[PATCH bpf-next] kbuild: Restore ability to build out-of-tree modules

[Leon Romanovsky <leon@kernel.org>]

From: Leon Romanovsky <leonro@nvidia.com>

The out-of-tree modules are built without vmlinux target and request
to recompile that target unconditionally causes to the following
compilation error.

[root@server kernel]# make
<..>
make -f ./scripts/Makefile.modpost
make -f ./scripts/Makefile.modfinal
make[3]: *** No rule to make target 'vmlinux', needed by '/my_temp/out-of-tree-module/kernel/test.ko'.  Stop.
make[2]: *** [scripts/Makefile.modpost:117: __modpost] Error 2
make[1]: *** [Makefile:1703: modules] Error 2
make[1]: Leaving directory '/usr/src/kernels/5.10.0-rc5_for_upstream_base_2020_11_29_11_34'
make: *** [Makefile:80: modules] Error 2

As a solution separate between build paths that has vmlinux target and paths without.

Fixes: 5f9ae91f7c0d ("kbuild: Build kernel module BTFs if BTF is enabled and pahole supports it")
Reported-by: Edward Srouji <edwards@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
---
Not proficient enough in Makefile, but it fixes the issue.
---
 scripts/Makefile.modfinal | 5 +++++
 scripts/Makefile.modpost  | 4 ++++
 2 files changed, 9 insertions(+)

diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
index 02b892421f7a..8a7d0604e7d0 100644
--- a/scripts/Makefile.modfinal
+++ b/scripts/Makefile.modfinal
@@ -48,9 +48,14 @@ if_changed_except = $(if $(call newer_prereqs_except,$(2))$(cmd-check),      \
 	$(cmd);                                                              \
 	printf '%s\n' 'cmd_$@ := $(make-cmd)' > $(dot-target).cmd, @:)

+ifdef MODPOST_VMLINUX
 # Re-generate module BTFs if either module's .ko or vmlinux changed
 $(modules): %.ko: %.o %.mod.o scripts/module.lds vmlinux FORCE
 	+$(call if_changed_except,ld_ko_o,vmlinux)
+else
+$(modules): %.ko: %.o %.mod.o scripts/module.lds FORCE
+	+$(call if_changed_except,ld_ko_o)
+endif
 ifdef CONFIG_DEBUG_INFO_BTF_MODULES
 	+$(if $(newer-prereqs),$(call cmd,btf_ko))
 endif
diff --git a/scripts/Makefile.modpost b/scripts/Makefile.modpost
index f54b6ac37ac2..f5aa5b422ad7 100644
--- a/scripts/Makefile.modpost
+++ b/scripts/Makefile.modpost
@@ -114,8 +114,12 @@ targets += $(output-symdump)

 __modpost: $(output-symdump)
 ifneq ($(KBUILD_MODPOST_NOFINAL),1)
+ifdef MODPOST_VMLINUX
+	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modfinal MODPOST_VMLINUX=1
+else
 	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modfinal
 endif
+endif

 PHONY += FORCE
 FORCE:
--
2.28.0