tcp: options clean up

This should fix the following bugs:
  * Connections with MD5 signatures produce invalid packets whenever SACK
    options are included
  * MD5 signatures are counted twice in the MSS calculations

Behaviour changes:
  * A SYN with MD5 + SACK + TS elicits a SYNACK with MD5 + SACK

    This is because we can't fit any SACK blocks in a packet with MD5 + TS
    options. There was discussion about disabling SACK rather than TS in
    order to fit in better with old, buggy kernels, but that was deemed to
    be unnecessary.

  * SYNs with MD5 don't include a TS option

    See above.

Additionally, it removes a bunch of duplicated logic for calculating options,
which should help avoid these sort of issues in the future.

Signed-off-by: Adam Langley <agl@imperialviolet.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 238 insertions, 196 deletions

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 31f5bbfc59bc..8983386356a5 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -50,6 +50,7 @@ extern atomic_t tcp_orphan_count;
 extern void tcp_time_wait(struct sock *sk, int state, int timeo);
 
 #define MAX_TCP_HEADER  (128 + MAX_HEADER)
+#define MAX_TCP_OPTION_SPACE 40
 
 /* 
  * Never offer a window over 32767 without using window scaling. Some
@@ -184,6 +185,7 @@ extern void tcp_time_wait(struct sock *sk, int state, int timeo);
 #define TCPOLEN_SACK_BASE_ALIGNED       4
 #define TCPOLEN_SACK_PERBLOCK           8
 #define TCPOLEN_MD5SIG_ALIGNED          20
+#define TCPOLEN_MSS_ALIGNED             4
 
 /* Flags in tp->nonagle */
 #define TCP_NAGLE_OFF           1       /* Nagle's algo is disabled */
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 958ff486165f..1fa683c0ba9b 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -345,28 +345,82 @@ static void tcp_init_nondata_skb(struct sk_buff *skb, u32 seq, u8 flags)
         TCP_SKB_CB(skb)->end_seq = seq;
 }
 
-static void tcp_build_and_update_options(__be32 *ptr, struct tcp_sock *tp,
-                                         __u32 tstamp, __u8 **md5_hash)
-{
-        if (tp->rx_opt.tstamp_ok) {
+#define OPTION_SACK_ADVERTISE   (1 << 0)
+#define OPTION_TS               (1 << 1)
+#define OPTION_MD5              (1 << 2)
+
+struct tcp_out_options {
+        u8 options;             /* bit field of OPTION_* */
+        u8 ws;                  /* window scale, 0 to disable */
+        u8 num_sack_blocks;     /* number of SACK blocks to include */
+        u16 mss;                /* 0 to disable */
+        __u32 tsval, tsecr;     /* need to include OPTION_TS */
+};
+
+static void tcp_options_write(__be32 *ptr, struct tcp_sock *tp,
+                              const struct tcp_out_options *opts,
+                              __u8 **md5_hash) {
+        if (unlikely(OPTION_MD5 & opts->options)) {
                 *ptr++ = htonl((TCPOPT_NOP << 24) |
                                (TCPOPT_NOP << 16) |
-                               (TCPOPT_TIMESTAMP << 8) |
-                               TCPOLEN_TIMESTAMP);
-                *ptr++ = htonl(tstamp);
-                *ptr++ = htonl(tp->rx_opt.ts_recent);
+                               (TCPOPT_MD5SIG << 8) |
+                               TCPOLEN_MD5SIG);
+                *md5_hash = (__u8 *)ptr;
+                ptr += 4;
+        } else {
+                *md5_hash = NULL;
         }
-        if (tp->rx_opt.eff_sacks) {
-                struct tcp_sack_block *sp = tp->rx_opt.dsack ? tp->duplicate_sack : tp->selective_acks;
+
+        if (likely(OPTION_TS & opts->options)) {
+                if (unlikely(OPTION_SACK_ADVERTISE & opts->options)) {
+                        *ptr++ = htonl((TCPOPT_SACK_PERM << 24) |
+                                       (TCPOLEN_SACK_PERM << 16) |
+                                       (TCPOPT_TIMESTAMP << 8) |
+                                       TCPOLEN_TIMESTAMP);
+                } else {
+                        *ptr++ = htonl((TCPOPT_NOP << 24) |
+                                       (TCPOPT_NOP << 16) |
+                                       (TCPOPT_TIMESTAMP << 8) |
+                                       TCPOLEN_TIMESTAMP);
+                }
+                *ptr++ = htonl(opts->tsval);
+                *ptr++ = htonl(opts->tsecr);
+        }
+
+        if (unlikely(opts->mss)) {
+                *ptr++ = htonl((TCPOPT_MSS << 24) |
+                               (TCPOLEN_MSS << 16) |
+                               opts->mss);
+        }
+
+        if (unlikely(OPTION_SACK_ADVERTISE & opts->options &&
+                     !(OPTION_TS & opts->options))) {
+                *ptr++ = htonl((TCPOPT_NOP << 24) |
+                               (TCPOPT_NOP << 16) |
+                               (TCPOPT_SACK_PERM << 8) |
+                               TCPOLEN_SACK_PERM);
+        }
+
+        if (unlikely(opts->ws)) {
+                *ptr++ = htonl((TCPOPT_NOP << 24) |
+                               (TCPOPT_WINDOW << 16) |
+                               (TCPOLEN_WINDOW << 8) |
+                               opts->ws);
+        }
+
+        if (unlikely(opts->num_sack_blocks)) {
+                struct tcp_sack_block *sp = tp->rx_opt.dsack ?
+                        tp->duplicate_sack : tp->selective_acks;
                 int this_sack;
 
