[PATCH v2 01/10] selftests/bpf: Fall back to distro build directory for test_kmods

[Ricardo B. Marlière]

When building against a distribution kernel there may be no in-tree build
directory. Prefer explicit O= first, then KBUILD_OUTPUT from the
environment, then an in-tree source tree if Module.symvers is present,
and finally /lib/modules/$(uname -r)/build.

Also skip test_kmods all/clean when the selected KDIR does not exist, so
selftests clean/install flows do not fail on systems without that host
module build directory.

Signed-off-by: Ricardo B. Marliere <rbm@xxxxxxxx>
---
 tools/testing/selftests/bpf/test_kmods/Makefile | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/test_kmods/Makefile b/tools/testing/selftests/bpf/test_kmods/Makefile
index 63c4d3f6a12f..a5216815d989 100644
--- a/tools/testing/selftests/bpf/test_kmods/Makefile
+++ b/tools/testing/selftests/bpf/test_kmods/Makefile
@@ -1,5 +1,6 @@
 TEST_KMOD_DIR := $(realpath $(dir $(abspath $(lastword $(MAKEFILE_LIST)))))
-KDIR ?= $(abspath $(TEST_KMOD_DIR)/../../../../..)
+SRCTREE_KDIR := $(abspath $(TEST_KMOD_DIR)/../../../../..)
+KDIR ?= $(if $(O),$(O),$(if $(KBUILD_OUTPUT),$(KBUILD_OUTPUT),$(if $(wildcard $(SRCTREE_KDIR)/Module.symvers),$(SRCTREE_KDIR),/lib/modules/$(shell uname -r)/build)))
 
 ifeq ($(V),1)
 Q =
@@ -14,8 +15,13 @@ $(foreach m,$(MODULES),$(eval obj-m += $(m:.ko=.o)))
 
 CFLAGS_bpf_testmod.o = -I$(src)
 
+# Ensure KDIR exists, otherwise skip module build and clean.
 all:
+ifneq ("$(wildcard $(KDIR))", "")
 	$(Q)$(MAKE) -C $(KDIR) M=$(TEST_KMOD_DIR) modules
+endif
 
 clean:
+ifneq ("$(wildcard $(KDIR))", "")
 	$(Q)$(MAKE) -C $(KDIR) M=$(TEST_KMOD_DIR) clean
+endif

-- 
2.53.0