[RFC PATCH bpf-next v5 4/8] selftests/bpf: generate the signing key and certificate once

$(VERIFICATION_CERT) and $(PRIVATE_KEY) come from a single genkey
invocation, but a rule with two normal targets runs its recipe once
per stale target - under -j both runs execute concurrently and the
openssl invocations race on the same output files. Rewrite it as a
pattern rule: pattern rules with multiple targets are implicitly
grouped even before make 4.3 introduced grouped (&:) targets - the
same workaround the test_kmods rule already uses.

Signed-off-by: Mykola Lysenko <[email protected]>
---
 tools/testing/selftests/bpf/Makefile | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile
index 565862247..774286a55 100644
--- a/tools/testing/selftests/bpf/Makefile
+++ b/tools/testing/selftests/bpf/Makefile
@@ -800,7 +800,11 @@ VERIFY_SIG_HDR := verification_cert.h
 VERIFICATION_CERT   := $(BUILD_DIR)/signing_key.der
 PRIVATE_KEY := $(BUILD_DIR)/signing_key.pem
 
-$(VERIFICATION_CERT) $(PRIVATE_KEY): $(VERIFY_SIG_SETUP)
+# One genkey run produces both files. A plain two-target rule is not
+# grouped - under -j make would run genkey twice concurrently and the
+# openssl invocations race; the pattern form is implicitly grouped even
+# with make < 4.3.
+$(BUILD_DIR)/%.pem $(BUILD_DIR)/%.der: $(VERIFY_SIG_SETUP)
 	$(Q)mkdir -p $(BUILD_DIR)
 	$(Q)$(VERIFY_SIG_SETUP) genkey $(BUILD_DIR)
 
-- 
2.43.0