[PATCH 3/3] selftests/bpf: Route test_kmods build artifacts to OUTPUT

[Ricardo B. Marlière (SUSE)]

When building bpf selftests with O=, the kernel module intermediates (.ko,
.o, .mod.*, .cmd files, Module.symvers, modules.order) produced by the
test_kmods sub-make are placed in the source tree instead of the
out-of-tree directory, because kbuild external-module builds always write
output to the M= source directory by default.

Introduce TEST_KMODS_OUTDIR in bpf/Makefile that resolves to
$(OUTPUT)/test_kmods when OUTPUT is set, and fall back to the in-tree
test_kmods/ directory for standalone builds. Update the pattern rule, the
RM invocation, and the copy rule to use this variable, and pass
OUTPUT=$(OUTPUT)/test_kmods to the test_kmods sub-make.

In test_kmods/Makefile, forward the received OUTPUT value as MO= to the
kernel Makefile. The top-level kernel Makefile maps MO= to
KBUILD_EXTMOD_OUTPUT, which redirects all external-module build output to
the specified directory. The kernel also creates that directory
automatically, so no explicit mkdir is needed.

Also update the CLEAN target in bpf/Makefile to propagate OUTPUT to both
the test_kmods and libarena sub-makes so that clean removes from the
correct location.

Signed-off-by: Ricardo B. Marlière (SUSE) <ricardo@xxxxxxxxxxxx>
---
 tools/testing/selftests/bpf/Makefile            | 16 ++++++++++------
 tools/testing/selftests/bpf/test_kmods/Makefile |  3 +++
 2 files changed, 13 insertions(+), 6 deletions(-)

diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
index 7d42632f9d42..b150a66f3c25 100644
--- a/tools/testing/selftests/bpf/Makefile
+++ b/tools/testing/selftests/bpf/Makefile
@@ -135,6 +135,9 @@ TEST_PROGS_EXTENDED := \
 TEST_KMODS := bpf_testmod.ko bpf_test_no_cfi.ko bpf_test_modorder_x.ko \
 	bpf_test_modorder_y.ko bpf_test_rqspinlock.ko
 TEST_KMOD_TARGETS = $(addprefix $(OUTPUT)/,$(TEST_KMODS))
+# Build directory for kernel modules: routed to $(OUTPUT)/test_kmods when
+# OUTPUT is set so that kbuild intermediates stay out of the source tree.
+TEST_KMODS_OUTDIR := $(if $(OUTPUT),$(OUTPUT)/test_kmods,test_kmods)
 
 # Compile but not part of 'make run_tests'
 TEST_GEN_PROGS_EXTENDED = \
@@ -163,8 +166,8 @@ override define CLEAN
 	$(Q)$(RM) -r $(TEST_GEN_FILES)
 	$(Q)$(RM) -r $(TEST_KMODS)
 	$(Q)$(RM) -r $(EXTRA_CLEAN)
-	$(Q)$(MAKE) -C test_kmods clean
-	$(Q)$(MAKE) -C libarena clean
+	$(Q)$(MAKE) -C test_kmods $(if $(OUTPUT),OUTPUT=$(OUTPUT)/test_kmods) clean
+	$(Q)$(MAKE) -C libarena $(if $(OUTPUT),OUTPUT=$(OUTPUT)/libarena) clean
 	$(Q)$(MAKE) docs-clean
 endef
 
@@ -301,17 +304,18 @@ $(OUTPUT)/sign-file: ../../../../scripts/sign-file.c
 # support that for regular rules. However, pattern matching rules are implicitly
 # treated as grouped even with older versions of make, so as a workaround, the
 # subst() turns the rule into a pattern matching rule
-$(addprefix test_kmods/,$(subst .ko,%ko,$(TEST_KMODS))): $(VMLINUX_BTF) $(RESOLVE_BTFIDS) $(wildcard test_kmods/Makefile test_kmods/*.[ch])
-	$(Q)$(RM) test_kmods/*.ko test_kmods/*.mod.o # force re-compilation
+$(addprefix $(TEST_KMODS_OUTDIR)/,$(subst .ko,%ko,$(TEST_KMODS))): $(VMLINUX_BTF) $(RESOLVE_BTFIDS) $(wildcard test_kmods/Makefile test_kmods/*.[ch])
+	$(Q)$(RM) $(TEST_KMODS_OUTDIR)/*.ko $(TEST_KMODS_OUTDIR)/*.mod.o # force re-compilation
 	$(Q)$(MAKE) $(submake_extras) -C test_kmods				\
 		$(if $(O),O=$(abspath $(O)))					\
 		$(if $(KBUILD_OUTPUT),KBUILD_OUTPUT=$(abspath $(KBUILD_OUTPUT)))\
+		$(if $(OUTPUT),OUTPUT=$(OUTPUT)/test_kmods)			\
 		RESOLVE_BTFIDS=$(RESOLVE_BTFIDS)				\
 		EXTRA_CFLAGS='' EXTRA_LDFLAGS=''
 
-$(TEST_KMOD_TARGETS): $(addprefix test_kmods/,$(TEST_KMODS))
+$(TEST_KMOD_TARGETS): $(addprefix $(TEST_KMODS_OUTDIR)/,$(TEST_KMODS))
 	$(call msg,MOD,,$@)
-	$(Q)$(if $(PERMISSIVE),if [ -f test_kmods/$(@F) ]; then )cp test_kmods/$(@F) $@$(if $(PERMISSIVE),; fi)
+	$(Q)$(if $(PERMISSIVE),if [ -f $(TEST_KMODS_OUTDIR)/$(@F) ]; then )cp $(TEST_KMODS_OUTDIR)/$(@F) $@$(if $(PERMISSIVE),; fi)
 
 
 DEFAULT_BPFTOOL := $(HOST_SCRATCH_DIR)/sbin/bpftool
diff --git a/tools/testing/selftests/bpf/test_kmods/Makefile b/tools/testing/selftests/bpf/test_kmods/Makefile
index 031c7454ce65..53e47e715ec4 100644
--- a/tools/testing/selftests/bpf/test_kmods/Makefile
+++ b/tools/testing/selftests/bpf/test_kmods/Makefile
@@ -32,14 +32,17 @@ PERMISSIVE := $(filter 0,$(BPF_STRICT_BUILD))
 all:
 ifeq ($(PERMISSIVE),)
 	$(Q)$(MAKE) -C $(KDIR) $(if $(KMOD_O_VALID),O=$(KMOD_O_VALID) KBUILD_OUTPUT=$(KMOD_O_VALID),KBUILD_OUTPUT=) \
+		$(if $(OUTPUT),MO=$(OUTPUT)) \
 		M=$(TEST_KMOD_DIR) modules
 else ifneq ("$(wildcard $(KDIR))", "")
 	$(Q)$(MAKE) -C $(KDIR) $(if $(KMOD_O_VALID),O=$(KMOD_O_VALID) KBUILD_OUTPUT=$(KMOD_O_VALID),KBUILD_OUTPUT=) \
+		$(if $(OUTPUT),MO=$(OUTPUT)) \
 		M=$(TEST_KMOD_DIR) modules
 endif
 
 clean:
 ifneq ("$(wildcard $(KDIR))", "")
 	$(Q)$(MAKE) -C $(KDIR) $(if $(KMOD_O_VALID),O=$(KMOD_O_VALID) KBUILD_OUTPUT=$(KMOD_O_VALID),KBUILD_OUTPUT=) \
+		$(if $(OUTPUT),MO=$(OUTPUT)) \
 		M=$(TEST_KMOD_DIR) clean
 endif

-- 
2.55.0