linux-kernel - Re: [PATCH RFC] kbuild: disable -Wc23-extensions from clang

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Message-ID: <20241125150700.GB2067874@thelio-3990X>
Date: Mon, 25 Nov 2024 08:07:00 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Thomas Weißschuh <linux@...ssschuh.net>
Cc: Masahiro Yamada <masahiroy@...nel.org>,
	Nicolas Schier <nicolas@...sle.eu>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>, linux-kbuild@...r.kernel.org,
	linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH RFC] kbuild: disable -Wc23-extensions from clang

Hi Thomas,

On Mon, Nov 25, 2024 at 10:03:00AM +0100, Thomas Weißschuh wrote:
> Some of the C23 extensions are also GNU extensions, which are allowed.
> Examples are declarations after labels or function definitions with
> omitted parameter names.
> 
> Also with the switch to C23 at some point in the future all those

We are likely a ways off from being able to switch to C23:

https://lore.kernel.org/10db3077-9409-446d-8e50-1a2a803db767@app.fastmail.com/

> warnings will be resolved automatically anyways.
> 
> Signed-off-by: Thomas Weißschuh <linux@...ssschuh.net>
> ---
> I had some patches failing the CI only with clang.
> And it feels like unnecessary busywork to fix these warnings.

A declaration after a label is a hard error with clang 17 and older, so
I think it is entirely appropriate to continue to warn on that:

https://godbolt.org/z/sj98GMhv3

While a function definition with omitted parameter names is always a
warning for the versions of clang supported by the kernel, I still think
those should be fixed for consistency sake, as I have really only seen
these occur in stub functions, which should match the definition of the
full function.

> ---
>  scripts/Makefile.extrawarn | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
> index 1d13cecc7cc7808610e635ddc03476cf92b3a8c1..0da75c9d84da4daf6936495a5ab96df75658da4f 100644
> --- a/scripts/Makefile.extrawarn
> +++ b/scripts/Makefile.extrawarn
> @@ -31,6 +31,8 @@ KBUILD_CFLAGS-$(CONFIG_CC_NO_ARRAY_BOUNDS) += -Wno-array-bounds
>  ifdef CONFIG_CC_IS_CLANG
>  # The kernel builds with '-std=gnu11' so use of GNU extensions is acceptable.
>  KBUILD_CFLAGS += -Wno-gnu
> +# Some allowed GNU extensions are also C23 extensions
> +KBUILD_CFLAGS += $(call cc-disable-warning, c23-extensions)
>  else
>  
>  # gcc inanely warns about local variables called 'main'
> 
> ---
> base-commit: 9f16d5e6f220661f73b36a4be1b21575651d8833
> change-id: 20241118-kbuild-c23-extensions-2560d140de36
> 
> Best regards,
> -- 
> Thomas Weißschuh <linux@...ssschuh.net>
>