selftests/bpf: extend BPF_SOCK_OPS_RTT_CB test for srtt and mrtt_us

Because srtt and mrtt_us are added as args in bpf_sock_ops at
BPF_SOCK_OPS_RTT_CB, a simple check is added to make sure they are both
non-zero.

$ ./test_progs -t tcp_rtt
  #373     tcp_rtt:OK
  Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED

Suggested-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Philo Lu <lulie@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240425161724.73707-3-lulie@linux.alibaba.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

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

@@ -10,6 +10,9 @@ struct tcp_rtt_storage {
 	__u32 delivered;
 	__u32 delivered_ce;
 	__u32 icsk_retransmits;
+
+	__u32 mrtt_us;	/* args[0] */
+	__u32 srtt;	/* args[1] */
 };
 
 static void send_byte(int fd)
@@ -83,6 +86,17 @@ static int verify_sk(int map_fd, int client_fd, const char *msg, __u32 invoked,
 		err++;
 	}
 
+	/* Precise values of mrtt and srtt are unavailable, just make sure they are nonzero */
+	if (val.mrtt_us == 0) {
+		log_err("%s: unexpected bpf_tcp_sock.args[0] (mrtt_us) %u == 0", msg, val.mrtt_us);
+		err++;
+	}
+
+	if (val.srtt == 0) {
+		log_err("%s: unexpected bpf_tcp_sock.args[1] (srtt) %u == 0", msg, val.srtt);
+		err++;
+	}
+
 	return err;
 }
 

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

@@ -10,6 +10,9 @@ struct tcp_rtt_storage {
 	__u32 delivered;
 	__u32 delivered_ce;
 	__u32 icsk_retransmits;
+
+	__u32 mrtt_us;	/* args[0] */
+	__u32 srtt;	/* args[1] */
 };
 
 struct {
@@ -55,5 +58,8 @@ int _sockops(struct bpf_sock_ops *ctx)
 	storage->delivered_ce = tcp_sk->delivered_ce;
 	storage->icsk_retransmits = tcp_sk->icsk_retransmits;
 
+	storage->mrtt_us = ctx->args[0];
+	storage->srtt = ctx->args[1];
+
 	return 1;
 }