Re: [PATCH v8 2/4] udp: create and send ICMPv4 to local peer when applicable

...
  +/**
 + * udp_send_conn_fail_icmp4() - Construct and send ICMPv4 to local peer
 + * @c:		Execution context
 + * @ee:	Extended error descriptor
 + * @ref:	epoll reference
 + * @in:	First bytes (max 8) of original UDP message body
 + * @dlen:	Length of the read part of original UDP message body
 + */
 +static void udp_send_conn_fail_icmp4(const struct ctx *c,
 +				     const struct sock_extended_err *ee,
 +				     const struct flowside *toside,
 +				     void *in, size_t dlen)
 +{
 +	struct in_addr oaddr = toside->oaddr.v4mapped.a4;
 +	struct in_addr eaddr = toside->eaddr.v4mapped.a4;
 +	in_port_t eport = toside->eport;
 +	in_port_t oport = toside->oport;
 +	struct {
 +		struct icmphdr icmp4h;
 +		struct iphdr ip4h;
 +		struct udphdr uh;
 +		char data[ICMP4_MAX_DLEN];
 +	} __attribute__((packed, aligned(__alignof__(max_align_t)))) msg;
 +	size_t msglen = sizeof(msg) - sizeof(msg.data) + dlen;
 +
 +	ASSERT(dlen <= ICMP4_MAX_DLEN);
 +	memset(&msg, 0, sizeof(msg));
 +	msg.icmp4h.type = ee->ee_type;
 +	msg.icmp4h.code = ee->ee_code;
 +	if (ee->ee_type == ICMP_DEST_UNREACH && ee->ee_code == ICMP_FRAG_NEEDED)
 +		msg.icmp4h.un.frag.mtu = htons((uint16_t) ee->ee_info);
 +
 +	/* Reconstruct the original headers as returned in the ICMP message */
 +	tap_push_ip4h(&msg.ip4h, eaddr, oaddr, dlen, IPPROTO_UDP);
 +	tap_push_uh4(&msg.uh, eaddr, eport, oaddr, oport, in, dlen);
 +	memcpy(&msg.data, in, dlen);
 +
 +	tap_icmp4_send(c, oaddr, eaddr, &msg, msglen);
 +} 
The destination IP of the origin packet might not be the source IP of an ICMP error
message, if a router sent this ICMP error message.

Increase local MTU and try this program:
```
#packet-too-big.py
#ip link set eth0 mtu 1520
from socket import *
import time
IP_RECVERR=0xb
IP_MTU_DISCOVER=0xa
IP_PMTUDISC_PROBE=0x3
with socket(AF_INET,SOCK_DGRAM,IPPROTO_UDP) as sock:
    sock.setsockopt(IPPROTO_IP,IP_RECVERR,1)
    sock.setsockopt(IPPROTO_IP,IP_MTU_DISCOVER,IP_PMTUDISC_PROBE)
    bigPacket=bytes(1480)
    sock.sendto(bigPacket,("151.101.1.6",443))
    time.sleep(0.1)
    print(sock.recvmsg(1480,1024,MSG_ERRQUEUE))

```

...
   	if (ref.type == EPOLL_TYPE_UDP_REPLY) {
  		flow_sidx_t sidx = flow_sidx_opposite(ref.flowside);
  		const struct flowside *toside = flowside_at_sidx(sidx);
 -
 -		udp_send_conn_fail_icmp4(c, ee, toside, data, rc);
 +		size_t dlen = rc;
 +
 +		if (hdr->cmsg_level == IPPROTO_IP) {
 +			dlen = MIN(dlen, ICMP4_MAX_DLEN);
 +			udp_send_conn_fail_icmp4(c, ee, toside, data, dlen);
 +		} else if (hdr->cmsg_level == IPPROTO_IPV6) {
 +			udp_send_conn_fail_icmp6(c, ee, toside, data,
 +						 dlen, sidx.flowi);
 +		}
  	} else {
  		trace("Ignoring received IP_RECVERR cmsg on listener socket");
  	} 
If the socket is dual-stack, cmsg_level may not match cmsg_data.
```
#dual-stack-test.py
from socket import *
import time
IP_RECVERR=0xb
with socket(AF_INET6,SOCK_DGRAM,IPPROTO_UDP) as sock:
    sock.setsockopt(IPPROTO_IP,IP_RECVERR,1)
    sock.setsockopt(IPPROTO_IP,IP_TTL,1)
    packet=bytes(8)
    sock.sendto(packet,("::ffff:151.101.1.6",443))
    time.sleep(0.1)
    print(sock.recvmsg(1472,1024,MSG_ERRQUEUE))

```