[PATCH] tcp: make GRUB honor TCP MSS option

[Ignat Korchagin]

Currently, GRUB neither parses nor announces TCP MSS option. According
to RFC 879 and its successors if the MSS option is not present the
default TCP segment size is 536.
That is the exact amount received for every TCP segment for an HTTP
download (tested with Debian Linux), that is the peer TCP stack sends
536 bytes at a time. Announcing larger TCP segment size makes peer TCP
stack sending larger segments which improves HTTP download speed.
Also, GRUB ignores TCP MSS option received from its peer and
calculating fragment length based only on network MTU. This could
crash potential TCP peers with small memory capacity receiving large
TCP segments when they do not expect them.

diff --git a/grub-core/net/tcp.c b/grub-core/net/tcp.c
index 1d90f1e..65f9637 100644
--- a/grub-core/net/tcp.c
+++ b/grub-core/net/tcp.c
@@ -66,6 +66,7 @@
   grub_uint32_t their_start_seq;
   grub_uint32_t their_cur_seq;
   grub_uint16_t my_window;
+  grub_uint16_t their_mss;
   struct unacked *unack_first;
   struct unacked *unack_last;
   grub_err_t (*recv_hook) (grub_net_tcp_socket_t sock, struct
grub_net_buff *nb,
@@ -484,6 +485,37 @@
   grub_priority_queue_destroy (sock->pq);
 }

+static grub_uint16_t
+our_mss (grub_net_tcp_socket_t sock)
+{
+  return (grub_uint16_t)(sock->inf->card->mtu - GRUB_NET_MAX_LINK_HEADER_SIZE
+       - GRUB_NET_OUR_MAX_IP_HEADER_SIZE - GRUB_NET_TCP_HEADER_SIZE - 40);
+}
+
+static grub_uint16_t
+get_mss (struct tcphdr *tcph)
+{
+  grub_uint8_t *ptr = (grub_uint8_t *)(tcph + 1);
+
+  while (ptr < (grub_uint8_t *)tcph + ((grub_be_to_cpu16
(tcph->flags) >> 12) * 4))
+    {
+      if (0 == *ptr || 1 == *ptr)
+        {
+          ptr++;
+          continue;
+        }
+
+      if (2 == *ptr || 4 == *(ptr + 1))
+        {
+          return grub_be_to_cpu16 (*((grub_uint16_t *)(ptr + 2)));
+        }
+
+      ptr += *(ptr + 1);
+    }
+
+  return 536;
+}
+
 grub_err_t
 grub_net_tcp_accept (grub_net_tcp_socket_t sock,
      grub_err_t (*recv_hook) (grub_net_tcp_socket_t sock,
@@ -497,13 +529,14 @@
 {
   struct grub_net_buff *nb_ack;
   struct tcphdr *tcph;
+  grub_uint32_t *mss;
   grub_err_t err;

   sock->recv_hook = recv_hook;
   sock->error_hook = error_hook;
   sock->fin_hook = fin_hook;
   sock->hook_data = hook_data;
-  nb_ack = grub_netbuff_alloc (sizeof (*tcph)
+  nb_ack = grub_netbuff_alloc (sizeof (*tcph) + 4
        + GRUB_NET_OUR_MAX_IP_HEADER_SIZE
        + GRUB_NET_MAX_LINK_HEADER_SIZE);
   if (!nb_ack)
@@ -516,17 +549,19 @@
       return err;
     }

-  err = grub_netbuff_put (nb_ack, sizeof (*tcph));
+  err = grub_netbuff_put (nb_ack, sizeof (*tcph) + 4);
   if (err)
     {
       grub_netbuff_free (nb_ack);
       return err;
     }
   tcph = (void *) nb_ack->data;
+  mss = (grub_uint32_t *)(tcph + 1);
   tcph->ack = grub_cpu_to_be32 (sock->their_cur_seq);
-  tcph->flags = grub_cpu_to_be16_compile_time ((5 << 12) | TCP_SYN | TCP_ACK);
+  tcph->flags = grub_cpu_to_be16_compile_time ((6 << 12) | TCP_SYN | TCP_ACK);
   tcph->window = grub_cpu_to_be16 (sock->my_window);
   tcph->urgent = 0;
+  *mss = grub_cpu_to_be32(0x02040000 | our_mss(sock));
   sock->established = 1;
   tcp_socket_register (sock);
   err = tcp_send (nb_ack, sock);
@@ -556,6 +591,7 @@
   static grub_uint16_t in_port = 21550;
   struct grub_net_buff *nb;
   struct tcphdr *tcph;
+  grub_uint32_t *mss;
   int i;
   grub_uint8_t *nbd;
   grub_net_link_level_address_t ll_target_addr;
@@ -593,7 +629,7 @@
   socket->fin_hook = fin_hook;
   socket->hook_data = hook_data;

-  nb = grub_netbuff_alloc (sizeof (*tcph) + 128);
+  nb = grub_netbuff_alloc (sizeof (*tcph) + 4 + 128);
   if (!nb)
     return NULL;
   err = grub_netbuff_reserve (nb, 128);
@@ -603,7 +639,7 @@
       return NULL;
     }

-  err = grub_netbuff_put (nb, sizeof (*tcph));
+  err = grub_netbuff_put (nb, sizeof (*tcph) + 4);
   if (err)
     {
       grub_netbuff_free (nb);
@@ -617,16 +653,18 @@
     }

   tcph = (void *) nb->data;
+  mss = (grub_uint32_t *)(tcph + 1);
   socket->my_start_seq = grub_get_time_ms ();
   socket->my_cur_seq = socket->my_start_seq + 1;
   socket->my_window = 8192;
   tcph->seqnr = grub_cpu_to_be32 (socket->my_start_seq);
   tcph->ack = grub_cpu_to_be32_compile_time (0);
-  tcph->flags = grub_cpu_to_be16_compile_time ((5 << 12) | TCP_SYN);
+  tcph->flags = grub_cpu_to_be16_compile_time ((6 << 12) | TCP_SYN);
   tcph->window = grub_cpu_to_be16 (socket->my_window);
   tcph->urgent = 0;
   tcph->src = grub_cpu_to_be16 (socket->in_port);
   tcph->dst = grub_cpu_to_be16 (socket->out_port);
+  *mss = grub_cpu_to_be32(0x02040000 | our_mss(socket));
   tcph->checksum = 0;
   tcph->checksum = grub_net_ip_transport_checksum (nb, GRUB_NET_IP_TCP,
    &socket->inf->address,
@@ -688,6 +726,9 @@
        - sizeof (*tcph));
   else
     fraglen = 1280 - GRUB_NET_OUR_IPV6_HEADER_SIZE;
+
+  if (fraglen > socket->their_mss)
+    fraglen = socket->their_mss;

   while (nb->tail - nb->data > fraglen)
     {
@@ -804,6 +845,7 @@
       {
  sock->their_start_seq = grub_be_to_cpu32 (tcph->seqnr);
  sock->their_cur_seq = sock->their_start_seq + 1;
+ sock->their_mss = get_mss(tcph);
  sock->established = 1;
       }

@@ -959,6 +1001,7 @@
  sock->their_cur_seq = sock->their_start_seq + 1;
  sock->my_cur_seq = sock->my_start_seq = grub_get_time_ms ();
  sock->my_window = 8192;
+ sock->their_mss = get_mss(tcph);

  sock->pq = grub_priority_queue_new (sizeof (struct grub_net_buff *),
     cmp);