kbuild: fix false-positive modpost warning when all symbols are trimmed

Nathan reports that the mips defconfig emits the following warning:

  WARNING: modpost: Symbol info of vmlinux is missing. Unresolved symbol check will be entirely skipped.

This false-positive happens when CONFIG_TRIM_UNUSED_KSYMS is enabled,
but no CONFIG option is set to 'm'.

Commit a0590473 ("nfs: fix PNFS_FLEXFILE_LAYOUT Kconfig default")
turned the last 'm' into 'y' for the mips defconfig, and uncovered
this issue.

In this case, the module feature itself is enabled, but we have no
module to build. As a result, CONFIG_TRIM_UNUSED_KSYMS drops all the
instances of EXPORT_SYMBOL. Then, modpost wrongly assumes vmlinux is
missing because vmlinux.symvers is empty. (As another false-positive
case, you can create a module that does not use any symbol of vmlinux).

The current behavior is to entirely suppress the unresolved symbol
warnings when vmlinux is missing just because there are too many.
I found the origin of this code in the historical git tree. [1]

If this is a matter of noisiness, I think modpost can display the
first 10 warnings, and the number of suppressed warnings at the end.

You will get a bit noisier logs when you run 'make modules' without
vmlinux, but such warnings are better to show because you never know
the resulting modules are actually loadable or not.

This commit changes the following:

 - If any of input *.symver files is missing, pass -w option to let
   the module build keep going with warnings instead of errors.

 - If there are too many (10+) unresolved symbol warnings, show only
   the first 10, and also the number of suppressed warnings.

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/history/history.git/commit/?id=1cc0e0529569bf6a94f6d49770aa6d4b599d2c46Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/Makefile.modpost

@@ -98,9 +98,11 @@ output-symdump := $(KBUILD_EXTMOD)/Module.symvers
 
 endif
 
+existing-input-symdump := $(wildcard $(input-symdump))
+
 # modpost options for modules (both in-kernel and external)
 MODPOST += \
-	$(addprefix -i ,$(wildcard $(input-symdump))) \
+	$(addprefix -i ,$(existing-input-symdump)) \
 	$(if $(KBUILD_NSDEPS),-d $(MODULES_NSDEPS)) \
 	$(if $(CONFIG_MODULE_ALLOW_MISSING_NAMESPACE_IMPORTS)$(KBUILD_NSDEPS),-N)
 
@@ -114,6 +116,7 @@ VPATH :=
 $(input-symdump):
 	@echo >&2 'WARNING: Symbol version dump "$@" is missing.'
 	@echo >&2 '         Modules may not have dependencies or modversions.'
+	@echo >&2 '         You may get many unresolved symbol warnings.'
 
 ifdef CONFIG_LTO_CLANG
 # With CONFIG_LTO_CLANG, .o files might be LLVM bitcode, so we need to run
@@ -134,7 +137,7 @@ endif
 modules := $(sort $(shell cat $(MODORDER)))
 
 # KBUILD_MODPOST_WARN can be set to avoid error out in case of undefined symbols
-ifneq ($(KBUILD_MODPOST_WARN),)
+ifneq ($(KBUILD_MODPOST_WARN)$(filter-out $(existing-input-symdump), $(input-symdump)),)
 MODPOST += -w
 endif
 

scripts/mod/modpost.c

@@ -23,8 +23,6 @@
 
 /* Are we using CONFIG_MODVERSIONS? */
 static int modversions = 0;
-/* Warn about undefined symbols? (do so if we have vmlinux) */
-static int have_vmlinux = 0;
 /* Is CONFIG_MODULE_SRCVERSION_ALL set? */
 static int all_versions = 0;
 /* If we are modposting external module set to 1 */
@@ -41,6 +39,13 @@ static int allow_missing_ns_imports;
 
 static bool error_occurred;
 
+/*
+ * Cut off the warnings when there are too many. This typically occurs when
+ * vmlinux is missing. ('make modules' without building vmlinux.)
+ */
+#define MAX_UNRESOLVED_REPORTS	10
+static unsigned int nr_unresolved;
+
 enum export {
 	export_plain,
 	export_gpl,
@@ -177,9 +182,6 @@ static struct module *new_module(const char *modname)
 	mod->next = modules;
 	modules = mod;
 
-	if (mod->is_vmlinux)
-		have_vmlinux = 1;
-
 	return mod;
 }
 
@@ -2141,7 +2143,7 @@ static void check_exports(struct module *mod)
 		const char *basename;
 		exp = find_symbol(s->name);
 		if (!exp || exp->module == mod) {
-			if (have_vmlinux && !s->weak)
+			if (!s->weak && nr_unresolved++ < MAX_UNRESOLVED_REPORTS)
 				modpost_log(warn_unresolved ? LOG_WARN : LOG_ERROR,
 					    "\"%s\" [%s.ko] undefined!\n",
 					    s->name, mod->name);
@@ -2545,13 +2547,6 @@ int main(int argc, char **argv)
 	if (files_source)
 		read_symbols_from_files(files_source);
 
-	/*
-	 * When there's no vmlinux, don't print warnings about
-	 * unresolved symbols (since there'll be too many ;)
-	 */
-	if (!have_vmlinux)
-		warn("Symbol info of vmlinux is missing. Unresolved symbol check will be entirely skipped.\n");
-
 	for (mod = modules; mod; mod = mod->next) {
 		char fname[PATH_MAX];
 
@@ -2595,6 +2590,10 @@ int main(int argc, char **argv)
 		}
 	}
 
+	if (nr_unresolved > MAX_UNRESOLVED_REPORTS)
+		warn("suppressed %u unresolved symbol warnings because there were too many)\n",
+		     nr_unresolved - MAX_UNRESOLVED_REPORTS);
+
 	free(buf.p);
 
 	return error_occurred ? 1 : 0;