On Wed, Mar 26, 2025 at 11:59:02AM -0400, Jon Maloy wrote:
...
  While developing traceroute forwarding tap-to-sock we
found that
 struct msghdr.msg_name for the ICMPs in the opposite direction always
 contains the destination address of the original UDP message, and not,
 as one might expect, the one of the host which created the error message.
 
 Study of the kernel code reveals that this address instead is appended
 as extra data after the received struct sock_extended_err area.
 
 We now change the ICMP receive code accordingly.
 
 Fixes: 55431f0077b6 ("udp: create and send ICMPv4 to local peer when
applicable")
 Fixes: 68b04182e07d ("udp: create and send ICMPv6 to local peer when
applicable")
 
 Signed-off-by: Jon Maloy <jmaloy(a)redhat.com> 
Reviewed-by: David Gibson <david(a)gibson.dropbear.id.au>

This will conflict boringly with my rename of the "send_conn_fail"
functions of course.  I think this is more important, so I suggest
this go first, and I'll rebase my patch.

...
  ---
 v2: Removed stray comment and unecessary initializations, as per
     feedback from David Gibson
 ---
  udp.c | 22 ++++++++++++----------
  1 file changed, 12 insertions(+), 10 deletions(-)
 
 diff --git a/udp.c b/udp.c
 index 80520cb..9d16529 100644
 --- a/udp.c
 +++ b/udp.c
 @@ -510,10 +510,13 @@ static void udp_send_conn_fail_icmp6(const struct ctx *c,
   */
  static int udp_sock_recverr(const struct ctx *c, union epoll_ref ref)
  {
 -	const struct sock_extended_err *ee;
 +	struct errhdr {
 +		struct sock_extended_err ee;
 +		union sockaddr_inany saddr;
 +	};
 +	const struct errhdr *eh;
  	const struct cmsghdr *hdr;
 -	union sockaddr_inany saddr;
 -	char buf[CMSG_SPACE(sizeof(*ee))];
 +	char buf[CMSG_SPACE(sizeof(struct errhdr))];
  	char data[ICMP6_MAX_DLEN];
  	int s = ref.fd;
  	struct iovec iov = {
 @@ -521,8 +524,6 @@ static int udp_sock_recverr(const struct ctx *c, union epoll_ref
ref)
  		.iov_len = sizeof(data)
  	};
  	struct msghdr mh = {
 -		.msg_name = &saddr,
 -		.msg_namelen = sizeof(saddr),
  		.msg_iov = &iov,
  		.msg_iovlen = 1,
  		.msg_control = buf,
 @@ -553,7 +554,7 @@ static int udp_sock_recverr(const struct ctx *c, union epoll_ref
ref)
  		return -1;
  	}
  
 -	ee = (const struct sock_extended_err *)CMSG_DATA(hdr);
 +	eh = (const struct errhdr *)CMSG_DATA(hdr);
  	if (ref.type == EPOLL_TYPE_UDP_REPLY) {
  		flow_sidx_t sidx = flow_sidx_opposite(ref.flowside);
  		const struct flowside *toside = flowside_at_sidx(sidx);
 @@ -561,18 +562,19 @@ static int udp_sock_recverr(const struct ctx *c, union epoll_ref
ref)
  
  		if (hdr->cmsg_level == IPPROTO_IP) {
  			dlen = MIN(dlen, ICMP4_MAX_DLEN);
 -			udp_send_conn_fail_icmp4(c, ee, toside, saddr.sa4.sin_addr,
 +			udp_send_conn_fail_icmp4(c, &eh->ee, toside,
 +						 eh->saddr.sa4.sin_addr,
  						 data, dlen);
  		} else if (hdr->cmsg_level == IPPROTO_IPV6) {
 -			udp_send_conn_fail_icmp6(c, ee, toside,
 -						 &saddr.sa6.sin6_addr,
 +			udp_send_conn_fail_icmp6(c, &eh->ee, toside,
 +						 &eh->saddr.sa6.sin6_addr,
  						 data, dlen, sidx.flowi);
  		}
  	} else {
  		trace("Ignoring received IP_RECVERR cmsg on listener socket");
  	}
  	debug("%s error on UDP socket %i: %s",
 -	      str_ee_origin(ee), s, strerror_(ee->ee_errno));
 +	      str_ee_origin(&eh->ee), s, strerror_(eh->ee.ee_errno));
  
  	return 1;
  } 
-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson