[PATCH v2 3/3] udp: Use dual stack sockets for port forwarding when possible

[David Gibson <david@gibson.dropbear.id.au>]

Platforms like Linux allow IPv6 sockets to listen for IPv4 connections as
well as native IPv6 connections.  By doing this we halve the number of
listening sockets we need (assuming passt/pasta is listening on the same
ports for IPv4 and IPv6).  When forwarding many ports (e.g. -u all) this
can significantly reduce the amount of kernel memory that passt consumes.

We've used such dual stack sockets for TCP since 8e914238b "tcp: Use dual
stack sockets for port forwarding when possible".  Add similar support for
UDP "listening" sockets.  Since UDP sockets don't use as much kernel memory
as TCP sockets this isn't as big a saving, but it's still significant.
When forwarding all TCP and UDP ports for both IPv4 & IPv6 (-t all -u all),
this reduces kernel memory usage from ~522 MiB to ~380MiB (kernel version
6.10.6 on Fedora 40, x86_64).

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 udp.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/udp.c b/udp.c
index 41a62475..bd9051e1 100644
--- a/udp.c
+++ b/udp.c
@@ -723,6 +723,25 @@ int udp_sock_init(const struct ctx *c, int ns, sa_family_t af,
 	else
 		uref.pif = PIF_HOST;
 
+	if (af == AF_UNSPEC && c->ifi4 && c->ifi6) {
+		int s;
+
+		/* Attempt to get a dual stack socket */
+		if (!ns) {
+			s = sock_l4(c, AF_UNSPEC, EPOLL_TYPE_UDP_LISTEN,
+				    addr, ifname, port, uref.u32);
+			udp_splice_init[V4][port] = s < 0 ? -1 : s;
+			udp_splice_init[V6][port] = s < 0 ? -1 : s;
+		} else {
+			s = sock_l4(c, AF_UNSPEC, EPOLL_TYPE_UDP_LISTEN,
+				    &in4addr_loopback, ifname, port, uref.u32);
+			udp_splice_ns[V4][port] = s < 0 ? -1 : s;
+			udp_splice_ns[V6][port] = s < 0 ? -1 : s;
+		}
+		if (IN_INTERVAL(0, FD_REF_MAX, s))
+			return 0;
+	}
+
 	if ((af == AF_INET || af == AF_UNSPEC) && c->ifi4) {
 		if (!ns) {
 			r4 = sock_l4(c, AF_INET, EPOLL_TYPE_UDP_LISTEN,
-- 
@@ -723,6 +723,25 @@ int udp_sock_init(const struct ctx *c, int ns, sa_family_t af,
 	else
 		uref.pif = PIF_HOST;
 
+	if (af == AF_UNSPEC && c->ifi4 && c->ifi6) {
+		int s;
+
+		/* Attempt to get a dual stack socket */
+		if (!ns) {
+			s = sock_l4(c, AF_UNSPEC, EPOLL_TYPE_UDP_LISTEN,
+				    addr, ifname, port, uref.u32);
+			udp_splice_init[V4][port] = s < 0 ? -1 : s;
+			udp_splice_init[V6][port] = s < 0 ? -1 : s;
+		} else {
+			s = sock_l4(c, AF_UNSPEC, EPOLL_TYPE_UDP_LISTEN,
+				    &in4addr_loopback, ifname, port, uref.u32);
+			udp_splice_ns[V4][port] = s < 0 ? -1 : s;
+			udp_splice_ns[V6][port] = s < 0 ? -1 : s;
+		}
+		if (IN_INTERVAL(0, FD_REF_MAX, s))
+			return 0;
+	}
+
 	if ((af == AF_INET || af == AF_UNSPEC) && c->ifi4) {
 		if (!ns) {
 			r4 = sock_l4(c, AF_INET, EPOLL_TYPE_UDP_LISTEN,
-- 
2.46.0