【Linux4.1.12源码分析】协议栈报文接收之传输层处理分析（UDP）

UDP报文的处理入口是udp_rcv函数，该函数是在ip_local_deliver_finish函数中被调用的。

1、udp_rcv函数

int udp_rcv(struct sk_buff *skb)
{
	return __udp4_lib_rcv(skb, &udp_table, IPPROTO_UDP);
}

2、__udp4_lib_rcv函数

int __udp4_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,
		   int proto)
{
	struct sock *sk;
	struct udphdr *uh;
	unsigned short ulen;
	struct rtable *rt = skb_rtable(skb);	//得到路由表项信息
	__be32 saddr, daddr;
	struct net *net = dev_net(skb->dev);

	/*
	 *  Validate the packet.
	 */
	if (!pskb_may_pull(skb, sizeof(struct udphdr)))		//检测报文长度
		goto drop;		/* No space for header. */

	uh   = udp_hdr(skb);
	ulen = ntohs(uh->len);
	saddr = ip_hdr(skb)->saddr;
	daddr = ip_hdr(skb)->daddr;

	if (ulen > skb->len)		//skb报文长度必须UDP头设定的长度，skb可能存在pad
		goto short_packet;

	if (proto == IPPROTO_UDP) {
		/* UDP validates ulen. */
		if (ulen < sizeof(*uh) || pskb_trim_rcsum(skb, ulen))	//裁剪skb报文
			goto short_packet;
		uh = udp_hdr(skb);
	}

	if (udp4_csum_init(skb, uh, proto))	//csum值校验，如果非零则表示csum错误
		goto csum_error;

	sk = skb_steal_sock(skb);	//IP层调用UDP的early_demux函数设置的sk
	if (sk) {
		struct dst_entry *dst = skb_dst(skb);
		int ret;

		if (unlikely(sk->sk_rx_dst != dst))
			udp_sk_rx_dst_set(sk, dst);

		ret = udp_queue_rcv_skb(sk, skb);	//sock处理skb报文
		sock_put(sk);
		/* a return value > 0 means to resubmit the input, but
		 * it wants the return to be -protocol, or 0
		 */
		if (ret > 0)
			return -ret;
		return 0;
	}

	if (rt->rt_flags & (RTCF_BROADCAST|RTCF_MULTICAST))
		return __udp4_lib_mcast_deliver(net, skb, uh,		//组播报文上送
						saddr, daddr, udptable, proto);

	sk = __udp4_lib_lookup_skb(skb, uh->source, uh->dest, udptable);	//全量检索sock
	if (sk) {
		int ret;

		if (inet_get_convert_csum(sk) && uh->check && !IS_UDPLITE(sk))
			skb_checksum_try_convert(skb, IPPROTO_UDP, uh->check,	//如果skb->ip_summed == CHECKSUM_NONE，重置csum
						 inet_compute_pseudo);

		ret = udp_queue_rcv_skb(sk, skb);	//sock处理skb报文
		sock_put(sk);

		/* a return value > 0 means to resubmit the input, but
		 * it wants the return to be -protocol, or 0
		 */
		if (ret > 0)
			return -ret;
		return 0;
	}

	if (!xfrm4_policy_check(NULL, XFRM_POLICY_IN, skb))	//ipset安全检测
		goto drop;
	nf_reset(skb);

	/* No socket. Drop packet silently, if checksum is wrong */
	if (udp_lib_checksum_complete(skb))	//没有socket，如果checksum出错则直接丢弃
		goto csum_error;

	UDP_INC_STATS_BH(net, UDP_MIB_NOPORTS, proto == IPPROTO_UDPLITE);
	icmp_send(skb, ICMP_DEST_UNREACH, ICMP_PORT_UNREACH, 0);	//发送icmp，通知发送方目的不可达

	/*
	 * Hmm.  We got an UDP packet to a port to which we
	 * don't wanna listen.  Ignore it.
	 */
	kfree_skb(skb);
	return 0;

short_packet:
	net_dbg_ratelimited("UDP%s: short packet: From %pI4:%u %d/%d to %pI4:%u\n",
			    proto == IPPROTO_UDPLITE ? "Lite" : "",
			    &saddr, ntohs(uh->source),
			    ulen, skb->len,
			    &daddr, ntohs(uh->dest));
	goto drop;

csum_error:
	/*
	 * RFC1122: OK.  Discards the bad packet silently (as far as
	 * the network is concerned, anyway) as per 4.1.3.4 (MUST).
	 */
	net_dbg_ratelimited("UDP%s: bad checksum. From %pI4:%u to %pI4:%u ulen %d\n",
			    proto == IPPROTO_UDPLITE ? "Lite" : "",
			    &saddr, ntohs(uh->source), &daddr, ntohs(uh->dest),
			    ulen);
	UDP_INC_STATS_BH(net, UDP_MIB_CSUMERRORS, proto == IPPROTO_UDPLITE);	//UDP csum错误统计
drop:
	UDP_INC_STATS_BH(net, UDP_MIB_INERRORS, proto == IPPROTO_UDPLITE);	//UDP丢包统计
	kfree_skb(skb);
	return 0;
}

