[PATCH 2/2] kbuild: add -Wundef to KBUILD_CPPFLAGS for W=1 builds

[Masahiro Yamada <masahiroy@kernel.org>]

The use of an undefined macro in an #if directive is warned, but only
in *.c files. No warning from other files such as *.S, *.lds.S.

Since -Wundef is a preprocessor-related warning, it should be added to
KBUILD_CPPFLAGS instead of KBUILD_CFLAGS.

My previous attempt [1] uncovered several warnings, and could not finish
fixing them all.

This commit adds -Wundef to KBUILD_CPPFLAGS for W=1 builds in order to
block new breakages. (The kbuild test robot tests with W=1)

We can fix the warnings one by one. After we fix all of them, we can
make this default in the top Makefile, and remove -Wundef from
KBUILD_CFLAGS.

[1]: https://lore.kernel.org/all/20221012180118.331005-2-masahiroy@kernel.org/

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
---

 scripts/Makefile.extrawarn | 1 +
 1 file changed, 1 insertion(+)

diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
index 6bbba36c5969..40cd13eca82e 100644
--- a/scripts/Makefile.extrawarn
+++ b/scripts/Makefile.extrawarn
@@ -38,6 +38,7 @@ KBUILD_CFLAGS += -Wno-sign-compare
 KBUILD_CFLAGS += -Wno-type-limits
 KBUILD_CFLAGS += -Wno-shift-negative-value
 
+KBUILD_CPPFLAGS += -Wundef
 KBUILD_CPPFLAGS += -DKBUILD_EXTRA_WARN1
 
 else
-- 
2.34.1