[PATCH 1/2] udp: correct source address for ICMP messages

[Jon Maloy <jmaloy@redhat.com>]

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@redhat.com>
---
 udp.c | 26 +++++++++++++++-----------
 1 file changed, 15 insertions(+), 11 deletions(-)

diff --git a/udp.c b/udp.c
index 80520cb..271e570 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 = {
@@ -521,11 +524,11 @@ 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_name = 0,
+		.msg_namelen = 0,
 		.msg_iov = &iov,
 		.msg_iovlen = 1,
-		.msg_control = buf,
+		.msg_control = buf,//(void *)&errhdr,
 		.msg_controllen = sizeof(buf),
 	};
 	ssize_t rc;
@@ -553,7 +556,7 @@ 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 +564,19 @@ 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;
 }
-- 
@@ -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 = {
@@ -521,11 +524,11 @@ 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_name = 0,
+		.msg_namelen = 0,
 		.msg_iov = &iov,
 		.msg_iovlen = 1,
-		.msg_control = buf,
+		.msg_control = buf,//(void *)&errhdr,
 		.msg_controllen = sizeof(buf),
 	};
 	ssize_t rc;
@@ -553,7 +556,7 @@ 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 +564,19 @@ 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;
 }
-- 
2.48.1