Re: [PATCH 1/2] Kbuild: enable -fms-extensions

[Nathan Chancellor <nathan@kernel.org>]

On Mon, Oct 20, 2025 at 04:22:27PM +0200, Rasmus Villemoes wrote:
> Once in a while, it turns out that enabling -fms-extensions could
> allow some slightly prettier code. But every time it has come up, the
> code that had to be used instead has been deemed "not too awful" and
> not worth introducing another compiler flag for.
> 
> That's probably true for each individual case, but then it's somewhat
> of a chicken/egg situation.
> 
> If we just "bite the bullet" as Linus says and enable it once and for
> all, it is available whenever a use case turns up, and no individual
> case has to justify it.
> 
> A lore.kernel.org search provides these examples:
> 
> - https://lore.kernel.org/lkml/200706301813.58435.agruen@suse.de/
> - https://lore.kernel.org/lkml/20180419152817.GD25406@bombadil.infradead.org/
> - https://lore.kernel.org/lkml/170622208395.21664.2510213291504081000@noble.neil.brown.name/
> - https://lore.kernel.org/lkml/87h6475w9q.fsf@prevas.dk/
> - https://lore.kernel.org/lkml/CAHk-=wjeZwww6Zswn6F_iZTpUihTSNKYppLqj36iQDDhfntuEw@mail.gmail.com/
> 
> Undoubtedly, there are more places in the code where this could also
> be used but where -fms-extensions just didn't come up in any
> discussion.
> 
> Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> ---
>  Makefile | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/Makefile b/Makefile
> index d14824792227..ef6f23ee8e7f 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -1061,6 +1061,15 @@ NOSTDINC_FLAGS += -nostdinc
>  # perform bounds checking.
>  KBUILD_CFLAGS += $(call cc-option, -fstrict-flex-arrays=3)
>  
> +# Allow including a tagged struct or union anonymously in another struct/union.
> +KBUILD_CFLAGS += -fms-extensions
> +
> +# For clang, the -fms-extensions flag is apparently not enough to
> +# express one's intention to make use of those extensions.
> +ifdef CONFIG_CC_IS_CLANG
> +KBUILD_CFLAGS += -Wno-microsoft-anon-tag
> +endif

I think this should go in the first 'ifdef CONFIG_CC_IS_CLANG' block in
scripts/Makefile.extrawarn below '-Wno-gnu' with a comment that is
similar in nature, which could even be combined like

  # The kernel builds with '-std-gnu11' and '-fms-extensions' so the use
  # of GNU and Microsoft extensions is acceptable.

Other than that, this seems fine to me.

>  # disable invalid "can't wrap" optimizations for signed / pointers
>  KBUILD_CFLAGS	+= -fno-strict-overflow
>  
> -- 
> 2.51.0
>