[RFC bpf-next 2/2] bpf: allow opt-out from using split BTF for modules

[Alan Maguire <alan.maguire@oracle.com>]

By having a BTF_BASE variable defaulting to using vmlinux
as base BTF, we allow module builders to build standalone
BTF such that it is generated independently and not
de-duplicated with core vmlinux BTF.  This allows such
modules to be more resilient to changes in vmlinux BTF
if they occur, as would happen if a change resulted in
a different vmlinux BTF id mapping.

Opt-out of split BTF is done via

 make BTF_BASE= M=path/2/module

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
---
 scripts/Makefile.modfinal | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
index 25bedd8..7294918 100644
--- a/scripts/Makefile.modfinal
+++ b/scripts/Makefile.modfinal
@@ -30,6 +30,8 @@ quiet_cmd_cc_o_c = CC [M]  $@
 
 ARCH_POSTLINK := $(wildcard $(srctree)/arch/$(SRCARCH)/Makefile.postlink)
 
+BTF_BASE := --btf_base vmlinux
+
 quiet_cmd_ld_ko_o = LD [M]  $@
       cmd_ld_ko_o +=							\
 	$(LD) -r $(KBUILD_LDFLAGS)					\
@@ -44,7 +46,7 @@ quiet_cmd_btf_ko = BTF [M] $@
 	elif [ -n "$(CONFIG_RUST)" ] && $(srctree)/scripts/is_rust_module.sh $@; then 		\
 		printf "Skipping BTF generation for %s because it's a Rust module\n" $@ 1>&2; \
 	else								\
-		LLVM_OBJCOPY="$(OBJCOPY)" $(PAHOLE) -J $(PAHOLE_FLAGS) --btf_base vmlinux $@; \
+		LLVM_OBJCOPY="$(OBJCOPY)" $(PAHOLE) -J $(PAHOLE_FLAGS) $(BTF_BASE) $@; \
 		$(RESOLVE_BTFIDS) -b vmlinux $@; 			\
 	fi;
 
-- 
1.8.3.1