selftests/bpf: Test an incomplete BPF CC

Test whether a TCP CC implemented in BPF providing neither cong_avoid()
nor cong_control() is correctly rejected. This check solely depends on
tcp_register_congestion_control() now, which is invoked during
bpf_map__attach_struct_ops().

Signed-off-by: Jörn-Thorben Hinz <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/testing/selftests/bpf/prog_tests/bpf_tcp_ca.c

···
 #include "bpf_tcp_nogpl.skel.h"
 #include "bpf_dctcp_release.skel.h"
 #include "tcp_ca_write_sk_pacing.skel.h"
+#include "tcp_ca_incompl_cong_ops.skel.h"
 
 #ifndef ENOTSUPP
 #define ENOTSUPP 524
···
 	tcp_ca_write_sk_pacing__destroy(skel);
 }
 
+static void test_incompl_cong_ops(void)
+{
+	struct tcp_ca_incompl_cong_ops *skel;
+	struct bpf_link *link;
+
+	skel = tcp_ca_incompl_cong_ops__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "open_and_load"))
+		return;
+
+	/* That cong_avoid() and cong_control() are missing is only reported at
+	 * this point:
+	 */
+	link = bpf_map__attach_struct_ops(skel->maps.incompl_cong_ops);
+	ASSERT_ERR_PTR(link, "attach_struct_ops");
+
+	bpf_link__destroy(link);
+	tcp_ca_incompl_cong_ops__destroy(skel);
+}
+
 void test_bpf_tcp_ca(void)
 {
 	if (test__start_subtest("dctcp"))
···
 		test_rel_setsockopt();
 	if (test__start_subtest("write_sk_pacing"))
 		test_write_sk_pacing();
+	if (test__start_subtest("incompl_cong_ops"))
+		test_incompl_cong_ops();
 }

tools/testing/selftests/bpf/progs/tcp_ca_incompl_cong_ops.c

···
+// SPDX-License-Identifier: GPL-2.0
+
+#include "vmlinux.h"
+
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+char _license[] SEC("license") = "GPL";
+
+static inline struct tcp_sock *tcp_sk(const struct sock *sk)
+{
+	return (struct tcp_sock *)sk;
+}
+
+SEC("struct_ops/incompl_cong_ops_ssthresh")
+__u32 BPF_PROG(incompl_cong_ops_ssthresh, struct sock *sk)
+{
+	return tcp_sk(sk)->snd_ssthresh;
+}
+
+SEC("struct_ops/incompl_cong_ops_undo_cwnd")
+__u32 BPF_PROG(incompl_cong_ops_undo_cwnd, struct sock *sk)
+{
+	return tcp_sk(sk)->snd_cwnd;
+}
+
+SEC(".struct_ops")
+struct tcp_congestion_ops incompl_cong_ops = {
+	/* Intentionally leaving out any of the required cong_avoid() and
+	 * cong_control() here.
+	 */
+	.ssthresh = (void *)incompl_cong_ops_ssthresh,
+	.undo_cwnd = (void *)incompl_cong_ops_undo_cwnd,
+	.name = "bpf_incompl_ops",
+};