tcp: fix fastopen code vs usec TS

After blamed commit, TFO client-ack-dropped-then-recovery-ms-timestamps
packetdrill test failed.

David Morley and Neal Cardwell started investigating and Neal pointed
that we had :

tcp_conn_request()
  tcp_try_fastopen()
   -> tcp_fastopen_create_child
     -> child = inet_csk(sk)->icsk_af_ops->syn_recv_sock()
       -> tcp_create_openreq_child()
          -> copy req_usec_ts from req:
          newtp->tcp_usec_ts = treq->req_usec_ts;
          // now the new TFO server socket always does usec TS, no matter
          // what the route options are...
  send_synack()
    -> tcp_make_synack()
        // disable tcp_rsk(req)->req_usec_ts if route option is not present:
        if (tcp_rsk(req)->req_usec_ts < 0)
                tcp_rsk(req)->req_usec_ts = dst_tcp_usec_ts(dst);

tcp_conn_request() has the initial dst, we can initialize
tcp_rsk(req)->req_usec_ts there instead of later in send_synack();

This means tcp_rsk(req)->req_usec_ts can be a boolean.

Many thanks to David an Neal for their help.

Fixes: 614e831 ("tcp: add support for usec resolution in TCP TS values")
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202310302216.f79d78bc-oliver.sang@intel.com
Suggested-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: David Morley <morleyd@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: edumazet

include/linux/tcp.h

@@ -152,7 +152,7 @@ struct tcp_request_sock {
 	u64				snt_synack; /* first SYNACK sent time */
 	bool				tfo_listener;
 	bool				is_mptcp;
-	s8				req_usec_ts;
+	bool				req_usec_ts;
 #if IS_ENABLED(CONFIG_MPTCP)
 	bool				drop_req;
 #endif

net/ipv4/syncookies.c

@@ -306,7 +306,7 @@ struct request_sock *cookie_tcp_reqsk_alloc(const struct request_sock_ops *ops,
 	treq->af_specific = af_ops;
 
 	treq->syn_tos = TCP_SKB_CB(skb)->ip_dsfield;
-	treq->req_usec_ts = -1;
+	treq->req_usec_ts = false;
 
 #if IS_ENABLED(CONFIG_MPTCP)
 	treq->is_mptcp = sk_is_mptcp(sk);

net/ipv4/tcp_input.c

@@ -7115,7 +7115,7 @@ int tcp_conn_request(struct request_sock_ops *rsk_ops,
 	req->syncookie = want_cookie;
 	tcp_rsk(req)->af_specific = af_ops;
 	tcp_rsk(req)->ts_off = 0;
-	tcp_rsk(req)->req_usec_ts = -1;
+	tcp_rsk(req)->req_usec_ts = false;
 #if IS_ENABLED(CONFIG_MPTCP)
 	tcp_rsk(req)->is_mptcp = 0;
 #endif
@@ -7143,9 +7143,10 @@ int tcp_conn_request(struct request_sock_ops *rsk_ops,
 	if (!dst)
 		goto drop_and_free;
 
-	if (tmp_opt.tstamp_ok)
+	if (tmp_opt.tstamp_ok) {
+		tcp_rsk(req)->req_usec_ts = dst_tcp_usec_ts(dst);
 		tcp_rsk(req)->ts_off = af_ops->init_ts_off(net, skb);
-
+	}
 	if (!want_cookie && !isn) {
 		int max_syn_backlog = READ_ONCE(net->ipv4.sysctl_max_syn_backlog);
 

net/ipv4/tcp_output.c

@@ -3693,8 +3693,6 @@ struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst,
 	mss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
 
 	memset(&opts, 0, sizeof(opts));
-	if (tcp_rsk(req)->req_usec_ts < 0)
-		tcp_rsk(req)->req_usec_ts = dst_tcp_usec_ts(dst);
 	now = tcp_clock_ns();
 #ifdef CONFIG_SYN_COOKIES
 	if (unlikely(synack_type == TCP_SYNACK_COOKIE && ireq->tstamp_ok))