                 *ptr++ = htonl((TCPOPT_NOP  << 24) |
                                (TCPOPT_NOP  << 16) |
                                (TCPOPT_SACK <<  8) |
-                               (TCPOLEN_SACK_BASE + (tp->rx_opt.eff_sacks *
+                               (TCPOLEN_SACK_BASE + (opts->num_sack_blocks *
                                                      TCPOLEN_SACK_PERBLOCK)));
 
-                for (this_sack = 0; this_sack < tp->rx_opt.eff_sacks; this_sack++) {
+                for (this_sack = 0; this_sack < opts->num_sack_blocks;
+                     ++this_sack) {
                         *ptr++ = htonl(sp[this_sack].start_seq);
                         *ptr++ = htonl(sp[this_sack].end_seq);
                 }
@@ -376,81 +430,137 @@ static void tcp_build_and_update_options(__be32 *ptr, struct tcp_sock *tp,
                         tp->rx_opt.eff_sacks--;
                 }
         }
+}
+
+static unsigned tcp_syn_options(struct sock *sk, struct sk_buff *skb,
+                                struct tcp_out_options *opts,
+                                struct tcp_md5sig_key **md5) {
+        struct tcp_sock *tp = tcp_sk(sk);
+        unsigned size = 0;
+
 #ifdef CONFIG_TCP_MD5SIG
-        if (md5_hash) {
-                *ptr++ = htonl((TCPOPT_NOP << 24) |
-                               (TCPOPT_NOP << 16) |
-                               (TCPOPT_MD5SIG << 8) |
-                               TCPOLEN_MD5SIG);
-                *md5_hash = (__u8 *)ptr;
+        *md5 = tp->af_specific->md5_lookup(sk, sk);
+        if (*md5) {
+                opts->options |= OPTION_MD5;
+                size += TCPOLEN_MD5SIG_ALIGNED;
         }
+#else
+        *md5 = NULL;
 #endif
+
+        /* We always get an MSS option.  The option bytes which will be seen in
+         * normal data packets should timestamps be used, must be in the MSS
+         * advertised.  But we subtract them from tp->mss_cache so that
+         * calculations in tcp_sendmsg are simpler etc.  So account for this
+         * fact here if necessary.  If we don't do this correctly, as a
+         * receiver we won't recognize data packets as being full sized when we
+         * should, and thus we won't abide by the delayed ACK rules correctly.
+         * SACKs don't matter, we never delay an ACK when we have any of those
+         * going out.  */
+        opts->mss = tcp_advertise_mss(sk);
+        size += TCPOLEN_MSS_ALIGNED;
+
+        if (likely(sysctl_tcp_timestamps && *md5 == NULL)) {
+                opts->options |= OPTION_TS;
+                opts->tsval = TCP_SKB_CB(skb)->when;
+                opts->tsecr = tp->rx_opt.ts_recent;
+                size += TCPOLEN_TSTAMP_ALIGNED;
+        }
+        if (likely(sysctl_tcp_window_scaling)) {
+                opts->ws = tp->rx_opt.rcv_wscale;
+                size += TCPOLEN_WSCALE_ALIGNED;
+        }
+        if (likely(sysctl_tcp_sack)) {
+                opts->options |= OPTION_SACK_ADVERTISE;
+                if (unlikely(!OPTION_TS & opts->options))
+                        size += TCPOLEN_SACKPERM_ALIGNED;
+        }
+
+        return size;
 }
 
