
and the stats below so that they do not get included in the final commit 
message.

> Cc: stable@...r.kernel.org
> Link: https://github.com/ClangBuiltLinux/linux/issues/1377
> [1]: https://reviews.llvm.org/D103048
> [2]: https://reviews.llvm.org/D52322
> Signed-off-by: Tor Vic <torvic9@...lbox.org>

Reviewed-by: Nathan Chancellor <nathan@...nel.org>
Tested-by: Nathan Chancellor <nathan@...nel.org>

Thank you for the patch!

> ---
>   arch/x86/Makefile | 5 +++--
>   1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 307529417021..cb5e8d39cac1 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -200,8 +200,9 @@ endif
>   KBUILD_LDFLAGS += -m elf_$(UTS_MACHINE)
> 
>   ifdef CONFIG_LTO_CLANG
> -KBUILD_LDFLAGS	+= -plugin-opt=-code-model=kernel \
> -		   -plugin-opt=-stack-alignment=$(if $(CONFIG_X86_32),4,8)
> +ifeq ($(shell test $(CONFIG_LLD_VERSION) -lt 130000; echo $$?),0)
> +KBUILD_LDFLAGS	+= -plugin-opt=-stack-alignment=$(if $(CONFIG_X86_32),4,8)
> +endif
>   endif
> 
>   ifdef CONFIG_X86_NEED_RELOCS
> 

Cheers,
Nathan