Add "connection reset" TCP handler for incoming SYN frames

etc/make/conf.mk

@@ -103,6 +103,7 @@ OBJS+=$(O)/sys/sys_net.o \
 	  $(O)/net/icmp.o \
 	  $(O)/net/udp.o \
 	  $(O)/net/raw.o \
+	  $(O)/net/tcp.o \
 	  $(O)/net/if.o \
 	  $(O)/net/util.o
 DIRS+=$(O)/net

include/net/inet.h

@@ -2,6 +2,7 @@
 #include "sys/types.h"
 
 #define INET_P_ICMP         1
+#define INET_P_TCP          6
 #define INET_P_UDP          17
 
 struct inet_frame {

include/net/tcp.h

@@ -0,0 +1,32 @@
+#pragma once
+#include "sys/types.h"
+
+struct tcp_frame {
+    uint16_t src_port;
+    uint16_t dst_port;
+    uint32_t seq;
+    uint32_t ack_seq;
+    uint8_t ns:1;
+    uint8_t zero:3;
+    uint8_t doff:4;
+    uint8_t fin:1;
+    uint8_t syn:1;
+    uint8_t rst:1;
+    uint8_t psh:1;
+    uint8_t ack:1;
+    uint8_t urg:1;
+    uint8_t ece:1;
+    uint8_t cwr:1;
+    uint16_t window;
+    uint16_t checksum;
+    uint16_t urg_ptr;
+} __attribute__((packed));
+
+struct eth_frame;
+struct inet_frame;
+struct packet;
+struct vfs_ioctx;
+struct ofile;
+struct sockaddr;
+
+void tcp_handle_frame(struct packet *p, struct eth_frame *eth, struct inet_frame *ip, void *data, size_t len);

net/inet.c

@@ -10,6 +10,7 @@
 #include "net/arp.h"
 #include "net/eth.h"
 #include "net/udp.h"
+#include "net/tcp.h"
 #include "net/if.h"
 
 uint16_t inet_checksum(const void *data, size_t len) {
@@ -106,6 +107,9 @@ void inet_handle_frame(struct packet *p, struct eth_frame *eth, void *data, size
     case INET_P_UDP:
         udp_handle_frame(p, eth, ip, data, len);
         break;
+    case INET_P_TCP:
+        tcp_handle_frame(p, eth, ip, data, len);
+        break;
     default:
         kwarn("%s: dropping unknown protocol %02x\n", p->dev->name, ip->proto);
         break;

net/tcp.c

@@ -0,0 +1,78 @@
+#include "net/packet.h"
+#include "sys/string.h"
+#include "sys/debug.h"
+#include "net/inet.h"
+#include "net/util.h"
+#include "net/eth.h"
+#include "net/tcp.h"
+#include "net/if.h"
+
+static uint16_t tcp_checksum(uint32_t daddr, uint32_t saddr, uint16_t tcp_length, void *tcp_data) {
+    uint32_t sum = 0;
+    uint32_t tmp = saddr;
+    sum += tmp & 0xFFFF;
+    sum += tmp >> 16;
+    tmp = daddr;
+    sum += tmp & 0xFFFF;
+    sum += tmp >> 16;
+    sum += 6;
+    sum += tcp_length;
+
+    for (int i = 0; i < tcp_length / 2; ++i) {
+        sum += ntohs(((uint16_t *) tcp_data)[i]);
+    }
+    sum = (sum >> 16) + (sum & 0xFFFF);
+    return ~(sum & 0xFFFF) & 0xFFFF;
+}
+
+// Reset a connection attempt given its SYN packet
+static void tcp_syn_reset(struct netdev *dev, uint32_t inaddr, struct tcp_frame *syn_frame) {
+    if (!(dev->flags & IF_F_HASIP)) {
+        return;
+    }
+
+    char packet[sizeof(struct eth_frame) + sizeof(struct inet_frame) + sizeof(struct tcp_frame)];
+    struct tcp_frame *tcp = (struct tcp_frame *) &packet[sizeof(struct eth_frame) + sizeof(struct inet_frame)];
+    memset(tcp, 0, sizeof(struct tcp_frame));
+
+    tcp->src_port = syn_frame->dst_port;
+    tcp->dst_port = syn_frame->src_port;
+    tcp->ack_seq = htonl(ntohl(syn_frame->seq) + 1);
+    tcp->doff = sizeof(struct tcp_frame) / 4;
+    tcp->rst = 1;
+    tcp->ack = 1;
+    tcp->window = syn_frame->window;
+
+    uint16_t checksum = tcp_checksum(inaddr, dev->inaddr, sizeof(struct tcp_frame), tcp);
+    tcp->checksum = htons(checksum);
+
+    debug_dump(DEBUG_DEFAULT, tcp, sizeof(struct tcp_frame));
+
+    inet_send_wrapped(dev, inaddr, INET_P_TCP, packet, sizeof(packet));
+}
+
+void tcp_handle_frame(struct packet *p, struct eth_frame *eth, struct inet_frame *ip, void *data, size_t len) {
+    if (len < sizeof(struct tcp_frame)) {
+        kwarn("%s: dropping undersized frame\n", p->dev->name);
+        return;
+    }
+
+    struct tcp_frame *tcp = data;
+    size_t frame_size = 4 * tcp->doff;
+
+    if (len < frame_size) {
+        kwarn("%s: dropping undersized frame\n", p->dev->name);
+        return;
+    }
+
+    data += frame_size;
+    len -= frame_size;
+
+    if (tcp->syn && !tcp->ack) {
+        // TODO: Check if it's an initial SYN
+        // Just reset the connection for now
+        tcp_syn_reset(p->dev, ntohl(ip->src_inaddr), tcp);
+    } else {
+        kwarn("%s: unhandled packet: syn=%d, ack=%d, psh=%d, rst=%d\n", tcp->syn, tcp->ack, tcp->psh, tcp->rst);
+    }
+}