selftests/net: Test tcp port reuse after unbinding a socket

Exercise the scenario described in detail in the cover letter:

  1) socket A: connect() from ephemeral port X
  2) socket B: explicitly bind() to port X
  3) check that port X is now excluded from ephemeral ports
  4) close socket B to release the port bind
  5) socket C: connect() from ephemeral port X

As well as a corner case to test that the connect-bind flag is cleared:

  1) connect() from ephemeral port X
  2) disconnect the socket with connect(AF_UNSPEC)
  3) bind() it explicitly to port X
  4) check that port X is now excluded from ephemeral ports

Reviewed-by: Kuniyuki Iwashima <[email protected]>
Signed-off-by: Jakub Sitnicki <[email protected]>
Link: https://patch.msgid.link/20250917-update-bind-bucket-state-on-unhash-v5-2-57168b661b47@cloudflare.com
Signed-off-by: Paolo Abeni <[email protected]>

Author: jsitnicki

tools/testing/selftests/net/Makefile

@@ -122,6 +122,7 @@ TEST_PROGS += broadcast_pmtu.sh
 TEST_PROGS += ipv6_force_forwarding.sh
 TEST_GEN_PROGS += ipv6_fragmentation
 TEST_PROGS += route_hint.sh
+TEST_GEN_PROGS += tcp_port_share
 
