Re: [PATCH] udp: Actually discard datagrams we can't forward

On Fri, May 02, 2025 at 11:06:40PM +0200, Stefano Brivio wrote:

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

Oof, yes, that's a somewhat embarrassing oversight. Reviewed-by: David Gibson

Signed-off-by: Stefano Brivio --- udp.c | 10 ++++++++++ 1 file changed, 10 insertions(+)

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"); } } }

-- David Gibson (he or they) | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you, not the other way | around. http://www.ozlabs.org/~dgibson