linux-kernel - Re: [PATCH v3 1/2] kbuild: move -Werror from KBUILD_CFLAGS to KBUILD

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Date:   Fri, 14 Oct 2022 09:03:29 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        llvm@...ts.linux.dev
Subject: Re: [PATCH v3 1/2] kbuild: move -Werror from KBUILD_CFLAGS to
 KBUILD_CPPFLAGS

On Thu, Oct 13, 2022 at 03:01:17AM +0900, Masahiro Yamada wrote:
> CONFIG_WERROR turns warnings into errors, which  happens only for *.c
> files because -Werror is added to KBUILD_CFLAGS.
> 
> Adding it to KBUILD_CPPFLAGS makes more sense because preprocessors
> understand the -Werror option.
> 
> For example, you can put a #warning directive in any preprocessed code.
> 
>     warning: #warning "this is a warning message" [-Wcpp]
> 
> If -Werror is added, it is promoted to an error.
> 
>     error: #warning "this is a warning message" [-Werror=cpp]
> 
> This commit moves -Werror to KBUILD_CPPFLAGS so it works in the same way
> for *.c, *.S, *.lds.S or whatever needs preprocessing.
> 
> Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
> ---
> 
> (no changes since v1)
> 
>  Makefile | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/Makefile b/Makefile
> index 85a63a1d29b3..790760d26ea0 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -859,7 +859,8 @@ stackp-flags-$(CONFIG_STACKPROTECTOR_STRONG)      := -fstack-protector-strong
>  
>  KBUILD_CFLAGS += $(stackp-flags-y)
>  
> -KBUILD_CFLAGS-$(CONFIG_WERROR) += -Werror
> +KBUILD_CPPFLAGS-$(CONFIG_WERROR) += -Werror
> +KBUILD_CPPFLAGS += $(KBUILD_CPPFLAGS-y)
>  KBUILD_CFLAGS-$(CONFIG_CC_NO_ARRAY_BOUNDS) += -Wno-array-bounds
>  
>  KBUILD_RUSTFLAGS-$(CONFIG_WERROR) += -Dwarnings
> -- 
> 2.34.1
> 
> 

For what it's worth, this is going to break 32-bit ARM builds with clang
plus the integrated assembler due to
https://github.com/ClangBuiltLinux/linux/issues/1315:

clang-16: error: argument unused during compilation: '-march=armv7-a' [-Werror,-Wunused-command-line-argument]

Ultimately, I want -Wunused-command-line-argument to be an error anyways
(https://github.com/ClangBuiltLinux/linux/issues/1587) but it would be
nice to get these cleaned up before this goes in.

Cheers,
Nathan