selftests/bpf: Verify bpf-tcp-cc rejects TCP_NODELAY

Add a bpf_tcp_ca selftest for the TCP_NODELAY restriction in
bpf-tcp-cc.

Update bpf_cubic to exercise init() and cwnd_event_tx_start(),
and check that both callbacks reject bpf_setsockopt(TCP_NODELAY)
with -EOPNOTSUPP.

Signed-off-by: KaFai Wan <kafai.wan@linux.dev>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20260421155804.135786-5-kafai.wan@linux.dev

Author: mannkafai

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

@@ -112,6 +112,10 @@ static void test_cubic(void)
 
 	ASSERT_EQ(cubic_skel->bss->bpf_cubic_acked_called, 1, "pkts_acked called");
 
+	ASSERT_TRUE(cubic_skel->bss->nodelay_init_reject, "init reject nodelay option");
+	ASSERT_TRUE(cubic_skel->bss->nodelay_cwnd_event_tx_start_reject,
+		    "cwnd_event_tx_start reject nodelay option");
+
 	bpf_link__destroy(link);
 	bpf_cubic__destroy(cubic_skel);
 }

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

@@ -16,6 +16,7 @@
 
 #include "bpf_tracing_net.h"
 #include <bpf/bpf_tracing.h>
+#include <errno.h>
 
 char _license[] SEC("license") = "GPL";
 
@@ -170,10 +171,18 @@ static void bictcp_hystart_reset(struct sock *sk)
 	ca->sample_cnt = 0;
 }
 
+bool nodelay_init_reject = false;
+bool nodelay_cwnd_event_tx_start_reject = false;
+
 SEC("struct_ops")
 void BPF_PROG(bpf_cubic_init, struct sock *sk)
 {
 	struct bpf_bictcp *ca = inet_csk_ca(sk);
+	int true_val = 1, ret;
+
+	ret = bpf_setsockopt(sk, SOL_TCP, TCP_NODELAY, &true_val, sizeof(true_val));
+	if (ret == -EOPNOTSUPP)
+		nodelay_init_reject = true;
 
 	bictcp_reset(ca);
 
@@ -189,8 +198,13 @@ void BPF_PROG(bpf_cubic_cwnd_event_tx_start, struct sock *sk)
 {
 	struct bpf_bictcp *ca = inet_csk_ca(sk);
 	__u32 now = tcp_jiffies32;
+	int true_val = 1, ret;
 	__s32 delta;
 
+	ret = bpf_setsockopt(sk, SOL_TCP, TCP_NODELAY, &true_val, sizeof(true_val));
+	if (ret == -EOPNOTSUPP)
+		nodelay_cwnd_event_tx_start_reject = true;
+
 	delta = now - tcp_sk(sk)->lsndtime;
 
 	/* We were application limited (idle) for a while.