linux-kernel - Re: [PATCH 1/3] x86: Propagate $(CLANG_FLAGS) to $(REALMODE

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Date:   Fri, 26 Mar 2021 22:38:31 +0900
From:   John Millikin <john@...n-millikin.com>
To:     Nathan Chancellor <nathan@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, Ard Biesheuvel <ardb@...nel.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com,
        Sedat Dilek <sedat.dilek@...il.com>
Subject: Re: [PATCH 1/3] x86: Propagate $(CLANG_FLAGS) to $(REALMODE_FLAGS)

On 3/26/21 09:04, Nathan Chancellor wrote:
> From: John Millikin <john@...n-millikin.com>
> 
> When cross-compiling with Clang, the `$(CLANG_FLAGS)' variable
> contains additional flags needed to build C and assembly sources
> for the target platform. Normally this variable is automatically
> included in `$(KBUILD_CFLAGS)' by via the top-level Makefile.
> 
> The x86 real-mode makefile builds `$(REALMODE_CFLAGS)' from a
> plain assignment and therefore drops the Clang flags. This causes
> Clang to not recognize x86-specific assembler directives:
> 
>   arch/x86/realmode/rm/header.S:36:1: error: unknown directive
>   .type real_mode_header STT_OBJECT ; .size real_mode_header, .-real_mode_header
>   ^
> 
> Explicit propagation of `$(CLANG_FLAGS)' to `$(REALMODE_CFLAGS)',
> which is inherited by real-mode make rules, fixes cross-compilation
> with Clang for x86 targets.
> 
> Relevant flags:
> 
> * `--target' sets the target architecture when cross-compiling. This
>   flag must be set for both compilation and assembly (`KBUILD_AFLAGS')
>   to support architecture-specific assembler directives.
> 
> * `-no-integrated-as' tells clang to assemble with GNU Assembler
>   instead of its built-in LLVM assembler. This flag is set by default
>   unless `LLVM_IAS=1' is set, because the LLVM assembler can't yet
>   parse certain GNU extensions.
> 
> Signed-off-by: John Millikin <john@...n-millikin.com>
> Tested-by: Sedat Dilek <sedat.dilek@...il.com>
> Signed-off-by: Nathan Chancellor <nathan@...nel.org>
> ---
>  arch/x86/Makefile | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 2d6d5a28c3bf..9a73e0cea19c 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -33,6 +33,7 @@ REALMODE_CFLAGS += -ffreestanding
>  REALMODE_CFLAGS += -fno-stack-protector
>  REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), -Wno-address-of-packed-member)
>  REALMODE_CFLAGS += $(call __cc-option, $(CC), $(REALMODE_CFLAGS), $(cc_stack_align4))
> +REALMODE_CFLAGS += $(CLANG_FLAGS)
>  export REALMODE_CFLAGS
>  
>  # BITS is used as extension for files which are available in a 32 bit
> 

Just wanted to say "thank you" for picking this up -- I ran into some real-life situations and didn't have time to debug why my patch was failing the Git validity check (presumed to be an email client misconfiguration).