[PATCH bpf-next v11 08/11] selftests/bpf: Allow test_progs to link with a partial object set

["Ricardo B. Marlière" <rbm@suse.com>]

When individual test files are skipped due to compilation failures, their
.test.o files are absent. The linker step currently lists all expected
.test.o files as explicit prerequisites, so make considers any missing one
an error.

In permissive mode, declare the test objects that already exist on disk
(via parse-time $(wildcard ...)) as normal prerequisites of the binary so
that modifications to a test source still trigger a relink, and keep the
full TRUNNER_TEST_OBJS list as order-only prerequisites so that initial
fresh builds still produce them and missing objects do not abort the link.
The recipe filter is split per mode: in permissive mode it combines a
recipe-time $(wildcard ...) (which catches objects freshly produced via
the order-only path on a fresh build) with $(filter-out
$(TRUNNER_TEST_OBJS),$^) (which keeps the non-test inputs from $^ but
drops the parse-time wildcard duplicates). This avoids passing the same
.test.o twice to the linker while still presenting test objects before
libbpf.a so that GNU ld, which scans static archives left-to-right, pulls
in archive members referenced exclusively by test objects (e.g.
ring_buffer__new from ringbuf.c). In default (strict) mode the recipe
remains the simple $(filter %.a %.o,$^) since TRUNNER_TEST_OBJS is part
of $^ exactly once.

Gate the partial-link behavior on $(if $(filter test_progs%,$1),...) so
it only applies to test_progs and its flavors. test_maps and similar
runners using strong cross-object references would link-fail with a
partial set and intentionally retain strict link semantics.

Note: adding a brand-new test_*.c file in permissive mode requires
removing the binary (or a clean rebuild) before the new test is linked
in, because the parse-time $(wildcard ...) is evaluated when the Makefile
is read and will not yet see the new .test.o. This is acceptable since
permissive mode targets tolerant CI builds rather than incremental
development.

Signed-off-by: Ricardo B. Marlière <rbm@suse.com>
---
 tools/testing/selftests/bpf/Makefile | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
index df173f9800f7..04634e6e0661 100644
--- a/tools/testing/selftests/bpf/Makefile
+++ b/tools/testing/selftests/bpf/Makefile
@@ -773,14 +773,15 @@ endif
 # some X.test.o files have runtime dependencies on Y.bpf.o files
 $(OUTPUT)/$(TRUNNER_BINARY): | $(TRUNNER_BPF_OBJS)
 
-$(OUTPUT)/$(TRUNNER_BINARY): $(TRUNNER_TEST_OBJS)			\
+$(OUTPUT)/$(TRUNNER_BINARY): $(if $(filter test_progs%,$1),$(if $(PERMISSIVE),$$(wildcard $(TRUNNER_TEST_OBJS)),$(TRUNNER_TEST_OBJS)),$(TRUNNER_TEST_OBJS))	\
 			     $(TRUNNER_EXTRA_OBJS) $$(BPFOBJ)		\
 			     $(TRUNNER_LIB_OBJS)			\
 			     $(TRUNNER_BPFTOOL)				\
 			     $(OUTPUT)/veristat				\
-			     | $(TRUNNER_BINARY)-extras
+			     | $(TRUNNER_BINARY)-extras			\
+			     $(if $(filter test_progs%,$1),$(if $(PERMISSIVE),$(TRUNNER_TEST_OBJS)))
 	$$(call msg,BINARY,,$$@)
-	$(Q)$$(CC) $$(CFLAGS) $$(filter %.a %.o,$$^) $$(LDLIBS) $$(LLVM_LDLIBS) $$(LDFLAGS) $$(LLVM_LDFLAGS) -o $$@
+	$(Q)$$(CC) $$(CFLAGS) $(if $(filter test_progs%,$1),$(if $(PERMISSIVE),$$(filter %.a %.o,$$(wildcard $(TRUNNER_TEST_OBJS)) $$(filter-out $(TRUNNER_TEST_OBJS),$$^)),$$(filter %.a %.o,$$^)),$$(filter %.a %.o,$$^)) $$(LDLIBS) $$(LLVM_LDLIBS) $$(LDFLAGS) $$(LLVM_LDFLAGS) -o $$@
 	$(Q)ln -sf $(if $2,..,.)/tools/build/bpftool/$(USE_BOOTSTRAP)bpftool \
 		   $(OUTPUT)/$(if $2,$2/)bpftool
 

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2.54.0