[PATCH RESEND bpf-next 1/2] kbuild: skip module BTF generation for out-of-tree external modules

[Andrii Nakryiko <andrii@xxxxxxxxxx>]

In some modes of operation, Kbuild allows to build modules without having
vmlinux image around. In such case, generation of module BTF is impossible.
This patch changes the behavior to emit a warning about impossibility of
generating kernel module BTF, instead of breaking the build. This is especially
important for out-of-tree external module builds.

In vmlinux-less mode:

$ make clean
$ make modules_prepare
$ touch drivers/acpi/button.c
$ make M=drivers/acpi
...
  CC [M]  drivers/acpi/button.o
  MODPOST drivers/acpi/Module.symvers
  LD [M]  drivers/acpi/button.ko
  BTF [M] drivers/acpi/button.ko
Skipping BTF generation for drivers/acpi/button.ko due to unavailability of vmlinux
...
$ readelf -S ~/linux-build/default/drivers/acpi/button.ko | grep BTF -A1
... empty ...

Now with normal build:

$ make all
...
LD [M]  drivers/acpi/button.ko
BTF [M] drivers/acpi/button.ko
...
$ readelf -S ~/linux-build/default/drivers/acpi/button.ko | grep BTF -A1
  [60] .BTF              PROGBITS         0000000000000000  00029310
       000000000000ab3f  0000000000000000           0     0     1

Reported-by: Bruce Allan <bruce.w.allan@xxxxxxxxx>
Fixes: 5f9ae91f7c0d ("kbuild: Build kernel module BTFs if BTF is enabled and pahole supports it")
Cc: Jessica Yu <jeyu@xxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
---
 scripts/Makefile.modfinal | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
index 02b892421f7a..d49ec001825d 100644
--- a/scripts/Makefile.modfinal
+++ b/scripts/Makefile.modfinal
@@ -38,7 +38,12 @@ quiet_cmd_ld_ko_o = LD [M]  $@
 	$(if $(ARCH_POSTLINK), $(MAKE) -f $(ARCH_POSTLINK) $@, true)
 
 quiet_cmd_btf_ko = BTF [M] $@
-      cmd_btf_ko = LLVM_OBJCOPY=$(OBJCOPY) $(PAHOLE) -J --btf_base vmlinux $@
+      cmd_btf_ko = 							\
+	if [ -f vmlinux ]; then						\
+		LLVM_OBJCOPY=$(OBJCOPY) $(PAHOLE) -J --btf_base vmlinux $@; \
+	else								\
+		printf "Skipping BTF generation for %s due to unavailability of vmlinux\n" $@ 1>&2; \
+	fi;
 
 # Same as newer-prereqs, but allows to exclude specified extra dependencies
 newer_prereqs_except = $(filter-out $(PHONY) $(1),$?)
@@ -49,7 +54,7 @@ if_changed_except = $(if $(call newer_prereqs_except,$(2))$(cmd-check),      \
 	printf '%s\n' 'cmd_$@ := $(make-cmd)' > $(dot-target).cmd, @:)
 
 # Re-generate module BTFs if either module's .ko or vmlinux changed
-$(modules): %.ko: %.o %.mod.o scripts/module.lds vmlinux FORCE
+$(modules): %.ko: %.o %.mod.o scripts/module.lds $(if $(KBUILD_BUILTIN),vmlinux) FORCE
 	+$(call if_changed_except,ld_ko_o,vmlinux)
 ifdef CONFIG_DEBUG_INFO_BTF_MODULES
 	+$(if $(newer-prereqs),$(call cmd,btf_ko))
-- 
2.24.1