3、udp_queue_rcv_skb函数

int udp_queue_rcv_skb(struct sock *sk, struct sk_buff *skb)
{
	struct udp_sock *up = udp_sk(sk);
	int rc;
	int is_udplite = IS_UDPLITE(sk);

	/*
	 *	Charge it to the socket, dropping if the queue is full.
	 */
	if (!xfrm4_policy_check(sk, XFRM_POLICY_IN, skb))	//ipset检测
		goto drop;
	nf_reset(skb);

	if (static_key_false(&udp_encap_needed) && up->encap_type) {
		int (*encap_rcv)(struct sock *sk, struct sk_buff *skb);

		/*
		 * This is an encapsulation socket so pass the skb to
		 * the socket's udp_encap_rcv() hook. Otherwise, just
		 * fall through and pass this up the UDP socket.
		 * up->encap_rcv() returns the following value:
		 * =0 if skb was successfully passed to the encap
		 *    handler or was discarded by it.
		 * >0 if skb should be passed on to UDP.
		 * <0 if skb should be resubmitted as proto -N
		 */

		/* if we're overly short, let UDP handle it */
		encap_rcv = ACCESS_ONCE(up->encap_rcv);
		if (skb->len > sizeof(struct udphdr) && encap_rcv) {
			int ret;

			/* Verify checksum before giving to encap */
			if (udp_lib_checksum_complete(skb))
				goto csum_error;

			ret = encap_rcv(sk, skb);	//报文处理，例如vxlan报文就是通过此方式实现
			if (ret <= 0) {
				UDP_INC_STATS_BH(sock_net(sk),
						 UDP_MIB_INDATAGRAMS,
						 is_udplite);
				return -ret;
			}
		}

		/* FALLTHROUGH -- it's a UDP Packet */
	}

	/*
	 * 	UDP-Lite specific tests, ignored on UDP sockets
	 */
	if ((is_udplite & UDPLITE_RECV_CC)  &&  UDP_SKB_CB(skb)->partial_cov) {

		/*
		 * MIB statistics other than incrementing the error count are
		 * disabled for the following two types of errors: these depend
		 * on the application settings, not on the functioning of the
		 * protocol stack as such.
		 *
		 * RFC 3828 here recommends (sec 3.3): "There should also be a
		 * way ... to ... at least let the receiving application block
		 * delivery of packets with coverage values less than a value
		 * provided by the application."
		 */
		if (up->pcrlen == 0) {          /* full coverage was set  */
			net_dbg_ratelimited("UDPLite: partial coverage %d while full coverage %d requested\n",
					    UDP_SKB_CB(skb)->cscov, skb->len);
			goto drop;
		}
		/* The next case involves violating the min. coverage requested
		 * by the receiver. This is subtle: if receiver wants x and x is
		 * greater than the buffersize/MTU then receiver will complain
		 * that it wants x while sender emits packets of smaller size y.
		 * Therefore the above ...()->partial_cov statement is essential.
		 */
		if (UDP_SKB_CB(skb)->cscov  <  up->pcrlen) {
			net_dbg_ratelimited("UDPLite: coverage %d too small, need min %d\n",
					    UDP_SKB_CB(skb)->cscov, up->pcrlen);
			goto drop;
		}
	}

	if (rcu_access_pointer(sk->sk_filter) &&
	    udp_lib_checksum_complete(skb))		//报文checksum检测
		goto csum_error;

	if (sk_rcvqueues_full(sk, sk->sk_rcvbuf)) {
		UDP_INC_STATS_BH(sock_net(sk), UDP_MIB_RCVBUFERRORS,
				 is_udplite);
		goto drop;
	}

	rc = 0;

	ipv4_pktinfo_prepare(sk, skb);
	bh_lock_sock(sk);
	if (!sock_owned_by_user(sk))
		rc = __udp_queue_rcv_skb(sk, skb);	//报文保存到sk的接收队列，并通知用户进程
	else if (sk_add_backlog(sk, skb, sk->sk_rcvbuf)) {
		bh_unlock_sock(sk);
		goto drop;
	}
	bh_unlock_sock(sk);

	return rc;

csum_error:
	UDP_INC_STATS_BH(sock_net(sk), UDP_MIB_CSUMERRORS, is_udplite);
drop:
	UDP_INC_STATS_BH(sock_net(sk), UDP_MIB_INERRORS, is_udplite);
	atomic_inc(&sk->sk_drops);
	kfree_skb(skb);
	return -1;
}

UDP有两条处理路径：1）放到sk的接收队列，通知等待进程；2）调用udp_sock的encap_rcv函数，类似vxlan的实现，可以直接在内核中进行处理。UDP处理中，checksum校验未展开分析，待后续单独分析。