[PATCH] kbuild: Enable -Wincompatible-function-pointer-types-strict in W=1

[Nathan Chancellor <nathan@kernel.org>]

-Wincompatible-function-pointer-types-strict aims to catch clang kernel
Control Flow Integrity (kCFI) violations at build time (rather than run
time) by validating function pointer assignments against the expected
prototype, similar to the existing -Wincompatible-function-pointer-types
that is considered a hard error in the kernel. The -strict variant
requires the types to match exactly, as opposed to just matching in
terms of ABI compatibility. This is primarily visible with int/unsigned
int in lieu of enum types or vice versa.

The tree is not completely clean, so this warning cannot currently be
enabled unconditionally. However, there are only warnings in one
subsystem ('drivers/counter'), so it is really close. In order to
benefit from CI infrastructure that tests with W=1, enable this warning
at that level, so that new instances have a chance of being caught and
fixed during development.

This should eventually be a hard error in a similar manner as
Wincompatible-function-pointer-types but some subsystems test
with W=1 + CONFIG_WERROR=n, so it would be rude to break their builds
when they do not care about warnings outside of their subsystem.

Link: https://github.com/ClangBuiltLinux/linux/issues/1750
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---
 scripts/Makefile.extrawarn | 1 +
 1 file changed, 1 insertion(+)

diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
index 2fe6f2828d37..9662f7fd6e39 100644
--- a/scripts/Makefile.extrawarn
+++ b/scripts/Makefile.extrawarn
@@ -108,6 +108,7 @@ KBUILD_CFLAGS += $(call cc-option, -Wformat-overflow)
 KBUILD_CFLAGS += $(call cc-option, -Wformat-truncation)
 KBUILD_CFLAGS += $(call cc-option, -Wstringop-overflow)
 KBUILD_CFLAGS += $(call cc-option, -Wstringop-truncation)
+KBUILD_CFLAGS += $(call cc-option, -Wincompatible-function-pointer-types-strict)
 
 KBUILD_CPPFLAGS += -Wundef
 KBUILD_CPPFLAGS += -DKBUILD_EXTRA_WARN1

---
base-commit: ce9ecca0238b140b88f43859b211c9fdfd8e5b70
change-id: 20231002-enable-wincompatible-function-pointer-types-strict-w-1-4a56b99b8c6f

Best regards,
-- 
Nathan Chancellor <nathan@kernel.org>