[PATCH v2] kbuild: Check for unknown options with cc-option usage in Kconfig and clang

[Stephen Boyd]

If the particular version of clang a user has doesn't enable
-Werror=unknown-warning-option by default, even though it is the
default[1], then make sure to pass the option to the Kconfig cc-option
command so that testing options from Kconfig files works properly.
Otherwise, depending on the default values setup in the clang toolchain
we will silently assume options such as -Wmaybe-uninitialized are
supported by clang, when they really aren't.

A compilation issue only started happening for me once commit
589834b3a009 ("kbuild: Add -Werror=unknown-warning-option to
CLANG_FLAGS") was applied on top of commit b303c6df80c9 ("kbuild:
compute false-positive -Wmaybe-uninitialized cases in Kconfig"). This
leads kbuild to try and test for the existence of the
-Wmaybe-uninitialized flag with the cc-option command in
scripts/Kconfig.include, and it doesn't see an error returned from the
option test so it sets the config value to Y. Then the Makefile tries to
pass the unknown option on the command line and
-Werror=unknown-warning-option catches the invalid option and breaks the
build. Before commit 589834b3a009 ("kbuild: Add
-Werror=unknown-warning-option to CLANG_FLAGS") the build works fine,
but any cc-option test of a warning option in Kconfig files silently
evaluates to true, even if the warning option flag isn't supported on
clang.

Note: This doesn't change cc-option usages in Makefiles because those
use a different rule that includes KBUILD_CFLAGS by default (see the
__cc-option command in scripts/Kbuild.incluide). The KBUILD_CFLAGS
variable already has the -Werror=unknown-warning-option flag set. Thanks
to Doug for pointing out the different rule.

[1] https://clang.llvm.org/docs/DiagnosticsReference.html#wunknown-warning-option
Cc: Peter Smith <peter.smith@xxxxxxxxxx>
Cc: Nathan Chancellor <natechancellor@xxxxxxxxx>
Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Cc: Douglas Anderson <dianders@xxxxxxxxxxxx>
Reviewed-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
Signed-off-by: Stephen Boyd <swboyd@xxxxxxxxxxxx>
---

Changes from v1:
 * Reworded commit text a bit
 * Added Reviewed-by tag

 Makefile                | 5 +++++
 scripts/Kconfig.include | 2 +-
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index 9be5834073f8..28177674178a 100644
--- a/Makefile
+++ b/Makefile
@@ -517,6 +517,8 @@ ifdef building_out_of_srctree
 	{ echo "# this is build directory, ignore it"; echo "*"; } > .gitignore
 endif
 
+KCONFIG_CC_OPTION_FLAGS := -Werror
+
 ifneq ($(shell $(CC) --version 2>&1 | head -n 1 | grep clang),)
 ifneq ($(CROSS_COMPILE),)
 CLANG_FLAGS	:= --target=$(notdir $(CROSS_COMPILE:%-=%))
@@ -531,11 +533,14 @@ ifeq ($(shell $(AS) --version 2>&1 | head -n 1 | grep clang),)
 CLANG_FLAGS	+= -no-integrated-as
 endif
 CLANG_FLAGS	+= -Werror=unknown-warning-option
+KCONFIG_CC_OPTION_FLAGS += -Werror=unknown-warning-option
 KBUILD_CFLAGS	+= $(CLANG_FLAGS)
 KBUILD_AFLAGS	+= $(CLANG_FLAGS)
 export CLANG_FLAGS
 endif
 
+export KCONFIG_CC_OPTION_FLAGS
+
 # The expansion should be delayed until arch/$(SRCARCH)/Makefile is included.
 # Some architectures define CROSS_COMPILE in arch/$(SRCARCH)/Makefile.
 # CC_VERSION_TEXT is referenced from Kconfig (so it needs export),
diff --git a/scripts/Kconfig.include b/scripts/Kconfig.include
index 8a5c4d645eb1..144e83e7cb81 100644
--- a/scripts/Kconfig.include
+++ b/scripts/Kconfig.include
@@ -25,7 +25,7 @@ failure = $(if-success,$(1),n,y)
 
 # $(cc-option,<flag>)
 # Return y if the compiler supports <flag>, n otherwise
-cc-option = $(success,$(CC) -Werror $(1) -E -x c /dev/null -o /dev/null)
+cc-option = $(success,$(CC) $(KCONFIG_CC_OPTION_FLAGS) $(1) -E -x c /dev/null -o /dev/null)
 
 # $(ld-option,<flag>)
 # Return y if the linker supports <flag>, n otherwise
-- 
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