-/* Construct a tcp options header for a SYN or SYN_ACK packet.
- * If this is every changed make sure to change the definition of
- * MAX_SYN_SIZE to match the new maximum number of options that you
- * can generate.
- *
- * Note - that with the RFC2385 TCP option, we make room for the
- * 16 byte MD5 hash. This will be filled in later, so the pointer for the
- * location to be filled is passed back up.
- */
-static void tcp_syn_build_options(__be32 *ptr, int mss, int ts, int sack,
-                                  int offer_wscale, int wscale, __u32 tstamp,
-                                  __u32 ts_recent, __u8 **md5_hash)
-{
-        /* We always get an MSS option.
-         * The option bytes which will be seen in normal data
-         * packets should timestamps be used, must be in the MSS
-         * advertised.  But we subtract them from tp->mss_cache so
-         * that calculations in tcp_sendmsg are simpler etc.
-         * So account for this fact here if necessary.  If we
-         * don't do this correctly, as a receiver we won't
-         * recognize data packets as being full sized when we
-         * should, and thus we won't abide by the delayed ACK
-         * rules correctly.
-         * SACKs don't matter, we never delay an ACK when we
-         * have any of those going out.
-         */
-        *ptr++ = htonl((TCPOPT_MSS << 24) | (TCPOLEN_MSS << 16) | mss);
-        if (ts) {
-                if (sack)
-                        *ptr++ = htonl((TCPOPT_SACK_PERM << 24) |
-                                       (TCPOLEN_SACK_PERM << 16) |
-                                       (TCPOPT_TIMESTAMP << 8) |
-                                       TCPOLEN_TIMESTAMP);
-                else
-                        *ptr++ = htonl((TCPOPT_NOP << 24) |
-                                       (TCPOPT_NOP << 16) |
-                                       (TCPOPT_TIMESTAMP << 8) |
-                                       TCPOLEN_TIMESTAMP);
-                *ptr++ = htonl(tstamp);         /* TSVAL */
-                *ptr++ = htonl(ts_recent);      /* TSECR */
-        } else if (sack)
-                *ptr++ = htonl((TCPOPT_NOP << 24) |
-                               (TCPOPT_NOP << 16) |
-                               (TCPOPT_SACK_PERM << 8) |
-                               TCPOLEN_SACK_PERM);
-        if (offer_wscale)
-                *ptr++ = htonl((TCPOPT_NOP << 24) |
-                               (TCPOPT_WINDOW << 16) |
-                               (TCPOLEN_WINDOW << 8) |
-                               (wscale));
+static unsigned tcp_synack_options(struct sock *sk,
+                                   struct request_sock *req,
+                                   unsigned mss, struct sk_buff *skb,
+                                   struct tcp_out_options *opts,
+                                   struct tcp_md5sig_key **md5) {
+        unsigned size = 0;
+        struct inet_request_sock *ireq = inet_rsk(req);
+        char doing_ts;
+
 #ifdef CONFIG_TCP_MD5SIG
-        /*
