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)2、__udp4_lib_rcv函数
{
return __udp4_lib_rcv(skb, &udp_table, IPPROTO_UDP);
}
int __udp4_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,3、udp_queue_rcv_skb函数
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;
}
int udp_queue_rcv_skb(struct sock *sk, struct sk_buff *skb)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。
{
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;
}