 # YNL files, must be before "include ..lib.mk"
 YNL_GEN_FILES := busy_poller

tools/testing/selftests/net/tcp_port_share.c

@@ -0,0 +1,258 @@
+// SPDX-License-Identifier: GPL-2.0 OR BSD-3-Clause
+// Copyright (c) 2025 Cloudflare, Inc.
+
+/* Tests for TCP port sharing (bind bucket reuse). */
+
+#include <arpa/inet.h>
+#include <net/if.h>
+#include <sys/ioctl.h>
+#include <fcntl.h>
+#include <sched.h>
+#include <stdlib.h>
+
+#include "../kselftest_harness.h"
+
+#define DST_PORT 30000
+#define SRC_PORT 40000
+
+struct sockaddr_inet {
+	union {
+		struct sockaddr_storage ss;
+		struct sockaddr_in6 v6;
+		struct sockaddr_in v4;
+		struct sockaddr sa;
+	};
+	socklen_t len;
+	char str[INET6_ADDRSTRLEN + __builtin_strlen("[]:65535") + 1];
+};
+
+const int one = 1;
+
+static int disconnect(int fd)
+{
+	return connect(fd, &(struct sockaddr){ AF_UNSPEC }, sizeof(struct sockaddr));
+}
+
+static int getsockname_port(int fd)
+{
+	struct sockaddr_inet addr = {};
+	int err;
+
+	addr.len = sizeof(addr);
+	err = getsockname(fd, &addr.sa, &addr.len);
+	if (err)
+		return -1;
+
+	switch (addr.sa.sa_family) {
+	case AF_INET:
+		return ntohs(addr.v4.sin_port);
+	case AF_INET6:
+		return ntohs(addr.v6.sin6_port);
+	default:
+		errno = EAFNOSUPPORT;
+		return -1;
+	}
+}
+
+static void make_inet_addr(int af, const char *ip, __u16 port,
+			   struct sockaddr_inet *addr)
+{
+	const char *fmt = "";
+
+	memset(addr, 0, sizeof(*addr));
+
+	switch (af) {
+	case AF_INET:
+		addr->len = sizeof(addr->v4);
+		addr->v4.sin_family = af;
+		addr->v4.sin_port = htons(port);
+		inet_pton(af, ip, &addr->v4.sin_addr);
+		fmt = "%s:%hu";
+		break;
+	case AF_INET6:
+		addr->len = sizeof(addr->v6);
+		addr->v6.sin6_family = af;
+		addr->v6.sin6_port = htons(port);
+		inet_pton(af, ip, &addr->v6.sin6_addr);
+		fmt = "[%s]:%hu";
+		break;
+	}
+
+	snprintf(addr->str, sizeof(addr->str), fmt, ip, port);
+}
+
+FIXTURE(tcp_port_share) {};
+
+FIXTURE_VARIANT(tcp_port_share) {
+	int domain;
+	/* IP to listen on and connect to */
+	const char *dst_ip;
+	/* Primary IP to connect from */
+	const char *src1_ip;
+	/* Secondary IP to connect from */
+	const char *src2_ip;
+	/* IP to bind to in order to block the source port */
+	const char *bind_ip;
+};
+
+FIXTURE_VARIANT_ADD(tcp_port_share, ipv4) {
+	.domain = AF_INET,
+	.dst_ip = "127.0.0.1",
+	.src1_ip = "127.1.1.1",
+	.src2_ip = "127.2.2.2",
+	.bind_ip = "127.3.3.3",
+};
+
+FIXTURE_VARIANT_ADD(tcp_port_share, ipv6) {
+	.domain = AF_INET6,
+	.dst_ip = "::1",
+	.src1_ip = "2001:db8::1",
+	.src2_ip = "2001:db8::2",
+	.bind_ip = "2001:db8::3",
+};
+
+FIXTURE_SETUP(tcp_port_share)
+{
+	int sc;
+
+	ASSERT_EQ(unshare(CLONE_NEWNET), 0);
+	ASSERT_EQ(system("ip link set dev lo up"), 0);
+	ASSERT_EQ(system("ip addr add dev lo 2001:db8::1/32 nodad"), 0);
+	ASSERT_EQ(system("ip addr add dev lo 2001:db8::2/32 nodad"), 0);
+	ASSERT_EQ(system("ip addr add dev lo 2001:db8::3/32 nodad"), 0);
+
+	sc = open("/proc/sys/net/ipv4/ip_local_port_range", O_WRONLY);
+	ASSERT_GE(sc, 0);
+	ASSERT_GT(dprintf(sc, "%hu %hu\n", SRC_PORT, SRC_PORT), 0);
+	ASSERT_EQ(close(sc), 0);
+}
+
+FIXTURE_TEARDOWN(tcp_port_share) {}
+
+/* Verify that an ephemeral port becomes available again after the socket
+ * bound to it and blocking it from reuse is closed.
+ */
+TEST_F(tcp_port_share, can_reuse_port_after_bind_and_close)
+{
+	const typeof(variant) v = variant;
+	struct sockaddr_inet addr;
+	int c1, c2, ln, pb;
+
+	/* Listen on <dst_ip>:<DST_PORT> */
+	ln = socket(v->domain, SOCK_STREAM, 0);
+	ASSERT_GE(ln, 0) TH_LOG("socket(): %m");
+	ASSERT_EQ(setsockopt(ln, SOL_SOCKET, SO_REUSEADDR, &one, sizeof(one)), 0);
+
+	make_inet_addr(v->domain, v->dst_ip, DST_PORT, &addr);
+	ASSERT_EQ(bind(ln, &addr.sa, addr.len), 0) TH_LOG("bind(%s): %m", addr.str);
+	ASSERT_EQ(listen(ln, 2), 0);
+
+	/* Connect from <src1_ip>:<SRC_PORT> */
+	c1 = socket(v->domain, SOCK_STREAM, 0);
+	ASSERT_GE(c1, 0) TH_LOG("socket(): %m");
+	ASSERT_EQ(setsockopt(c1, SOL_IP, IP_BIND_ADDRESS_NO_PORT, &one, sizeof(one)), 0);
+
+	make_inet_addr(v->domain, v->src1_ip, 0, &addr);
+	ASSERT_EQ(bind(c1, &addr.sa, addr.len), 0) TH_LOG("bind(%s): %m", addr.str);