-         * If MD5 is enabled, then we set the option, and include the size
-         * (always 18). The actual MD5 hash is added just before the
-         * packet is sent.
-         */
-        if (md5_hash) {
-                *ptr++ = htonl((TCPOPT_NOP << 24) |
-                               (TCPOPT_NOP << 16) |
-                               (TCPOPT_MD5SIG << 8) |
-                               TCPOLEN_MD5SIG);
-                *md5_hash = (__u8 *)ptr;
+        *md5 = tcp_rsk(req)->af_specific->md5_lookup(sk, req);
+        if (*md5) {
+                opts->options |= OPTION_MD5;
+                size += TCPOLEN_MD5SIG_ALIGNED;
         }
+#else
+        *md5 = NULL;
 #endif
+
+        /* we can't fit any SACK blocks in a packet with MD5 + TS
+           options. There was discussion about disabling SACK rather than TS in
+           order to fit in better with old, buggy kernels, but that was deemed
+           to be unnecessary. */
+        doing_ts = ireq->tstamp_ok && !(*md5 && ireq->sack_ok);
+
+        opts->mss = mss;
+        size += TCPOLEN_MSS_ALIGNED;
+
+        if (likely(ireq->wscale_ok)) {
+                opts->ws = ireq->rcv_wscale;
+                size += TCPOLEN_WSCALE_ALIGNED;
+        }
+        if (likely(doing_ts)) {
+                opts->options |= OPTION_TS;
+                opts->tsval = TCP_SKB_CB(skb)->when;
+                opts->tsecr = req->ts_recent;
+                size += TCPOLEN_TSTAMP_ALIGNED;
+        }
+        if (likely(ireq->sack_ok)) {
+                opts->options |= OPTION_SACK_ADVERTISE;
+                if (unlikely(!doing_ts))
+                        size += TCPOLEN_SACKPERM_ALIGNED;
+        }
+
+        return size;
+}
+
+static unsigned tcp_established_options(struct sock *sk, struct sk_buff *skb,
+                                        struct tcp_out_options *opts,
+                                        struct tcp_md5sig_key **md5) {
+        struct tcp_skb_cb *tcb = skb ? TCP_SKB_CB(skb) : NULL;
+        struct tcp_sock *tp = tcp_sk(sk);
+        unsigned size = 0;
+
+#ifdef CONFIG_TCP_MD5SIG
+        *md5 = tp->af_specific->md5_lookup(sk, sk);
+        if (unlikely(*md5)) {
+                opts->options |= OPTION_MD5;
+                size += TCPOLEN_MD5SIG_ALIGNED;
+        }
+#else
+        *md5 = NULL;
+#endif
+
+        if (likely(tp->rx_opt.tstamp_ok)) {
+                opts->options |= OPTION_TS;
+                opts->tsval = tcb ? tcb->when : 0;
+                opts->tsecr = tp->rx_opt.ts_recent;
+                size += TCPOLEN_TSTAMP_ALIGNED;
+        }
+
+        if (unlikely(tp->rx_opt.eff_sacks)) {
+                const unsigned remaining = MAX_TCP_OPTION_SPACE - size;
+                opts->num_sack_blocks =
+                        min_t(unsigned, tp->rx_opt.eff_sacks,
+                              (remaining - TCPOLEN_SACK_BASE_ALIGNED) /
+                              TCPOLEN_SACK_PERBLOCK);
+                size += TCPOLEN_SACK_BASE_ALIGNED +
+                        opts->num_sack_blocks * TCPOLEN_SACK_PERBLOCK;
+        }
+
+        return size;
 }
 
