[PATCH net] tcp: properly scale window in tcp_v[46]_reqsk_send_ack()

[Eric Dumazet]

From: Eric Dumazet <eduma...@google.com>

When sending an ack in SYN_RECV state, we must scale the offered
window if wscale option was negotiated and accepted.

Tested:
 Following packetdrill test demonstrates the issue :

0.000 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0

+0 bind(3, ..., ...) = 0
+0 listen(3, 1) = 0

// Establish a connection.
+0 < S 0:0(0) win 20000 <mss 1000,sackOK,wscale 7, nop, TS val 100 ecr 0>
+0 > S. 0:0(0) ack 1 win 28960 <mss 1460,sackOK, TS val 100 ecr 100, nop, 
wscale 7>

+0 < . 1:11(10) ack 1 win 156 <nop,nop,TS val 99 ecr 100>
// check that window is properly scaled !
+0 > . 1:1(0) ack 1 win 226 <nop,nop,TS val 200 ecr 100>


Signed-off-by: Eric Dumazet <eduma...@google.com>
Cc: Yuchung Cheng <ych...@google.com>
Cc: Neal Cardwell <ncardw...@google.com>
---
 net/ipv4/tcp_ipv4.c |    8 +++++++-
 net/ipv6/tcp_ipv6.c |    8 +++++++-
 2 files changed, 14 insertions(+), 2 deletions(-)


diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 32b048e524d6..7158d4f8dae4 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -814,8 +814,14 @@ static void tcp_v4_reqsk_send_ack(const struct sock *sk, 
struct sk_buff *skb,
        u32 seq = (sk->sk_state == TCP_LISTEN) ? tcp_rsk(req)->snt_isn + 1 :
                                             tcp_sk(sk)->snd_nxt;
 
+       /* RFC 7323 2.3
+        * The window field (SEG.WND) of every outgoing segment, with the
+        * exception of <SYN> segments, MUST be right-shifted by
+        * Rcv.Wind.Shift bits:
+        */
        tcp_v4_send_ack(sock_net(sk), skb, seq,
-                       tcp_rsk(req)->rcv_nxt, req->rsk_rcv_wnd,
+                       tcp_rsk(req)->rcv_nxt,
+                       req->rsk_rcv_wnd >> inet_rsk(req)->rcv_wscale,
                        tcp_time_stamp,
                        req->ts_recent,
                        0,
diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index 33df8b8575cc..94f4f89d73e7 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -944,9 +944,15 @@ static void tcp_v6_reqsk_send_ack(const struct sock *sk, 
struct sk_buff *skb,
        /* sk->sk_state == TCP_LISTEN -> for regular TCP_SYN_RECV
         * sk->sk_state == TCP_SYN_RECV -> for Fast Open.
         */
+       /* RFC 7323 2.3
+        * The window field (SEG.WND) of every outgoing segment, with the
+        * exception of <SYN> segments, MUST be right-shifted by
+        * Rcv.Wind.Shift bits:
+        */
        tcp_v6_send_ack(sk, skb, (sk->sk_state == TCP_LISTEN) ?
                        tcp_rsk(req)->snt_isn + 1 : tcp_sk(sk)->snd_nxt,
-                       tcp_rsk(req)->rcv_nxt, req->rsk_rcv_wnd,
+                       tcp_rsk(req)->rcv_nxt,
+                       req->rsk_rcv_wnd >> inet_rsk(req)->rcv_wscale,
                        tcp_time_stamp, req->ts_recent, sk->sk_bound_dev_if,
                        tcp_v6_md5_do_lookup(sk, &ipv6_hdr(skb)->daddr),
                        0, 0);