+
+	make_inet_addr(v->domain, v->dst_ip, DST_PORT, &addr);
+	ASSERT_EQ(connect(c1, &addr.sa, addr.len), 0) TH_LOG("connect(%s): %m", addr.str);
+	ASSERT_EQ(getsockname_port(c1), SRC_PORT);
+
+	/* Bind to <bind_ip>:<SRC_PORT>. Block the port from reuse. */
+	pb = socket(v->domain, SOCK_STREAM, 0);
+	ASSERT_GE(pb, 0) TH_LOG("socket(): %m");
+	ASSERT_EQ(setsockopt(pb, SOL_SOCKET, SO_REUSEADDR, &one, sizeof(one)), 0);
+
+	make_inet_addr(v->domain, v->bind_ip, SRC_PORT, &addr);
+	ASSERT_EQ(bind(pb, &addr.sa, addr.len), 0) TH_LOG("bind(%s): %m", addr.str);
+
+	/* Try to connect from <src2_ip>:<SRC_PORT>. Expect failure. */
+	c2 = socket(v->domain, SOCK_STREAM, 0);
+	ASSERT_GE(c2, 0) TH_LOG("socket");
+	ASSERT_EQ(setsockopt(c2, SOL_IP, IP_BIND_ADDRESS_NO_PORT, &one, sizeof(one)), 0);
+
+	make_inet_addr(v->domain, v->src2_ip, 0, &addr);
+	ASSERT_EQ(bind(c2, &addr.sa, addr.len), 0) TH_LOG("bind(%s): %m", addr.str);
+
+	make_inet_addr(v->domain, v->dst_ip, DST_PORT, &addr);
+	ASSERT_EQ(connect(c2, &addr.sa, addr.len), -1) TH_LOG("connect(%s)", addr.str);
+	ASSERT_EQ(errno, EADDRNOTAVAIL) TH_LOG("%m");
+
+	/* Unbind from <bind_ip>:<SRC_PORT>. Unblock the port for reuse. */
+	ASSERT_EQ(close(pb), 0);
+
+	/* Connect again from <src2_ip>:<SRC_PORT> */
+	EXPECT_EQ(connect(c2, &addr.sa, addr.len), 0) TH_LOG("connect(%s): %m", addr.str);
+	EXPECT_EQ(getsockname_port(c2), SRC_PORT);
+
+	ASSERT_EQ(close(c2), 0);
+	ASSERT_EQ(close(c1), 0);
+	ASSERT_EQ(close(ln), 0);
+}
+
+/* Verify that a socket auto-bound during connect() blocks port reuse after
+ * disconnect (connect(AF_UNSPEC)) followed by an explicit port bind().
+ */
+TEST_F(tcp_port_share, port_block_after_disconnect)
+{
+	const typeof(variant) v = variant;
+	struct sockaddr_inet addr;
+	int c1, c2, ln, pb;
+
+	/* Listen on <dst_ip>:<DST_PORT> */
+	ln = socket(v->domain, SOCK_STREAM, 0);
+	ASSERT_GE(ln, 0) TH_LOG("socket(): %m");
+	ASSERT_EQ(setsockopt(ln, SOL_SOCKET, SO_REUSEADDR, &one, sizeof(one)), 0);
+
+	make_inet_addr(v->domain, v->dst_ip, DST_PORT, &addr);
+	ASSERT_EQ(bind(ln, &addr.sa, addr.len), 0) TH_LOG("bind(%s): %m", addr.str);
+	ASSERT_EQ(listen(ln, 2), 0);
+
+	/* Connect from <src1_ip>:<SRC_PORT> */
+	c1 = socket(v->domain, SOCK_STREAM, 0);
+	ASSERT_GE(c1, 0) TH_LOG("socket(): %m");
+	ASSERT_EQ(setsockopt(c1, SOL_IP, IP_BIND_ADDRESS_NO_PORT, &one, sizeof(one)), 0);
+
+	make_inet_addr(v->domain, v->src1_ip, 0, &addr);
+	ASSERT_EQ(bind(c1, &addr.sa, addr.len), 0) TH_LOG("bind(%s): %m", addr.str);
+
+	make_inet_addr(v->domain, v->dst_ip, DST_PORT, &addr);
+	ASSERT_EQ(connect(c1, &addr.sa, addr.len), 0) TH_LOG("connect(%s): %m", addr.str);
+	ASSERT_EQ(getsockname_port(c1), SRC_PORT);
+
+	/* Disconnect the socket and bind it to <bind_ip>:<SRC_PORT> to block the port */
+	ASSERT_EQ(disconnect(c1), 0) TH_LOG("disconnect: %m");
+	ASSERT_EQ(setsockopt(c1, SOL_SOCKET, SO_REUSEADDR, &one, sizeof(one)), 0);
+
+	make_inet_addr(v->domain, v->bind_ip, SRC_PORT, &addr);
+	ASSERT_EQ(bind(c1, &addr.sa, addr.len), 0) TH_LOG("bind(%s): %m", addr.str);
+
+	/* Trigger port-addr bucket state update with another bind() and close() */
+	pb = socket(v->domain, SOCK_STREAM, 0);
+	ASSERT_GE(pb, 0) TH_LOG("socket(): %m");
+	ASSERT_EQ(setsockopt(pb, SOL_SOCKET, SO_REUSEADDR, &one, sizeof(one)), 0);
+
+	make_inet_addr(v->domain, v->bind_ip, SRC_PORT, &addr);
+	ASSERT_EQ(bind(pb, &addr.sa, addr.len), 0) TH_LOG("bind(%s): %m", addr.str);
+
+	ASSERT_EQ(close(pb), 0);
+
+	/* Connect from <src2_ip>:<SRC_PORT>. Expect failure. */
+	c2 = socket(v->domain, SOCK_STREAM, 0);
+	ASSERT_GE(c2, 0) TH_LOG("socket: %m");
+	ASSERT_EQ(setsockopt(c2, SOL_IP, IP_BIND_ADDRESS_NO_PORT, &one, sizeof(one)), 0);
+
+	make_inet_addr(v->domain, v->src2_ip, 0, &addr);
+	ASSERT_EQ(bind(c2, &addr.sa, addr.len), 0) TH_LOG("bind(%s): %m", addr.str);
+
+	make_inet_addr(v->domain, v->dst_ip, DST_PORT, &addr);
+	EXPECT_EQ(connect(c2, &addr.sa, addr.len), -1) TH_LOG("connect(%s)", addr.str);
+	EXPECT_EQ(errno, EADDRNOTAVAIL) TH_LOG("%m");
+
+	ASSERT_EQ(close(c2), 0);
+	ASSERT_EQ(close(c1), 0);
+	ASSERT_EQ(close(ln), 0);
+}
+
+TEST_HARNESS_MAIN