udp: Handle errors on UDP sockets

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@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

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;
 

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>";
+}

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