UDP and/or TCP can be disabled with the --no-udp and --no-tcp options.
However, when this is specified, it's still possible to configure forwarded
ports for the disabled protocol.  In some cases this will open sockets and
perform other actions, which might not be safe since the entire protocol
won't be initialised.

Check for this case, and explicitly forbid it.

Signed-off-by: David Gibson <david(a)gibson.dropbear.id.au>
---
 conf.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/conf.c b/conf.c
index 3c38cebc..629eb897 100644
--- a/conf.c
+++ b/conf.c
@@ -132,6 +132,11 @@ static void conf_ports(const struct ctx *c, char optname, const char
*optarg,
 		return;
 	}
 
+	if ((optname == 't' || optname == 'T') && c->no_tcp)
+		die("TCP port forwarding requested but TCP is disabled");
+	if ((optname == 'u' || optname == 'U') && c->no_udp)
+		die("UDP port forwarding requested but UDP is disabled");
+
 	if (!strcmp(optarg, "auto")) {
 		if (fwd->mode)
 			goto mode_conflict;
-- 
2.45.2

We abort the UDP socket handler if the no_udp flag is set.  But if UDP
was disabled we should never have had a UDP socket to trigger the handler
in the first place.  If we somehow did, ignoring it here isn't really going
to help because aborting without doing anything is likely to lead to an
epoll loop.  The same is the case for the TCP socket and timer handlers and
the no_tcp flag.

Change these checks on the flag to ASSERT()s.  Similarly add ASSERT()s to
several other entry points to the protocol specific code which should never
be called if the protocol is disabled.

Signed-off-by: David Gibson <david(a)gibson.dropbear.id.au>
---
 tcp.c | 14 +++++++++++---
 udp.c | 13 +++++++++++--
 2 files changed, 22 insertions(+), 5 deletions(-)

diff --git a/tcp.c b/tcp.c
index 75b959a2..de3f9f64 100644
--- a/tcp.c
+++ b/tcp.c
@@ -2286,7 +2286,9 @@ void tcp_listen_handler(struct ctx *c, union epoll_ref ref,
 	union flow *flow;
 	int s;
 
-	if (c->no_tcp || !(flow = flow_alloc()))
+	ASSERT(!c->no_tcp);
+
+	if (!(flow = flow_alloc()))
 		return;
 
 	s = accept4(ref.fd, &sa.sa, &sl, SOCK_NONBLOCK);
@@ -2339,8 +2341,7 @@ void tcp_timer_handler(struct ctx *c, union epoll_ref ref)
 	struct itimerspec check_armed = { { 0 }, { 0 } };
 	struct tcp_tap_conn *conn = CONN(ref.flow);
 
-	if (c->no_tcp)
-		return;
+	ASSERT(!c->no_tcp);
 
 	/* We don't reset timers on ~ACK_FROM_TAP_DUE, ~ACK_TO_TAP_DUE. If the
 	 * timer is currently armed, this event came from a previous setting,
@@ -2398,6 +2399,7 @@ void tcp_sock_handler(struct ctx *c, union epoll_ref ref, uint32_t
events)
 {
 	struct tcp_tap_conn *conn = CONN(ref.flowside.flow);
 
+	ASSERT(!c->no_tcp);
 	ASSERT(conn->f.type == FLOW_TCP);
 	ASSERT(conn->f.pif[ref.flowside.side] != PIF_TAP);
 
@@ -2497,6 +2499,8 @@ int tcp_sock_init(const struct ctx *c, sa_family_t af, const void
*addr,
 {
 	int r4 = FD_REF_MAX + 1, r6 = FD_REF_MAX + 1;
 
+	ASSERT(!c->no_tcp);
+
 	if (af == AF_UNSPEC && c->ifi4 && c->ifi6)
 		/* Attempt to get a dual stack socket */
 		if (tcp_sock_init_af(c, AF_UNSPEC, port, addr, ifname) >= 0)
@@ -2574,6 +2578,8 @@ static void tcp_ns_sock_init6(const struct ctx *c, in_port_t port)
  */
 void tcp_ns_sock_init(const struct ctx *c, in_port_t port)
 {
+	ASSERT(!c->no_tcp);
+
 	if (c->ifi4)
 		tcp_ns_sock_init4(c, port);
 	if (c->ifi6)
@@ -2661,6 +2667,8 @@ int tcp_init(struct ctx *c)
 {
 	unsigned b;
 
