selftests/bpf: Build testing_helpers.o out of tree

Add $(OUTPUT) prefix to testing_helpers.o, so it can be built out of
tree when necessary. At the moment, in addition to being built in-tree
even when out-of-tree is required, testing_helpers.o is not built with
the right recipe when cross-building.

For consistency the other helpers, cgroup_helpers and trace_helpers, can
also be passed as objects instead of source. Use *_HELPERS variable to
keep the Makefile readable.

Fixes: f87c1930 ("selftests/bpf: Merge test_stub.c into testing_helpers.c")
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211201145101.823159-1-jean-philippe@linaro.org

tools/testing/selftests/bpf/Makefile

@@ -192,22 +192,26 @@ TEST_GEN_PROGS_EXTENDED += $(DEFAULT_BPFTOOL)
 
 $(TEST_GEN_PROGS) $(TEST_GEN_PROGS_EXTENDED): $(BPFOBJ)
 
-$(OUTPUT)/test_dev_cgroup: cgroup_helpers.c testing_helpers.o
-$(OUTPUT)/test_skb_cgroup_id_user: cgroup_helpers.c testing_helpers.o
-$(OUTPUT)/test_sock: cgroup_helpers.c testing_helpers.o
-$(OUTPUT)/test_sock_addr: cgroup_helpers.c testing_helpers.o
-$(OUTPUT)/test_sockmap: cgroup_helpers.c testing_helpers.o
-$(OUTPUT)/test_tcpnotify_user: cgroup_helpers.c trace_helpers.c testing_helpers.o
-$(OUTPUT)/get_cgroup_id_user: cgroup_helpers.c testing_helpers.o
-$(OUTPUT)/test_cgroup_storage: cgroup_helpers.c testing_helpers.o
-$(OUTPUT)/test_sock_fields: cgroup_helpers.c testing_helpers.o
-$(OUTPUT)/test_sysctl: cgroup_helpers.c testing_helpers.o
-$(OUTPUT)/test_tag: testing_helpers.o
-$(OUTPUT)/test_lirc_mode2_user: testing_helpers.o
-$(OUTPUT)/xdping: testing_helpers.o
-$(OUTPUT)/flow_dissector_load: testing_helpers.o
-$(OUTPUT)/test_maps: testing_helpers.o
-$(OUTPUT)/test_verifier: testing_helpers.o
+CGROUP_HELPERS	:= $(OUTPUT)/cgroup_helpers.o
+TESTING_HELPERS	:= $(OUTPUT)/testing_helpers.o
+TRACE_HELPERS	:= $(OUTPUT)/trace_helpers.o
+
+$(OUTPUT)/test_dev_cgroup: $(CGROUP_HELPERS) $(TESTING_HELPERS)
+$(OUTPUT)/test_skb_cgroup_id_user: $(CGROUP_HELPERS) $(TESTING_HELPERS)
+$(OUTPUT)/test_sock: $(CGROUP_HELPERS) $(TESTING_HELPERS)
+$(OUTPUT)/test_sock_addr: $(CGROUP_HELPERS) $(TESTING_HELPERS)
+$(OUTPUT)/test_sockmap: $(CGROUP_HELPERS) $(TESTING_HELPERS)
+$(OUTPUT)/test_tcpnotify_user: $(CGROUP_HELPERS) $(TESTING_HELPERS) $(TRACE_HELPERS)
+$(OUTPUT)/get_cgroup_id_user: $(CGROUP_HELPERS) $(TESTING_HELPERS)
+$(OUTPUT)/test_cgroup_storage: $(CGROUP_HELPERS) $(TESTING_HELPERS)
+$(OUTPUT)/test_sock_fields: $(CGROUP_HELPERS) $(TESTING_HELPERS)
+$(OUTPUT)/test_sysctl: $(CGROUP_HELPERS) $(TESTING_HELPERS)
+$(OUTPUT)/test_tag: $(TESTING_HELPERS)
+$(OUTPUT)/test_lirc_mode2_user: $(TESTING_HELPERS)
+$(OUTPUT)/xdping: $(TESTING_HELPERS)
+$(OUTPUT)/flow_dissector_load: $(TESTING_HELPERS)
+$(OUTPUT)/test_maps: $(TESTING_HELPERS)
+$(OUTPUT)/test_verifier: $(TESTING_HELPERS)
 
 BPFTOOL ?= $(DEFAULT_BPFTOOL)
 $(DEFAULT_BPFTOOL): $(wildcard $(BPFTOOLDIR)/*.[ch] $(BPFTOOLDIR)/Makefile)    \
@@ -536,8 +540,8 @@ $(OUTPUT)/bench_bpf_loop.o: $(OUTPUT)/bpf_loop_bench.skel.h
 $(OUTPUT)/bench.o: bench.h testing_helpers.h $(BPFOBJ)
 $(OUTPUT)/bench: LDLIBS += -lm
 $(OUTPUT)/bench: $(OUTPUT)/bench.o \
-		 $(OUTPUT)/testing_helpers.o \
-		 $(OUTPUT)/trace_helpers.o \
+		 $(TESTING_HELPERS) \
+		 $(TRACE_HELPERS) \
 		 $(OUTPUT)/bench_count.o \
 		 $(OUTPUT)/bench_rename.o \
 		 $(OUTPUT)/bench_trigger.o \