udp: Actually discard datagrams we can't forward2025_05_03.587980c

Given that udp_sock_fwd() now loops on udp_peek_addr() to get endpoint
addresses for datagrams, if we can't forward one of these datagrams,
we need to make sure we actually discard it. Otherwise, with MSG_PEEK,
we won't dequeue and loop on it forever.

For example, if we fail to create a socket for a new flow, because,
say, the destination of an inbound packet is multicast, and we can't
bind() to a multicast address, the loop will look like this:

18.0563: Flow 0 (NEW): FREE -> NEW
18.0563: Flow 0 (INI): NEW -> INI
18.0563: Flow 0 (INI): HOST [127.0.0.1]:42487 -> [127.0.0.1]:9997 => ?
18.0563: Flow 0 (TGT): INI -> TGT
18.0563: Flow 0 (TGT): HOST [127.0.0.1]:42487 -> [ff02::c]:9997 => SPLICE [0.0.0.0]:42487 -> [88.198.0.164]:9997
18.0563: Flow 0 (UDP flow): TGT -> TYPED
18.0564: Flow 0 (UDP flow): HOST [127.0.0.1]:42487 -> [ff02::c]:9997 => SPLICE [0.0.0.0]:42487 -> [88.198.0.164]:9997
18.0564: Flow 0 (UDP flow): Couldn't open flow specific socket: Invalid argument
18.0564: Flow 0 (FREE): TYPED -> FREE
18.0564: Flow 0 (FREE): HOST [127.0.0.1]:42487 -> [ff02::c]:9997 => SPLICE [0.0.0.0]:42487 -> [88.198.0.164]:9997
18.0564: Discarding datagram without flow
18.0564: Flow 0 (NEW): FREE -> NEW
18.0564: Flow 0 (INI): NEW -> INI
18.0564: Flow 0 (INI): HOST [127.0.0.1]:42487 -> [127.0.0.1]:9997 => ?
18.0564: Flow 0 (TGT): INI -> TGT
18.0564: Flow 0 (TGT): HOST [127.0.0.1]:42487 -> [ff02::c]:9997 => SPLICE [0.0.0.0]:42487 -> [88.198.0.164]:9997
18.0564: Flow 0 (UDP flow): TGT -> TYPED
18.0564: Flow 0 (UDP flow): HOST [127.0.0.1]:42487 -> [ff02::c]:9997 => SPLICE [0.0.0.0]:42487 -> [88.198.0.164]:9997
18.0564: Flow 0 (UDP flow): Couldn't open flow specific socket: Invalid argument
18.0564: Flow 0 (FREE): TYPED -> FREE
18.0564: Flow 0 (FREE): HOST [127.0.0.1]:42487 -> [ff02::c]:9997 => SPLICE [0.0.0.0]:42487 -> [88.198.0.164]:9997
18.0564: Discarding datagram without flow

and seen from strace:

epoll_wait(3, [{events=EPOLLIN, data=0x1076c00000705}], 8, 1000) = 1
recvmsg(7, {msg_name={sa_family=AF_INET6, sin6_port=htons(55899), sin6_flowinfo=htonl(0), inet_pton(AF_INET6, "fe80::26e8:53ff:fef3:13b6", &sin6_addr), sin6_scope_id=if_nametoindex("wlp4s0")}, msg_namelen=28, msg_iov=NULL, msg_iovlen=0, msg_control=[{cmsg_len=36, cmsg_level=SOL_IPV6, cmsg_type=0x32, cmsg_data="\xff\x02\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x0c\x03\x00\x00\x00"}], msg_controllen=40, msg_flags=MSG_TRUNC}, MSG_PEEK|MSG_DONTWAIT) = 0
socket(AF_INET6, SOCK_DGRAM|SOCK_NONBLOCK, IPPROTO_UDP) = 12
setsockopt(12, SOL_IPV6, IPV6_V6ONLY, [1], 4) = 0
setsockopt(12, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
setsockopt(12, SOL_IPV6, IPV6_RECVERR, [1], 4) = 0
setsockopt(12, SOL_IPV6, IPV6_RECVPKTINFO, [1], 4) = 0
bind(12, {sa_family=AF_INET6, sin6_port=htons(1900), sin6_flowinfo=htonl(0), inet_pton(AF_INET6, "ff02::c", &sin6_addr), sin6_scope_id=0}, 28) = -1 EINVAL (Invalid argument)
close(12)                               = 0
recvmsg(7, {msg_name={sa_family=AF_INET6, sin6_port=htons(55899), sin6_flowinfo=htonl(0), inet_pton(AF_INET6, "fe80::26e8:53ff:fef3:13b6", &sin6_addr), sin6_scope_id=if_nametoindex("wlp4s0")}, msg_namelen=28, msg_iov=NULL, msg_iovlen=0, msg_control=[{cmsg_len=36, cmsg_level=SOL_IPV6, cmsg_type=0x32, cmsg_data="\xff\x02\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x0c\x03\x00\x00\x00"}], msg_controllen=40, msg_flags=MSG_TRUNC}, MSG_PEEK|MSG_DONTWAIT) = 0
socket(AF_INET6, SOCK_DGRAM|SOCK_NONBLOCK, IPPROTO_UDP) = 12
setsockopt(12, SOL_IPV6, IPV6_V6ONLY, [1], 4) = 0
setsockopt(12, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
setsockopt(12, SOL_IPV6, IPV6_RECVERR, [1], 4) = 0
setsockopt(12, SOL_IPV6, IPV6_RECVPKTINFO, [1], 4) = 0
bind(12, {sa_family=AF_INET6, sin6_port=htons(1900), sin6_flowinfo=htonl(0), inet_pton(AF_INET6, "ff02::c", &sin6_addr), sin6_scope_id=0}, 28) = -1 EINVAL (Invalid argument)
close(12)                               = 0

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

1 files changed, 10 insertions, 0 deletions

diff --git a/udp.c b/udp.c
index f5a5cd1..ca28b37 100644
--- a/udp.c
+++ b/udp.c
@@ -828,6 +828,7 @@ void udp_sock_fwd(const struct ctx *c, int s, uint8_t frompif,
 	int rc;
 
 	while ((rc = udp_peek_addr(s, &src, &dst)) != 0) {
+		bool discard = false;
 		flow_sidx_t tosidx;
 		uint8_t topif;
 
@@ -861,8 +862,17 @@ void udp_sock_fwd(const struct ctx *c, int s, uint8_t frompif,
 			flow_err(uflow,
 				 "No support for forwarding UDP from %s to %s",
 				 pif_name(frompif), pif_name(topif));
+			discard = true;
 		} else {
 			debug("Discarding datagram without flow");
+			discard = true;
+		}
+
+		if (discard) {
+			struct msghdr msg = { 0 };
+
+			if (recvmsg(s, &msg, MSG_DONTWAIT) < 0)
+				debug_perror("Failed to discard datagram");
 		}
 	}
 }