selftests/bpf: Make skeleton headers order-only prerequisites of .test.d

The .test.d dependency files are generated by the C preprocessor and list
the headers each test file actually #includes. Skeleton headers appear in
those generated lists, so the .test.o -> .skel.h dependency is already
tracked by the .d file content.

Making skeletons order-only prerequisites of .test.d means that a missing
or skipped skeleton does not prevent .test.d generation, and regenerating a
skeleton does not force .test.d to be recreated. This avoids unnecessary
recompilation and, more importantly, avoids build errors when a skeleton
was intentionally skipped due to a BPF compilation failure.

$$(BPFOBJ) is intentionally kept as a normal prerequisite: a libbpf
rebuild legitimately invalidates .test.d, since libbpf header changes
can affect the headers .test.o sees. Only the skeleton headers are
moved to order-only.

Note that adding a new BPF skeleton via a modified existing local header
still works correctly: GNU make builds order-only prerequisites that do
not exist (the order-only qualifier only suppresses timestamp-driven
rebuilds, not existence-driven builds), so a brand-new .skel.h listed in
TRUNNER_BPF_SKELS is generated even when .test.d is otherwise up to date.
The modified local header invalidates .test.o through the previously
included .d content, forcing a recompile that regenerates .test.d with
the new .skel.h dependency captured by gcc -MMD.

Signed-off-by: Ricardo B. Marlière <rbm@suse.com>
Link: https://lore.kernel.org/r/20260602-selftests-bpf_misconfig-v12-5-27f898b3ba26@suse.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
index 49f6a5503e8409e61a1b28f52f693b590079203f..09de69a811129d62854d449a049e197470bfec9a 100644 (file)
--- a/tools/testing/selftests/bpf/Makefile
+++ b/tools/testing/selftests/bpf/Makefile
@@ -726,11 +726,11 @@ $(TRUNNER_TEST_OBJS): $(TRUNNER_OUTPUT)/%.test.o:                 \
 $(TRUNNER_TEST_OBJS:.o=.d): $(TRUNNER_OUTPUT)/%.test.d:                        \
                            $(TRUNNER_TESTS_DIR)/%.c                    \
                            $(TRUNNER_EXTRA_HDRS)                       \
+                           $$(BPFOBJ) | $(TRUNNER_OUTPUT)              \
                            $(TRUNNER_BPF_SKELS)                        \
                            $(TRUNNER_BPF_LSKELS)                       \
                            $(TRUNNER_BPF_LSKELS_SIGNED)                \
-                           $(TRUNNER_BPF_SKELS_LINKED)                 \
-                           $$(BPFOBJ) | $(TRUNNER_OUTPUT)
+                           $(TRUNNER_BPF_SKELS_LINKED)
 
 ifeq ($(filter clean docs-clean emit_tests,$(MAKECMDGOALS)),)
 include $(wildcard $(TRUNNER_TEST_OBJS:.o=.d))