[RFC bpf-next 08/13] kbuild, bpf: add module-specific pahole/resolve_btfids flags for base reference BTF

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

Support creation of module BTF along with base reference BTF;
the latter is stored in a .BTF.base_ref ELF section and supplements
split BTF references to base BTF with information about base types,
allowing for later reconciliation of split BTF with a (possibly
changed) base.  resolve_btfids uses the "-r" option to specify
that the BTF.ids section should be populated with split BTF
relative to the added .BTF.base_ref section rather than relative
to the vmlinux base.

Modules using base reference BTF can be built via

BTF_BASE_REF=1 make -C. -M=path2/module

The default is still to use split BTF relative to vmlinux.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
---
 scripts/Makefile.btf      | 7 +++++++
 scripts/Makefile.modfinal | 4 ++--
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/scripts/Makefile.btf b/scripts/Makefile.btf
index 9694ca3c5252..c8212b2ab7ca 100644
--- a/scripts/Makefile.btf
+++ b/scripts/Makefile.btf
@@ -19,4 +19,11 @@ pahole-flags-$(call test-ge, $(pahole-ver), 126)	= -j --btf_features=encode_forc
 
 pahole-flags-$(CONFIG_PAHOLE_HAS_LANG_EXCLUDE)		+= --lang_exclude=rust
 
+ifeq ($(BTF_BASE_REF),1)
+module-pahole-flags-$(call test-ge, $(pahole-ver), 126)	+= --btf_features=base_ref
+module-resolve-btfids-flags-$(call test-ge, $(pahole-ver), 126) = -r
+endif
+
 export PAHOLE_FLAGS := $(pahole-flags-y)
+export MODULE_PAHOLE_FLAGS := $(module-pahole-flags-y)
+export MODULE_RESOLVE_BTFIDS_FLAGS := $(module-resolve-btfids-flags-y)
diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
index 8568d256d6fb..22f5bb0a60a6 100644
--- a/scripts/Makefile.modfinal
+++ b/scripts/Makefile.modfinal
@@ -39,8 +39,8 @@ quiet_cmd_btf_ko = BTF [M] $@
 	if [ ! -f vmlinux ]; then					\
 		printf "Skipping BTF generation for %s due to unavailability of vmlinux\n" $@ 1>&2; \
 	else								\
-		LLVM_OBJCOPY="$(OBJCOPY)" $(PAHOLE) -J $(PAHOLE_FLAGS) --btf_base vmlinux $@; \
-		$(RESOLVE_BTFIDS) -b vmlinux $@; 			\
+		LLVM_OBJCOPY="$(OBJCOPY)" $(PAHOLE) -J $(PAHOLE_FLAGS) $(MODULE_PAHOLE_FLAGS) --btf_base vmlinux $@; \
+		$(RESOLVE_BTFIDS) $(MODULE_RESOLVE_BTFIDS_FLAGS) -b vmlinux $@; 			\
 	fi;
 
 # Same as newer-prereqs, but allows to exclude specified extra dependencies
-- 
2.39.3