 /* This routine actually transmits TCP packets queued in by
@@ -471,13 +581,11 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
         struct inet_sock *inet;
         struct tcp_sock *tp;
         struct tcp_skb_cb *tcb;
-        int tcp_header_size;
-#ifdef CONFIG_TCP_MD5SIG
+        struct tcp_out_options opts;
+        unsigned tcp_options_size, tcp_header_size;
         struct tcp_md5sig_key *md5;
         __u8 *md5_hash_location;
-#endif
         struct tcphdr *th;
-        int sysctl_flags;
         int err;
 
         BUG_ON(!skb || !tcp_skb_pcount(skb));
@@ -500,52 +608,18 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
         inet = inet_sk(sk);
         tp = tcp_sk(sk);
         tcb = TCP_SKB_CB(skb);
-        tcp_header_size = tp->tcp_header_len;
-
-#define SYSCTL_FLAG_TSTAMPS     0x1
-#define SYSCTL_FLAG_WSCALE      0x2
-#define SYSCTL_FLAG_SACK        0x4
+        memset(&opts, 0, sizeof(opts));
 
-        sysctl_flags = 0;
-        if (unlikely(tcb->flags & TCPCB_FLAG_SYN)) {
-                tcp_header_size = sizeof(struct tcphdr) + TCPOLEN_MSS;
-                if (sysctl_tcp_timestamps) {
-                        tcp_header_size += TCPOLEN_TSTAMP_ALIGNED;
-                        sysctl_flags |= SYSCTL_FLAG_TSTAMPS;
-                }
-                if (sysctl_tcp_window_scaling) {
-                        tcp_header_size += TCPOLEN_WSCALE_ALIGNED;
-                        sysctl_flags |= SYSCTL_FLAG_WSCALE;
-                }
-                if (sysctl_tcp_sack) {
-                        sysctl_flags |= SYSCTL_FLAG_SACK;
-                        if (!(sysctl_flags & SYSCTL_FLAG_TSTAMPS))
-                                tcp_header_size += TCPOLEN_SACKPERM_ALIGNED;
-                }
-        } else if (unlikely(tp->rx_opt.eff_sacks)) {
-                /* A SACK is 2 pad bytes, a 2 byte header, plus
-                 * 2 32-bit sequence numbers for each SACK block.
-                 */
-                tcp_header_size += (TCPOLEN_SACK_BASE_ALIGNED +
-                                    (tp->rx_opt.eff_sacks *
-                                     TCPOLEN_SACK_PERBLOCK));
-        }
+        if (unlikely(tcb->flags & TCPCB_FLAG_SYN))
+                tcp_options_size = tcp_syn_options(sk, skb, &opts, &md5);
+        else
+                tcp_options_size = tcp_established_options(sk, skb, &opts,
+                                                           &md5);
+        tcp_header_size = tcp_options_size + sizeof(struct tcphdr);
 
         if (tcp_packets_in_flight(tp) == 0)
                 tcp_ca_event(sk, CA_EVENT_TX_START);
 
-#ifdef CONFIG_TCP_MD5SIG
-        /*
-         * Are we doing MD5 on this segment? If so - make
-         * room for it.
-         */
-        md5 = tp->af_specific->md5_lookup(sk, sk);
-        if (md5) {
-                tcp_header_size += TCPOLEN_MD5SIG_ALIGNED;
-                sk->sk_route_caps &= ~NETIF_F_GSO_MASK;
-        }
-#endif
-
         skb_push(skb, tcp_header_size);
         skb_reset_transport_header(skb);
         skb_set_owner_w(skb, sk);
@@ -576,33 +650,14 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
                 th->urg                 = 1;
         }
 
