[PATCH] modpost: add an option to suppress 'exported twice' warnings

[Masahiro Yamada]

Since commit "modpost: do not set ->preloaded for symbols from
Module.symvers", the modpost always warns about symbols exported
multiple times.

Generally, I believe it is a good thing to show a warning when the
same symbol name is exported twice. This avoids the accidental symbol
conflict.

However, in some cases, we build an external module to provide a
different version/variant of the in-kernel module, overriding the
same set of exported symbols.

At least, there is one use-case in the upstream code;
tools/testing/nvdimm/libnvdimm.ko replaces drivers/nvdimm/libnvdimm.ko
in order to link it against mocked version of core kernel symbols.

Now, this emits a lots of 'exported twice' warnings:

  https://lkml.org/lkml/2019/10/31/627

To suppress those, add a new option KBUILD_DUPLICATED_EXPORTS_NO_WARN.

If you intentionally override the existing symbols, you can pass it
from the command line:

  make M=tools/testing/nvdimm KBUILD_DUPLICATED_EXPORTS_NO_WARN=1

Or, more conveniently, you can add it to the module Makefile, so
you can still do:

  make M=tools/testing/nvdimm

without sprinkling the warnings.

Reported-by: Jeff Moyer <jmoyer@xxxxxxxxxx>
Signed-off-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
---

Jeff Moyer,
Dan Williams,

Please check if this patch solves the nvdimm build issue.


 Documentation/kbuild/kbuild.rst | 7 +++++++
 scripts/Makefile.modpost        | 1 +
 scripts/mod/modpost.c           | 9 +++++++--
 tools/testing/nvdimm/Kbuild     | 4 ++++
 4 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/Documentation/kbuild/kbuild.rst b/Documentation/kbuild/kbuild.rst
index f1e5dce86af7..8ff0935f9e7f 100644
--- a/Documentation/kbuild/kbuild.rst
+++ b/Documentation/kbuild/kbuild.rst
@@ -223,6 +223,13 @@ KBUILD_SIGN_PIN
 This variable allows a passphrase or PIN to be passed to the sign-file
 utility when signing kernel modules, if the private key requires such.
 
+KBUILD_DUPLICATED_EXPORTS_NO_WARN
+---------------------------------
+This variable can be set to suppress warnings about symbols exported
+multiple times. This is useful when you are building an external module
+to provide a different version of an in-tree module.
+
+
 KBUILD_MODPOST_WARN
 -------------------
 KBUILD_MODPOST_WARN can be set to avoid errors in case of undefined
diff --git a/scripts/Makefile.modpost b/scripts/Makefile.modpost
index 8359f8af5ee6..52e50ac45688 100644
--- a/scripts/Makefile.modpost
+++ b/scripts/Makefile.modpost
@@ -53,6 +53,7 @@ MODPOST = scripts/mod/modpost						\
 	$(if $(KBUILD_EXTMOD),$(addprefix -e ,$(KBUILD_EXTRA_SYMBOLS)))	\
 	$(if $(KBUILD_EXTMOD),-o $(modulesymfile))			\
 	$(if $(CONFIG_SECTION_MISMATCH_WARN_ONLY),,-E)			\
+	$(if $(KBUILD_DUPLICATED_EXPORTS_NO_WARN),-W)			\
 	$(if $(KBUILD_MODPOST_WARN),-w)
 
 ifdef MODPOST_VMLINUX
diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
index f70b924f379f..858568f4edb4 100644
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -33,6 +33,8 @@ static int external_module = 0;
 static int vmlinux_section_warnings = 1;
 /* Only warn about unresolved symbols */
 static int warn_unresolved = 0;
+/* Warn about symbols exported multiple times */
+static int warn_duplicated_exports = 1;
 /* How a symbol is exported */
 static int sec_mismatch_count = 0;
 static int sec_mismatch_fatal = 0;
@@ -382,7 +384,7 @@ static struct symbol *sym_add_exported(const char *name, struct module *mod,
 	if (!s) {
 		s = new_symbol(name, mod, export);
 	} else {
-		if (!s->preloaded) {
+		if (warn_duplicated_exports && !s->preloaded) {
 			warn("%s: '%s' exported twice. Previous export was in %s%s\n",
 			     mod->name, name, s->module->name,
 			     is_vmlinux(s->module->name) ? "" : ".ko");
@@ -2559,7 +2561,7 @@ int main(int argc, char **argv)
 	struct ext_sym_list *extsym_iter;
 	struct ext_sym_list *extsym_start = NULL;
 
-	while ((opt = getopt(argc, argv, "i:e:mnsT:o:awEd:")) != -1) {
+	while ((opt = getopt(argc, argv, "i:e:mnsT:o:awWEd:")) != -1) {
 		switch (opt) {
 		case 'i':
 			kernel_read = optarg;
@@ -2594,6 +2596,9 @@ int main(int argc, char **argv)
 		case 'w':
 			warn_unresolved = 1;
 			break;
+		case 'W':
+			warn_duplicated_exports = 0;
+			break;
 		case 'E':
 			sec_mismatch_fatal = 1;
 			break;
diff --git a/tools/testing/nvdimm/Kbuild b/tools/testing/nvdimm/Kbuild
index c4a9196d794c..c848374f3b0b 100644
--- a/tools/testing/nvdimm/Kbuild
+++ b/tools/testing/nvdimm/Kbuild
@@ -93,4 +93,8 @@ libnvdimm-y += dimm_devs.o
 libnvdimm-y += libnvdimm_test.o
 libnvdimm-y += config_check.o
 
+# This intentionally overrides the original libnvdimm.ko
+# Suppress 'exported twice' warnings from the modpost.
+KBUILD_DUPLICATED_EXPORTS_NO_WARN := 1
+
 obj-m += test/
-- 
2.17.1