Kbuild: enable -fms-extensions

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>
Acked-by: David Sterba <dsterba@suse.com>
Link: https://patch.msgid.link/20251020142228.1819871-2-linux@rasmusvillemoes.dk
[nathan: Move disabled clang warning to scripts/Makefile.extrawarn and
         adjust comment]
Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Author: Villemoes

Makefile

@@ -1061,6 +1061,9 @@ 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
+
 # disable invalid "can't wrap" optimizations for signed / pointers
 KBUILD_CFLAGS	+= -fno-strict-overflow
 

scripts/Makefile.extrawarn

@@ -28,8 +28,10 @@ endif
 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.
+# The kernel builds with '-std=gnu11' and '-fms-extensions' so use of GNU and
+# Microsoft extensions is acceptable.
 KBUILD_CFLAGS += -Wno-gnu
+KBUILD_CFLAGS += -Wno-microsoft-anon-tag
 
 # Clang checks for overflow/truncation with '%p', while GCC does not:
 # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111219