-        if (unlikely(tcb->flags & TCPCB_FLAG_SYN)) {
-                tcp_syn_build_options((__be32 *)(th + 1),
-                                      tcp_advertise_mss(sk),
-                                      (sysctl_flags & SYSCTL_FLAG_TSTAMPS),
-                                      (sysctl_flags & SYSCTL_FLAG_SACK),
-                                      (sysctl_flags & SYSCTL_FLAG_WSCALE),
-                                      tp->rx_opt.rcv_wscale,
-                                      tcb->when,
-                                      tp->rx_opt.ts_recent,
-
-#ifdef CONFIG_TCP_MD5SIG
-                                      md5 ? &md5_hash_location :
-#endif
-                                      NULL);
-        } else {
-                tcp_build_and_update_options((__be32 *)(th + 1),
-                                             tp, tcb->when,
-#ifdef CONFIG_TCP_MD5SIG
-                                             md5 ? &md5_hash_location :
-#endif
-                                             NULL);
+        tcp_options_write((__be32 *)(th + 1), tp, &opts, &md5_hash_location);
+        if (likely((tcb->flags & TCPCB_FLAG_SYN) == 0))
                 TCP_ECN_send(sk, skb, tcp_header_size);
-        }
 
 #ifdef CONFIG_TCP_MD5SIG
         /* Calculate the MD5 hash, as we have all we need now */
         if (md5) {
+                sk->sk_route_caps &= ~NETIF_F_GSO_MASK;
                 tp->af_specific->calc_md5_hash(md5_hash_location,
                                                md5, sk, NULL, skb);
         }
@@ -626,10 +681,6 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
         tcp_enter_cwr(sk, 1);
 
         return net_xmit_eval(err);
-
-#undef SYSCTL_FLAG_TSTAMPS
-#undef SYSCTL_FLAG_WSCALE
-#undef SYSCTL_FLAG_SACK
 }
 
 /* This routine just queue's the buffer
@@ -970,6 +1021,9 @@ unsigned int tcp_current_mss(struct sock *sk, int large_allowed)
         u32 mss_now;
         u16 xmit_size_goal;
         int doing_tso = 0;
+        unsigned header_len;
+        struct tcp_out_options opts;
+        struct tcp_md5sig_key *md5;
 
         mss_now = tp->mss_cache;
 
@@ -982,14 +1036,16 @@ unsigned int tcp_current_mss(struct sock *sk, int large_allowed)
                         mss_now = tcp_sync_mss(sk, mtu);
         }
 
-        if (tp->rx_opt.eff_sacks)
-                mss_now -= (TCPOLEN_SACK_BASE_ALIGNED +
-                            (tp->rx_opt.eff_sacks * TCPOLEN_SACK_PERBLOCK));
-
-#ifdef CONFIG_TCP_MD5SIG
-        if (tp->af_specific->md5_lookup(sk, sk))
-                mss_now -= TCPOLEN_MD5SIG_ALIGNED;
-#endif
+        header_len = tcp_established_options(sk, NULL, &opts, &md5) +
+                     sizeof(struct tcphdr);
+        /* The mss_cache is sized based on tp->tcp_header_len, which assumes
+         * some common options. If this is an odd packet (because we have SACK
+         * blocks etc) then our calculated header_len will be different, and
+         * we have to adjust mss_now correspondingly */
+        if (header_len != tp->tcp_header_len) {
+                int delta = (int) header_len - tp->tcp_header_len;
+                mss_now -= delta;
+        }
 
         xmit_size_goal = mss_now;
 
@@ -2179,11 +2235,10 @@ struct sk_buff *tcp_make_synack(struct sock *sk, struct dst_entry *dst,
         struct tcp_sock *tp = tcp_sk(sk);
         struct tcphdr *th;
         int tcp_header_size;
+        struct tcp_out_options opts;
         struct sk_buff *skb;
-#ifdef CONFIG_TCP_MD5SIG
         struct tcp_md5sig_key *md5;
         __u8 *md5_hash_location;
-#endif
 
         skb = sock_wmalloc(sk, MAX_TCP_HEADER + 15, 1, GFP_ATOMIC);
         if (skb == NULL)
@@ -2194,18 +2249,27 @@ struct sk_buff *tcp_make_synack(struct sock *sk, struct dst_entry *dst,
 
         skb->dst = dst_clone(dst);
 
