linux-kernel - Re: [PATCH v2] arch/x86: Propagate $(CLANG_FLAGS) to $(REALMODE

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Date:   Wed, 3 Feb 2021 13:19:34 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     John Millikin <jmillikin@...il.com>
Cc:     Nathan Chancellor <natechancellor@...il.com>, hpa@...or.com,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        John Millikin <john@...n-millikin.com>
Subject: Re: [PATCH v2] arch/x86: Propagate $(CLANG_FLAGS) to
 $(REALMODE_FLAGS)

On Sat, Dec 26, 2020 at 05:41:25PM +0900, John Millikin wrote:
> 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>
> ---
> Changes in v2:
>   - Reworded the commit message to highlight that this is for
>     cross-compilation.
>   - Removed the `ifdef CONFIG_CC_IS_CLANG' guard.
> 
>  arch/x86/Makefile | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 7116da3980be..412b849063ec 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -33,6 +33,7 @@ REALMODE_CFLAGS += -ffreestanding
>  REALMODE_CFLAGS += -fno-stack-protector

This one too:

checking file arch/x86/Makefile
patch: **** malformed patch at line 62:  REALMODE_CFLAGS += -fno-stack-protector

-- 
Regards/Gruss,
    Boris.

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