tcp: use tcp_v4_send_synack on first SYN-ACK

To avoid large code duplication in IPv6, we need to first simplify
the complicate SYN-ACK sending code in tcp_v4_conn_request().

To use tcp_v4(6)_send_synack() to send all SYN-ACKs, we need to
initialize the mini socket's receive window before trying to
create the child socket and/or building the SYN-ACK packet. So we move
that initialization from tcp_make_synack() to tcp_v4_conn_request()
as a new function tcp_openreq_init_req_rwin().

After this refactoring the SYN-ACK sending code is simpler and easier
to implement Fast Open for IPv6.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Daniel Lee <longinus00@gmail.com>
Signed-off-by: Jerry Chu <hkchu@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 31 insertions, 0 deletions

diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index 05c1b155251d..e68e0d4af6c9 100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -362,6 +362,37 @@ void tcp_twsk_destructor(struct sock *sk)
 }
 EXPORT_SYMBOL_GPL(tcp_twsk_destructor);
 
+void tcp_openreq_init_rwin(struct request_sock *req,
+                           struct sock *sk, struct dst_entry *dst)
+{
+        struct inet_request_sock *ireq = inet_rsk(req);
+        struct tcp_sock *tp = tcp_sk(sk);
+        __u8 rcv_wscale;
+        int mss = dst_metric_advmss(dst);
+
+        if (tp->rx_opt.user_mss && tp->rx_opt.user_mss < mss)
+                mss = tp->rx_opt.user_mss;
+
+        /* Set this up on the first call only */
+        req->window_clamp = tp->window_clamp ? : dst_metric(dst, RTAX_WINDOW);
+
+        /* limit the window selection if the user enforce a smaller rx buffer */
+        if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
+            (req->window_clamp > tcp_full_space(sk) || req->window_clamp == 0))
+                req->window_clamp = tcp_full_space(sk);
+
+        /* tcp_full_space because it is guaranteed to be the first packet */
+        tcp_select_initial_window(tcp_full_space(sk),
+                mss - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0),
+                &req->rcv_wnd,
+                &req->window_clamp,
+                ireq->wscale_ok,
+                &rcv_wscale,
+                dst_metric(dst, RTAX_INITRWND));
+        ireq->rcv_wscale = rcv_wscale;
+}
+EXPORT_SYMBOL(tcp_openreq_init_rwin);
+
 static inline void TCP_ECN_openreq_child(struct tcp_sock *tp,
                                          struct request_sock *req)
 {