linux-kernel - Re: [PATCH 1/3] Makefile.compiler: s/KBUILD_CFLAGS/KBUILD

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Date:   Fri, 26 Aug 2022 11:21:43 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Tom Rix <trix@...hat.com>, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev, x86@...nel.org,
        Dmitrii Bundin <dmitrii.bundin.a@...il.com>,
        Fangrui Song <maskray@...gle.com>,
        Alexey Alexandrov <aalexand@...gle.com>,
        Bill Wendling <morbo@...gle.com>,
        Greg Thelen <gthelen@...gle.com>
Subject: Re: [PATCH 1/3] Makefile.compiler: s/KBUILD_CFLAGS/KBUILD_AFLAGS/
 for as-option

Hi Nick,

I think the title would be a little more readable if it was:

Makefile.compiler: Use KBUILD_AFLAGS for as-option

On Fri, Aug 26, 2022 at 11:10:33AM -0700, Nick Desaulniers wrote:
> as-instr uses KBUILD_AFLAGS, but as-option uses KBUILD_CFLAGS.  This can
> cause as-option to fail unexpectedly because clang will emit
> -Werror,-Wunused-command-line-argument for various -m and -f flags for
> assembler sources.
> 
> Callers of as-option (and as-instr) likely want to be adding flags to
> KBUILD_AFLAGS/aflags-y, not KBUILD_CFLAGS/cflags-y.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/1699
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> ---
>  arch/x86/boot/compressed/Makefile | 5 +++--
>  scripts/Makefile.compiler         | 6 +++---
>  2 files changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> index 35ce1a64068b..fb3db714a028 100644
> --- a/arch/x86/boot/compressed/Makefile
> +++ b/arch/x86/boot/compressed/Makefile
> @@ -48,8 +48,6 @@ KBUILD_CFLAGS += -Wno-pointer-sign
>  KBUILD_CFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
>  KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
>  KBUILD_CFLAGS += -D__DISABLE_EXPORTS
> -# Disable relocation relaxation in case the link is not PIE.
> -KBUILD_CFLAGS += $(call as-option,-Wa$(comma)-mrelax-relocations=no)
>  KBUILD_CFLAGS += -include $(srctree)/include/linux/hidden.h
>  
>  # sev.c indirectly inludes inat-table.h which is generated during
> @@ -58,6 +56,9 @@ KBUILD_CFLAGS += -include $(srctree)/include/linux/hidden.h
>  CFLAGS_sev.o += -I$(objtree)/arch/x86/lib/
>  
>  KBUILD_AFLAGS  := $(KBUILD_CFLAGS) -D__ASSEMBLY__
> +# Disable relocation relaxation in case the link is not PIE.
> +KBUILD_AFLAGS += $(call as-option,-Wa$(comma)-mrelax-relocations=no)
> +

Commit 09e43968db40 ("x86/boot/compressed: Disable relocation
relaxation") added this to address
https://github.com/ClangBuiltLinux/linux/issues/1121, is it correct to
move it to only being used for the .S files in arch/x86/boot/compressed?

>  GCOV_PROFILE := n
>  UBSAN_SANITIZE :=n
>  
> diff --git a/scripts/Makefile.compiler b/scripts/Makefile.compiler
> index 94d0d40cddb3..d1739f0d3ce3 100644
> --- a/scripts/Makefile.compiler
> +++ b/scripts/Makefile.compiler
> @@ -29,13 +29,13 @@ try-run = $(shell set -e;		\
>  	fi)
>  
>  # as-option
> -# Usage: cflags-y += $(call as-option,-Wa$(comma)-isa=foo,)
> +# Usage: aflags-y += $(call as-option,-Wa$(comma)-isa=foo,)
>  
>  as-option = $(call try-run,\
> -	$(CC) $(KBUILD_CFLAGS) $(1) -c -x assembler /dev/null -o "$$TMP",$(1),$(2))
> +	$(CC) $(KBUILD_AFLAGS) $(1) -c -x assembler /dev/null -o "$$TMP",$(1),$(2))
>  
>  # as-instr
> -# Usage: cflags-y += $(call as-instr,instr,option1,option2)
> +# Usage: aflags-y += $(call as-instr,instr,option1,option2)
>  
>  as-instr = $(call try-run,\
>  	printf "%b\n" "$(1)" | $(CC) $(KBUILD_AFLAGS) -c -x assembler -o "$$TMP" -,$(2),$(3))
> -- 
> 2.37.2.672.g94769d06f0-goog
>