+	ASSERT(!c->no_tcp);
+
 	for (b = 0; b < TCP_HASH_TABLE_SIZE; b++)
 		tc_hash[b] = FLOW_SIDX_NONE;
 
diff --git a/udp.c b/udp.c
index 187483f0..fbf3ce73 100644
--- a/udp.c
+++ b/udp.c
@@ -749,7 +749,9 @@ static int udp_sock_recv(const struct ctx *c, int s, uint32_t events,
 	 */
 	int n = (c->mode == MODE_PASTA ? 1 : UDP_MAX_FRAMES);
 
-	if (c->no_udp || !(events & EPOLLIN))
+	ASSERT(!c->no_udp);
+
+	if (!(events & EPOLLIN))
 		return 0;
 
 	n = recvmmsg(s, mmh, n, 0, NULL);
@@ -849,10 +851,11 @@ int udp_tap_handler(struct ctx *c, uint8_t pif,
 	in_port_t src, dst;
 	socklen_t sl;
 
-	(void)c;
 	(void)saddr;
 	(void)pif;
 
+	ASSERT(!c->no_udp);
+
 	uh = packet_get(p, idx, 0, sizeof(*uh), NULL);
 	if (!uh)
 		return 1;
@@ -1026,6 +1029,8 @@ int udp_sock_init(const struct ctx *c, int ns, sa_family_t af,
 				     .orig = true, .port = port };
 	int s, r4 = FD_REF_MAX + 1, r6 = FD_REF_MAX + 1;
 
+	ASSERT(!c->no_udp);
+
 	if (ns)
 		uref.pif = PIF_SPLICE;
 	else
@@ -1214,6 +1219,8 @@ void udp_timer(struct ctx *c, const struct timespec *now)
 	unsigned int i;
 	long *word, tmp;
 
+	ASSERT(!c->no_udp);
+
 	if (c->mode == MODE_PASTA) {
 		if (c->udp.fwd_out.f.mode == FWD_AUTO) {
 			fwd_scan_ports_udp(&c->udp.fwd_out.f, &c->udp.fwd_in.f,
@@ -1257,6 +1264,8 @@ v6:
  */
 int udp_init(struct ctx *c)
 {
+	ASSERT(!c->no_udp);
+
 	udp_iov_init(c);
 
 	udp_invert_portmap(&c->udp.fwd_in);
-- 
2.45.2

Currently we ignore all events other than EPOLLIN on UDP sockets.  This
means that if we ever receive an EPOLLERR event, we'll enter an infinite
loop on epoll, because we'll never do anything to clear the error.

Luckily that doesn't seem to have happened in practice, but it's certainly
fragile.  Furthermore changes in how we handle UDP sockets with the flow
table mean we will start receiving error events.

Add handling of EPOLLERR events.  For now we just read the error from the
error queue (thereby clearing the error state) and print a debug message.
We can add more substantial handling of specific events in future if we
want to.

Signed-off-by: David Gibson <david(a)gibson.dropbear.id.au>
---
 udp.c  | 60 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 util.c | 29 ++++++++++++++++++++++++++++
 util.h |  3 +++
 3 files changed, 92 insertions(+)

diff --git a/udp.c b/udp.c
index fbf3ce73..150f970a 100644
--- a/udp.c
+++ b/udp.c
@@ -110,6 +110,7 @@
 #include <sys/socket.h>
 #include <sys/uio.h>
 #include <time.h>
+#include <linux/errqueue.h>
 
 #include "checksum.h"
 #include "util.h"
@@ -728,6 +729,59 @@ static void udp_tap_prepare(const struct ctx *c, const struct mmsghdr
*mmh,
 	(*tap_iov)[UDP_IOV_PAYLOAD].iov_len = l4len;
 }
 
