udp: Propagate errors on listening and brand new socketsHEAD2025_04_15.2340bbfmaster

udp_sock_recverr() processes errors on UDP sockets and attempts to
propagate them as ICMP packets on the tap interface.  To do this it
currently requires the flow with which the error is associated as a
parameter.  If that's missing it will clear the error condition, but not
propagate it.

That means that we largely ignore errors on "listening" sockets.  It also
means we may discard some errors on flow specific sockets if they occur
very shortly after the socket is created.  In udp_flush_flow() we need to
clear any datagrams received between bind() and connect() which might not
be associated with the "final" flow for the socket.  If we get errors
before that point we'll ignore them in the same way because we don't know
the flow they're associated with in advance.

This can happen in practice if we have errors which occur almost
immediately after connect(), such as ECONNREFUSED when we connect() to a
local address where nothing is listening.

Between the extended error message itself and the PKTINFO information we
do actually have enough information to find the correct flow.  So, rather
than ignoring errors where we don't have a flow "hint", determine the flow
the hard way in udp_sock_recverr().

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
[sbrivio: Change warn() to debug() in udp_sock_recverr()]
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 32 insertions, 9 deletions

diff --git a/udp.c b/udp.c
index e8240fe..57769d0 100644
--- a/udp.c
+++ b/udp.c
@@ -504,27 +504,34 @@ static int udp_pktinfo(struct msghdr *msg, union inany_addr *dst)
  * @c:		Execution context
  * @s:		Socket to receive errors from
  * @sidx:	Flow and side of @s, or FLOW_SIDX_NONE if unknown
+ * @pif:	Interface on which the error occurred
+ *              (only used if @sidx == FLOW_SIDX_NONE)
+ * @port:	Local port number of @s (only used if @sidx == FLOW_SIDX_NONE)
  *
  * Return: 1 if error received and processed, 0 if no more errors in queue, < 0
  *         if there was an error reading the queue
  *
  * #syscalls recvmsg
  */
-static int udp_sock_recverr(const struct ctx *c, int s, flow_sidx_t sidx)
+static int udp_sock_recverr(const struct ctx *c, int s, flow_sidx_t sidx,
+			    uint8_t pif, in_port_t port)
 {
 	struct errhdr {
 		struct sock_extended_err ee;
 		union sockaddr_inany saddr;
 	};
 	char buf[PKTINFO_SPACE + CMSG_SPACE(sizeof(struct errhdr))];
+	const struct errhdr *eh = NULL;
 	char data[ICMP6_MAX_DLEN];
-	const struct errhdr *eh;
 	struct cmsghdr *hdr;
 	struct iovec iov = {
 		.iov_base = data,
 		.iov_len = sizeof(data)
 	};
+	union sockaddr_inany src;
 	struct msghdr mh = {
+		.msg_name = &src,
+		.msg_namelen = sizeof(src),
 		.msg_iov = &iov,
 		.msg_iovlen = 1,
 		.msg_control = buf,
@@ -554,7 +561,7 @@ static int udp_sock_recverr(const struct ctx *c, int s, flow_sidx_t sidx)
 		      hdr->cmsg_type == IP_RECVERR) ||
 		     (hdr->cmsg_level == IPPROTO_IPV6 &&
 		      hdr->cmsg_type == IPV6_RECVERR))
-		    break;
+			break;
 	}
 
 	if (!hdr) {
@@ -568,8 +575,19 @@ static int udp_sock_recverr(const struct ctx *c, int s, flow_sidx_t sidx)
 	      str_ee_origin(&eh->ee), s, strerror_(eh->ee.ee_errno));
 
 	if (!flow_sidx_valid(sidx)) {
-		trace("Ignoring received IP_RECVERR cmsg on listener socket");
-		return 1;
+		/* No hint from the socket, determine flow from addresses */
+		union inany_addr dst;
+
+		if (udp_pktinfo(&mh, &dst) < 0) {
+			debug("Missing PKTINFO on UDP error");
+			return 1;
+		}
+
+		sidx = flow_lookup_sa(c, IPPROTO_UDP, pif, &src, &dst, port);
+		if (!flow_sidx_valid(sidx)) {
+			debug("Ignoring UDP error without flow");
+			return 1;
+		}
 	}
 
 	tosidx = flow_sidx_opposite(sidx);
@@ -597,10 +615,14 @@ static int udp_sock_recverr(const struct ctx *c, int s, flow_sidx_t sidx)
  * @c:		Execution context
  * @s:		Socket to receive errors from
  * @sidx:	Flow and side of @s, or FLOW_SIDX_NONE if unknown
+ * @pif:	Interface on which the error occurred
+ *              (only used if @sidx == FLOW_SIDX_NONE)
+ * @port:	Local port number of @s (only used if @sidx == FLOW_SIDX_NONE)
  *
  * Return: Number of errors handled, or < 0 if we have an unrecoverable error
  */
-static int udp_sock_errs(const struct ctx *c, int s, flow_sidx_t sidx)
+static int udp_sock_errs(const struct ctx *c, int s, flow_sidx_t sidx,
+			 uint8_t pif, in_port_t port)
 {
 	unsigned n_err = 0;
 	socklen_t errlen;
@@ -609,7 +631,7 @@ static int udp_sock_errs(const struct ctx *c, int s, flow_sidx_t sidx)
 	ASSERT(!c->no_udp);
 
 	/* Empty the error queue */
-	while ((rc = udp_sock_recverr(c, s, sidx)) > 0)
+	while ((rc = udp_sock_recverr(c, s, sidx, pif, port)) > 0)
 		n_err += rc;
 
 	if (rc < 0)
@@ -776,7 +798,8 @@ void udp_sock_fwd(const struct ctx *c, int s, uint8_t frompif,
 			trace("Error peeking at socket address: %s",
 			      strerror_(-rc));
 			/* Clear errors & carry on */
-			if (udp_sock_errs(c, s, FLOW_SIDX_NONE) < 0) {
+			if (udp_sock_errs(c, s, FLOW_SIDX_NONE,
+					  frompif, port) < 0) {
 				err(
 "UDP: Unrecoverable error on listening socket: (%s port %hu)",
 				    pif_name(frompif), port);
@@ -837,7 +860,7 @@ void udp_sock_handler(const struct ctx *c, union epoll_ref ref,
 	ASSERT(!c->no_udp && uflow);
 
 	if (events & EPOLLERR) {
-		if (udp_sock_errs(c, ref.fd, ref.flowside) < 0) {
+		if (udp_sock_errs(c, ref.fd, ref.flowside, PIF_NONE, 0) < 0) {
 			flow_err(uflow, "Unrecoverable error on flow socket");
 			goto fail;
 		}