modpost: propagate W=1 build option to modpost

"No build warning" is a strong requirement these days, so you must fix
all issues before enabling a new warning flag.

We often add a new warning to W=1 first so that the kbuild test robot
blocks new breakages.

This commit allows modpost to show extra warnings only when W=1
(or KBUILD_EXTRA_WARN=1) is given.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

scripts/Makefile.modpost

@@ -47,6 +47,7 @@ modpost-args =										\
 	$(if $(KBUILD_MODPOST_WARN),-w)							\
 	$(if $(KBUILD_NSDEPS),-d $(MODULES_NSDEPS))					\
 	$(if $(CONFIG_MODULE_ALLOW_MISSING_NAMESPACE_IMPORTS)$(KBUILD_NSDEPS),-N)	\
+	$(if $(findstring 1, $(KBUILD_EXTRA_WARN)),-W)					\
 	-o $@
 
 modpost-deps := $(MODPOST)

scripts/mod/modpost.c

@@ -42,6 +42,8 @@ static bool allow_missing_ns_imports;
 
 static bool error_occurred;
 
+static bool extra_warn;
+
 /*
  * Cut off the warnings when there are too many. This typically occurs when
  * vmlinux is missing. ('make modules' without building vmlinux.)
@@ -2199,7 +2201,7 @@ int main(int argc, char **argv)
 	LIST_HEAD(dump_lists);
 	struct dump_list *dl, *dl2;
 
-	while ((opt = getopt(argc, argv, "ei:mnT:o:awENd:")) != -1) {
+	while ((opt = getopt(argc, argv, "ei:mnT:o:aWwENd:")) != -1) {
 		switch (opt) {
 		case 'e':
 			external_module = true;
@@ -2224,6 +2226,9 @@ int main(int argc, char **argv)
 		case 'T':
 			files_source = optarg;
 			break;
+		case 'W':
+			extra_warn = true;
+			break;
 		case 'w':
 			warn_unresolved = true;
 			break;