+/**
+ * udp_sock_recverr() - Receive and clear an error from a socket
+ * @s:		Socket to receive from
+ *
+ * Return: true if errors received and processed, false if no more errors
+ *
+ * #syscalls recvmsg
+ */
+static bool udp_sock_recverr(int s)
+{
+	const struct sock_extended_err *ee;
+	const struct cmsghdr *hdr;
+	char buf[CMSG_SPACE(sizeof(*ee))];
+	struct msghdr mh = {
+		.msg_name = NULL,
+		.msg_namelen = 0,
+		.msg_iov = NULL,
+		.msg_iovlen = 0,
+		.msg_control = buf,
+		.msg_controllen = sizeof(buf),
+	};
+	ssize_t rc;
+
+	rc = recvmsg(s, &mh, MSG_ERRQUEUE);
+	if (rc < 0) {
+		if (errno != EAGAIN && errno != EWOULDBLOCK)
+			err_perror("Failed to read error queue");
+		return false;
+	}
+
+	if (!(mh.msg_flags & MSG_ERRQUEUE)) {
+		err("Missing MSG_ERRQUEUE flag reading error queue");
+		return false;
+	}
+
+	hdr = CMSG_FIRSTHDR(&mh);
+	if (!((hdr->cmsg_level == IPPROTO_IP &&
+	       hdr->cmsg_type == IP_RECVERR) ||
+	      (hdr->cmsg_level == IPPROTO_IPV6 &&
+	       hdr->cmsg_type == IPV6_RECVERR))) {
+		err("Unexpected cmsg reading error queue");
+		return false;
+	}
+
+	ee = (const struct sock_extended_err *)CMSG_DATA(hdr);
+
+	/* TODO: When possible propagate and otherwise handle errors */
+	debug("%s error on UDP socket %i: %s",
+	      str_ee_origin(ee), s, strerror(ee->ee_errno));
+
+	return true;
+}
+
 /**
  * udp_sock_recv() - Receive datagrams from a socket
  * @c:		Execution context
@@ -751,6 +805,12 @@ static int udp_sock_recv(const struct ctx *c, int s, uint32_t
events,
 
 	ASSERT(!c->no_udp);
 
+	/* Clear any errors first */
+	if (events & EPOLLERR) {
+		while (udp_sock_recverr(s))
+			;
+	}
+
 	if (!(events & EPOLLIN))
 		return 0;
 
diff --git a/util.c b/util.c
index 428847d2..1569f1c0 100644
--- a/util.c
+++ b/util.c
@@ -25,6 +25,7 @@
 #include <time.h>
 #include <errno.h>
 #include <stdbool.h>
+#include <linux/errqueue.h>
 
 #include "util.h"
 #include "iov.h"
@@ -97,6 +98,14 @@ static int sock_l4_sa(const struct ctx *c, enum epoll_type type,
 	if (setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &y, sizeof(y)))
 		debug("Failed to set SO_REUSEADDR on socket %i", fd);
 
+	if (proto == IPPROTO_UDP) {
+		int level = af == AF_INET ? IPPROTO_IP : IPPROTO_IPV6;
+		int opt = af == AF_INET ? IP_RECVERR : IPV6_RECVERR;
+
+		if (setsockopt(fd, level, opt, &y, sizeof(y)))
+			die_perror("Failed to set RECVERR on socket %i", fd);
+	}
+
 	if (ifname && *ifname) {
 		/* Supported since kernel version 5.7, commit c427bfec18f2
 		 * ("net: core: enable SO_BINDTODEVICE for non-root users"). If
@@ -654,3 +663,23 @@ const char *sockaddr_ntop(const void *sa, char *dst, socklen_t size)
 
 	return dst;
 }
+
+/** str_ee_origin() - Convert socket extended error origin to a string
+ * @ee:		Socket extended error structure
+ *
+ * Return: Static string describing error origin
+ */
+const char *str_ee_origin(const struct sock_extended_err *ee)
+{
+	const char *const desc[] = {
+		[SO_EE_ORIGIN_NONE]  = "<no origin>",
+		[SO_EE_ORIGIN_LOCAL] = "Local",
+		[SO_EE_ORIGIN_ICMP]  = "ICMP",
+		[SO_EE_ORIGIN_ICMP6] = "ICMPv6",
+	};
+
+	if (ee->ee_origin < ARRAY_SIZE(desc))
+		return desc[ee->ee_origin];
+
+	return "<invalid>";
+}
diff --git a/util.h b/util.h
index d0150396..1d479ddf 100644
--- a/util.h
+++ b/util.h
@@ -194,7 +194,10 @@ static inline const char *af_name(sa_family_t af)
 
 #define SOCKADDR_STRLEN		MAX(SOCKADDR_INET_STRLEN, SOCKADDR_INET6_STRLEN)
 
+struct sock_extended_err;
+
 const char *sockaddr_ntop(const void *sa, char *dst, socklen_t size);
+const char *str_ee_origin(const struct sock_extended_err *ee);
 
 /**
  * mod_sub() - Modular arithmetic subtraction
-- 
2.45.2