-        tcp_header_size = (sizeof(struct tcphdr) + TCPOLEN_MSS +
-                           (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0) +
-                           (ireq->wscale_ok ? TCPOLEN_WSCALE_ALIGNED : 0) +
-                           /* SACK_PERM is in the place of NOP NOP of TS */
-                           ((ireq->sack_ok && !ireq->tstamp_ok) ? TCPOLEN_SACKPERM_ALIGNED : 0));
+        if (req->rcv_wnd == 0) { /* ignored for retransmitted syns */
+                __u8 rcv_wscale;
+                /* Set this up on the first call only */
+                req->window_clamp = tp->window_clamp ? : dst_metric(dst, RTAX_WINDOW);
+                /* tcp_full_space because it is guaranteed to be the first packet */
+                tcp_select_initial_window(tcp_full_space(sk),
+                        dst_metric(dst, RTAX_ADVMSS) - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0),
+                        &req->rcv_wnd,
+                        &req->window_clamp,
+                        ireq->wscale_ok,
+                        &rcv_wscale);
+                ireq->rcv_wscale = rcv_wscale;
+        }
+
+        memset(&opts, 0, sizeof(opts));
+        TCP_SKB_CB(skb)->when = tcp_time_stamp;
+        tcp_header_size = tcp_synack_options(sk, req,
+                                             dst_metric(dst, RTAX_ADVMSS),
+                                             skb, &opts, &md5) +
+                          sizeof(struct tcphdr);
 
-#ifdef CONFIG_TCP_MD5SIG
-        /* Are we doing MD5 on this segment? If so - make room for it */
-        md5 = tcp_rsk(req)->af_specific->md5_lookup(sk, req);
-        if (md5)
-                tcp_header_size += TCPOLEN_MD5SIG_ALIGNED;
-#endif
         skb_push(skb, tcp_header_size);
         skb_reset_transport_header(skb);
 
@@ -2223,19 +2287,6 @@ struct sk_buff *tcp_make_synack(struct sock *sk, struct dst_entry *dst,
                              TCPCB_FLAG_SYN | TCPCB_FLAG_ACK);
         th->seq = htonl(TCP_SKB_CB(skb)->seq);
         th->ack_seq = htonl(tcp_rsk(req)->rcv_isn + 1);
-        if (req->rcv_wnd == 0) { /* ignored for retransmitted syns */
-                __u8 rcv_wscale;
-                /* Set this up on the first call only */
-                req->window_clamp = tp->window_clamp ? : dst_metric(dst, RTAX_WINDOW);
-                /* tcp_full_space because it is guaranteed to be the first packet */
-                tcp_select_initial_window(tcp_full_space(sk),
-                        dst_metric(dst, RTAX_ADVMSS) - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0),
-                        &req->rcv_wnd,
-                        &req->window_clamp,
-                        ireq->wscale_ok,
-                        &rcv_wscale);
-                ireq->rcv_wscale = rcv_wscale;
-        }
 
         /* RFC1323: The window in SYN & SYN/ACK segments is never scaled. */
         th->window = htons(min(req->rcv_wnd, 65535U));
@@ -2244,18 +2295,7 @@ struct sk_buff *tcp_make_synack(struct sock *sk, struct dst_entry *dst,
                 TCP_SKB_CB(skb)->when = cookie_init_timestamp(req);
         else
 #endif
-        TCP_SKB_CB(skb)->when = tcp_time_stamp;
-        tcp_syn_build_options((__be32 *)(th + 1), dst_metric(dst, RTAX_ADVMSS), ireq->tstamp_ok,
-                              ireq->sack_ok, ireq->wscale_ok, ireq->rcv_wscale,
-                              TCP_SKB_CB(skb)->when,
-                              req->ts_recent,
-                              (
-#ifdef CONFIG_TCP_MD5SIG
-                               md5 ? &md5_hash_location :
-#endif
-                               NULL)
-                              );
-
+        tcp_options_write((__be32 *)(th + 1), tp, &opts, &md5_hash_location);
         th->doff = (tcp_header_size >> 2);
         TCP_INC_STATS(sock_net(sk), TCP_MIB_OUTSEGS);