scripts/nsdeps: support nsdeps for external module builds

scripts/nsdeps is written to take care of only in-tree modules.
Perhaps, this is not a bug, but just a design. At least,
Documentation/core-api/symbol-namespaces.rst focuses on in-tree modules.

Having said that, some people already tried nsdeps for external modules.
So, it would be nice to support it.
Reported-by: Steve French <smfrench@gmail.com>
Reported-by: Jessica Yu <jeyu@kernel.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Jessica Yu <jeyu@kernel.org>
Acked-by: Jessica Yu <jeyu@kernel.org>
Reviewed-by: Matthias Maennich <maennich@google.com>
Tested-by: Matthias Maennich <maennich@google.com>

Documentation/core-api/symbol-namespaces.rst

@@ -152,3 +152,6 @@ in-tree modules::
 	- notice the warning of modpost telling about a missing import
 	- run `make nsdeps` to add the import to the correct code location
 
+You can also run nsdeps for external module builds. A typical usage is::
+
+	$ make -C <path_to_kernel_src> M=$PWD nsdeps

Makefile

@@ -1008,6 +1008,7 @@ endif
 PHONY += prepare0
 
 export MODORDER := $(extmod-prefix)modules.order
+export MODULES_NSDEPS := $(extmod-prefix)modules.nsdeps
 
 ifeq ($(KBUILD_EXTMOD),)
 core-y		+= kernel/ certs/ mm/ fs/ ipc/ security/ crypto/ block/
@@ -1620,7 +1621,7 @@ _emodinst_post: _emodinst_
 	$(call cmd,depmod)
 
 clean-dirs := $(KBUILD_EXTMOD)
-clean: rm-files := $(KBUILD_EXTMOD)/Module.symvers
+clean: rm-files := $(KBUILD_EXTMOD)/Module.symvers $(KBUILD_EXTMOD)/modules.nsdeps
 
 PHONY += /
 /:

scripts/Makefile.modpost

@@ -66,7 +66,7 @@ __modpost:
 else
 
 MODPOST += $(subst -i,-n,$(filter -i,$(MAKEFLAGS))) -s -T - \
-	$(if $(KBUILD_NSDEPS),-d modules.nsdeps)
+	$(if $(KBUILD_NSDEPS),-d $(MODULES_NSDEPS))
 
 ifeq ($(KBUILD_EXTMOD),)
 MODPOST += $(wildcard vmlinux)

scripts/nsdeps

@@ -21,6 +21,12 @@ if [ "$SPATCH_VERSION_NUM" -lt "$SPATCH_REQ_VERSION_NUM" ] ; then
 	exit 1
 fi
 
+if [ "$KBUILD_EXTMOD" ]; then
+	src_prefix=
+else
+	src_prefix=$srctree/
+fi
+
 generate_deps_for_ns() {
 	$SPATCH --very-quiet --in-place --sp-file \
 		$srctree/scripts/coccinelle/misc/add_namespace.cocci -D ns=$1 $2
@@ -32,7 +38,7 @@ generate_deps() {
 	local namespaces="$*"
 	local mod_source_files="`cat $mod.mod | sed -n 1p                      \
 					      | sed -e 's/\.o/\.c/g'           \
-					      | sed "s|[^ ]* *|${srctree}/&|g"`"
+					      | sed "s|[^ ]* *|${src_prefix}&|g"`"
 	for ns in $namespaces; do
 		echo "Adding namespace $ns to module $mod.ko."
 		generate_deps_for_ns $ns "$mod_source_files"
@@ -54,4 +60,4 @@ generate_deps() {
 while read line
 do
 	generate_deps $line
-done < modules.nsdeps
+done < $MODULES_NSDEPS