Re: [PATCH 2/2] kbuild: Use '-fms-anonymous-structs' if it is available

Kees Cook
2026-02-23T22:33:36Z

On Mon, Feb 23, 2026 at 12:10:29PM -0700, Nathan Chancellor wrote:
> Clang recently added '-fms-anonymous-structs' [1] to specifically enable
> the Microsoft tagged anonymous structure / union extension, for which
> the kernel added '-fms-extensions' in commit c4781dc3d1cf ("Kbuild:
> enable -fms-extensions"). Switch to this more narrow option if it is
> available, which would have helped avoid the issue addressed by
> commit a6773e6932cb ("jfs: Rename _inline to avoid conflict with clang's
> '-fms-extensions'"). GCC has talked about adding a similar flag [2] as
> well but potentially naming it differently.
>
> Move the selection of the flag to Kconfig to make it easier to use
> cc-option (as CC_FLAGS_DIALECT may be used in arch Makefiles, which may
> be too early for cc-option in Kbuild) and customize based on compiler
> flag names.
>
> Link: https://github.com/llvm/llvm-project/commit/c391efe6fb67329d8e2fd231692cc6b0ea902956 [1]
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123623 [2]
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
> Makefile | 2 +-
> init/Kconfig | 5 +++++
> 2 files changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/Makefile b/Makefile
> index 06ff3032a6bc..1d5c0ab9ed5c 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -791,7 +791,7 @@ endif
>
> CC_FLAGS_DIALECT := -std=gnu11
> # Allow including a tagged struct or union anonymously in another struct/union.
> -CC_FLAGS_DIALECT += -fms-extensions
> +CC_FLAGS_DIALECT += $(CONFIG_CC_MS_EXTENSIONS)
> # Clang enables warnings about GNU and Microsoft extensions by default, disable
> # them because this is expected with the above options.
> ifdef CONFIG_CC_IS_CLANG
> diff --git a/init/Kconfig b/init/Kconfig
> index c25869cf59c1..c4282d049463 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -960,6 +960,11 @@ config CC_IMPLICIT_FALLTHROUGH
> default "-Wimplicit-fallthrough=5" if CC_IS_GCC && $(cc-option,-Wimplicit-fallthrough=5)
> default "-Wimplicit-fallthrough" if CC_IS_CLANG && $(cc-option,-Wunreachable-code-fallthrough)
>
> +config CC_MS_EXTENSIONS
> + string
> + default "-fms-anonymous-structs" if $(cc-option,-fms-anonymous-structs)
> + default "-fms-extensions"
> +
> # Currently, disable gcc-10+ array-bounds globally.
> # It's still broken in gcc-13, so no upper bound yet.
> config GCC10_NO_ARRAY_BOUNDS

Series looks good to me. One style question, why go the round-trip with
Kconfig instead of doing it all with a fallback in the Makefile:

CC_FLAGS_DIALECT += $(call cc-option,-fms-anonymous-structs,-fms-extensions)

--
Kees Cook