[tcp] Allow sufficient headroom for TCP headers

TCP currently neglects to allow sufficient space for its own headers
when allocating I/O buffers.  This problem is masked by the fact that
the maximum link-layer header size (802.11) is substantially larger
than the common Ethernet link-layer header.

Fix by allowing sufficient space for any TCP headers, as well as the
network-layer and link-layer headers.

Reported-by: Scott K Logan <logans@cottsay.net>
Debugged-by: Scott K Logan <logans@cottsay.net>
Tested-by: Scott K Logan <logans@cottsay.net>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/tcp.h

  */
 #define TCP_MSL ( 2 * 60 * TICKS_PER_SEC )
 
+/**
+ * TCP maximum header length
+ *
+ */
+#define TCP_MAX_HEADER_LEN					\
+	( MAX_LL_NET_HEADER_LEN +				\
+	  sizeof ( struct tcp_header ) +			\
+	  sizeof ( struct tcp_mss_option ) +			\
+	  sizeof ( struct tcp_timestamp_padded_option ) )
+
 /**
  * Compare TCP sequence numbers
  *

src/net/tcp.c

 		start_timer ( &tcp->timer );
 
 	/* Allocate I/O buffer */
-	iobuf = alloc_iob ( len + MAX_LL_NET_HEADER_LEN );
+	iobuf = alloc_iob ( len + TCP_MAX_HEADER_LEN );
 	if ( ! iobuf ) {
 		DBGC ( tcp, "TCP %p could not allocate iobuf for %08x..%08x "
 		       "%08x\n", tcp, tcp->snd_seq, ( tcp->snd_seq + seq_len ),
 		       tcp->rcv_ack );
 		return -ENOMEM;
 	}
-	iob_reserve ( iobuf, MAX_LL_NET_HEADER_LEN );
+	iob_reserve ( iobuf, TCP_MAX_HEADER_LEN );
 
 	/* Fill data payload from transmit queue */
 	tcp_process_tx_queue ( tcp, len, iobuf, 0 );
 	int rc;
 
 	/* Allocate space for dataless TX buffer */
-	iobuf = alloc_iob ( MAX_LL_NET_HEADER_LEN );
+	iobuf = alloc_iob ( TCP_MAX_HEADER_LEN );
 	if ( ! iobuf ) {
 		DBGC ( tcp, "TCP %p could not allocate iobuf for RST "
 		       "%08x..%08x %08x\n", tcp, ntohl ( in_tcphdr->ack ),
 		       ntohl ( in_tcphdr->ack ), ntohl ( in_tcphdr->seq ) );
 		return -ENOMEM;
 	}
-	iob_reserve ( iobuf, MAX_LL_NET_HEADER_LEN );
+	iob_reserve ( iobuf, TCP_MAX_HEADER_LEN );
 
 	/* Construct RST response */
 	tcphdr = iob_push ( iobuf, sizeof ( *tcphdr ) );