[PATCH bpf-next v6 02/11] selftests/bpf: Fix test_kmods KDIR to honor O= and distro kernels

[Ricardo B. Marlière]

test_kmods/Makefile always pointed KDIR at the kernel source tree root,
ignoring O= and KBUILD_OUTPUT. On distro kernels where the source tree has
not been built, the Makefile had no fallback and would fail
unconditionally.

When O= or KBUILD_OUTPUT is set, pass it through so kbuild can locate the
correct kernel build directory (Module.symvers, scripts, etc.). Note that
the module artifacts themselves still land in the M= directory, which is
test_kmods/; O= only controls where kbuild finds its build infrastructure.
Fall back to /lib/modules/$(uname -r)/build when neither an explicit build
directory nor an in-tree Module.symvers is present.

Guard both all and clean against a missing KDIR so the step is silently
skipped rather than fatal. Make the parent Makefile's cp conditional so it
does not abort when modules were not built.

Signed-off-by: Ricardo B. Marlière <rbm@xxxxxxxx>
---
 tools/testing/selftests/bpf/Makefile            |  4 ++--
 tools/testing/selftests/bpf/test_kmods/Makefile | 24 +++++++++++++++++++++---
 2 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
index c8ba8ccc2c6d..52176dc226f1 100644
--- a/tools/testing/selftests/bpf/Makefile
+++ b/tools/testing/selftests/bpf/Makefile
@@ -296,7 +296,7 @@ $(addprefix test_kmods/,$(subst .ko,%ko,$(TEST_KMODS))): $(VMLINUX_BTF) $(RESOLV
 
 $(TEST_KMOD_TARGETS): $(addprefix test_kmods/,$(TEST_KMODS))
 	$(call msg,MOD,,$@)
-	$(Q)cp test_kmods/$(@F) $@
+	$(Q)$(if $(PERMISSIVE),if [ -f test_kmods/$(@F) ]; then )cp test_kmods/$(@F) $@$(if $(PERMISSIVE),; fi)
 
 
 DEFAULT_BPFTOOL := $(HOST_SCRATCH_DIR)/sbin/bpftool
@@ -711,7 +711,7 @@ $(TRUNNER_LIB_OBJS): $(TRUNNER_OUTPUT)/%.o:$(TOOLSDIR)/lib/%.c
 $(TRUNNER_BINARY)-extras: $(TRUNNER_EXTRA_FILES) | $(TRUNNER_OUTPUT)
 ifneq ($2:$(OUTPUT),:$(shell pwd))
 	$$(call msg,EXT-COPY,$(TRUNNER_BINARY),$(TRUNNER_EXTRA_FILES))
-	$(Q)rsync -aq $$^ $(TRUNNER_OUTPUT)/
+	$(Q)rsync -aq $(if $(PERMISSIVE),--ignore-missing-args) $$^ $(TRUNNER_OUTPUT)/
 endif
 
 # some X.test.o files have runtime dependencies on Y.bpf.o files
diff --git a/tools/testing/selftests/bpf/test_kmods/Makefile b/tools/testing/selftests/bpf/test_kmods/Makefile
index 63c4d3f6a12f..10f202bd0d8c 100644
--- a/tools/testing/selftests/bpf/test_kmods/Makefile
+++ b/tools/testing/selftests/bpf/test_kmods/Makefile
@@ -1,5 +1,7 @@
 TEST_KMOD_DIR := $(realpath $(dir $(abspath $(lastword $(MAKEFILE_LIST)))))
-KDIR ?= $(abspath $(TEST_KMOD_DIR)/../../../../..)
+SRCTREE_KDIR := $(abspath $(TEST_KMOD_DIR)/../../../../..)
+KMOD_O := $(or $(O),$(KBUILD_OUTPUT))
+KDIR ?= $(if $(KMOD_O),$(SRCTREE_KDIR),$(if $(wildcard $(SRCTREE_KDIR)/Module.symvers),$(SRCTREE_KDIR),/lib/modules/$(shell uname -r)/build))
 
 ifeq ($(V),1)
 Q =
@@ -14,8 +16,24 @@ $(foreach m,$(MODULES),$(eval obj-m += $(m:.ko=.o)))
 
 CFLAGS_bpf_testmod.o = -I$(src)
 
+# When BPF_STRICT_BUILD != 0, a missing KDIR is fatal (the default).
+# When permissive, skip silently.
+PERMISSIVE := $(filter 0,$(BPF_STRICT_BUILD))
+
 all:
-	$(Q)$(MAKE) -C $(KDIR) M=$(TEST_KMOD_DIR) modules
+ifeq ($(PERMISSIVE),)
+	$(Q)$(MAKE) -C $(KDIR) $(if $(KMOD_O),O=$(KMOD_O),KBUILD_OUTPUT=) \
+		M=$(TEST_KMOD_DIR) modules
+else ifneq ("$(wildcard $(KDIR))", "")
+	$(Q)$(MAKE) -C $(KDIR) $(if $(KMOD_O),O=$(KMOD_O),KBUILD_OUTPUT=) \
+		M=$(TEST_KMOD_DIR) modules
+endif
 
 clean:
-	$(Q)$(MAKE) -C $(KDIR) M=$(TEST_KMOD_DIR) clean
+ifeq ($(PERMISSIVE),)
+	$(Q)$(MAKE) -C $(KDIR) $(if $(KMOD_O),O=$(KMOD_O),KBUILD_OUTPUT=) \
+		M=$(TEST_KMOD_DIR) clean
+else ifneq ("$(wildcard $(KDIR))", "")
+	$(Q)$(MAKE) -C $(KDIR) $(if $(KMOD_O),O=$(KMOD_O),KBUILD_OUTPUT=) \
+		M=$(TEST_KMOD_DIR) clean
+endif

-- 
2.53.0