Merge branch 'bpf-sock-migration'

Kuniyuki Iwashima says:

====================
The SO_REUSEPORT option allows sockets to listen on the same port and to
accept connections evenly. However, there is a defect in the current
implementation [1]. When a SYN packet is received, the connection is tied
to a listening socket. Accordingly, when the listener is closed, in-flight
requests during the three-way handshake and child sockets in the accept
queue are dropped even if other listeners on the same port could accept
such connections.

This situation can happen when various server management tools restart
server (such as nginx) processes. For instance, when we change nginx
configurations and restart it, it spins up new workers that respect the
new configuration and closes all listeners on the old workers, resulting
in the in-flight ACK of 3WHS is responded by RST.

To avoid such a situation, users have to know deeply how the kernel handles
SYN packets and implement connection draining by eBPF [2]:

  1. Stop routing SYN packets to the listener by eBPF.
  2. Wait for all timers to expire to complete requests
  3. Accept connections until EAGAIN, then close the listener.

  or

  1. Start counting SYN packets and accept syscalls using the eBPF map.
  2. Stop routing SYN packets.
  3. Accept connections up to the count, then close the listener.

In either way, we cannot close a listener immediately. However, ideally,
the application need not drain the not yet accepted sockets because 3WHS
and tying a connection to a listener are just the kernel behaviour. The
root cause is within the kernel, so the issue should be addressed in kernel
space and should not be visible to user space. This patchset fixes it so
that users need not take care of kernel implementation and connection
draining. With this patchset, the kernel redistributes requests and
connections from a listener to the others in the same reuseport group
at/after close or shutdown syscalls.

Although some software does connection draining, there are still merits in
migration. For some security reasons, such as replacing TLS certificates,
we may want to apply new settings as soon as possible and/or we may not be
able to wait for connection draining. The sockets in the accept queue have
not started application sessions yet. So, if we do not drain such sockets,
they can be handled by the newer listeners and could have a longer
lifetime. It is difficult to drain all connections in every case, but we
can decrease such aborted connections by migration. In that sense,
migration is always better than draining.

Moreover, auto-migration simplifies user space logic and also works well in
a case where we cannot modify and build a server program to implement the
workaround.

Note that the source and destination listeners MUST have the same settings
at the socket API level; otherwise, applications may face inconsistency and
cause errors. In such a case, we have to use the eBPF program to select a
specific listener or to cancel migration.

Special thanks to Martin KaFai Lau for bouncing ideas and exchanging code
snippets along the way.

Link:
 [1] The SO_REUSEPORT socket option
 https://lwn.net/Articles/542629/

 [2] Re: [PATCH 1/1] net: Add SO_REUSEPORT_LISTEN_OFF socket option as drain mode
 https://lore.kernel.org/netdev/1458828813.10868.65.camel@edumazet-glaptop3.roam.corp.google.com/

Changelog:
 v8:
  * Make reuse const in reuseport_sock_index()
  * Don't use __reuseport_add_sock() in reuseport_alloc()
  * Change the arg of the second memcpy() in reuseport_grow()
  * Fix coding style to use goto in reuseport_alloc()
  * Keep sk_refcnt uninitialized in inet_reqsk_clone()
  * Initialize ireq_opt and ipv6_opt separately in reqsk_migrate_reset()

  [ This series does not include a stats patch suggested by Yuchung Cheng
    not to drop Acked-by/Reviewed-by tags and save reviewer's time. I will
    post the patch as a follow up after this series is merged. ]

 v7:
 https://lore.kernel.org/bpf/20210521182104.18273-1-kuniyu@amazon.co.jp/
  * Prevent attaching/detaching a bpf prog via shutdowned socket
  * Fix typo in commit messages
  * Split selftest into subtests

 v6:
 https://lore.kernel.org/bpf/20210517002258.75019-1-kuniyu@amazon.co.jp/
  * Change description in ip-sysctl.rst
  * Test IPPROTO_TCP before reading tfo_listener
  * Move reqsk_clone() to inet_connection_sock.c and rename to
    inet_reqsk_clone()
  * Pass req->rsk_listener to inet_csk_reqsk_queue_drop() and
    reqsk_queue_removed() in the migration path of receiving ACK
  * s/ARG_PTR_TO_SOCKET/PTR_TO_SOCKET/ in sk_reuseport_is_valid_access()
  * In selftest, use atomic ops to increment global vars, drop ACK by XDP,
    enable force fastopen, use "skel->bss" instead of "skel->data"

 v5:
 https://lore.kernel.org/bpf/20210510034433.52818-1-kuniyu@amazon.co.jp/
  * Move initializtion of sk_node from 6th to 5th patch
  * Initialize sk_refcnt in reqsk_clone()
  * Modify some definitions in reqsk_timer_handler()
  * Validate in which path/state migration happens in selftest

 v4:
 https://lore.kernel.org/bpf/20210427034623.46528-1-kuniyu@amazon.co.jp/
  * Make some functions and variables 'static' in selftest
  * Remove 'scalability' from the cover letter

 v3:
 https://lore.kernel.org/bpf/20210420154140.80034-1-kuniyu@amazon.co.jp/
  * Add sysctl back for reuseport_grow()
  * Add helper functions to manage socks[]
  * Separate migration related logic into functions: reuseport_resurrect(),
    reuseport_stop_listen_sock(), reuseport_migrate_sock()
  * Clone request_sock to be migrated
  * Migrate request one by one
  * Pass child socket to eBPF prog

 v2:
 https://lore.kernel.org/netdev/20201207132456.65472-1-kuniyu@amazon.co.jp/
  * Do not save closed sockets in socks[]
  * Revert 607904c357c61adf20b8fd18af765e501d61a385
  * Extract inet_csk_reqsk_queue_migrate() into a single patch
  * Change the spin_lock order to avoid lockdep warning
  * Add static to __reuseport_select_sock
  * Use refcount_inc_not_zero() in reuseport_select_migrated_sock()
  * Set the default attach type in bpf_prog_load_check_attach()
  * Define new proto of BPF_FUNC_get_socket_cookie
  * Fix test to be compiled successfully
  * Update commit messages

 v1:
 https://lore.kernel.org/netdev/20201201144418.35045-1-kuniyu@amazon.co.jp/
  * Remove the sysctl option
  * Enable migration if eBPF progam is not attached
  * Add expected_attach_type to check if eBPF program can migrate sockets
  * Add a field to tell migration type to eBPF program
  * Support BPF_FUNC_get_socket_cookie to get the cookie of sk
  * Allocate an empty skb if skb is NULL
  * Pass req_to_sk(req)->sk_hash because listener's hash is zero
  * Update commit messages and coverletter

 RFC:
 https://lore.kernel.org/netdev/20201117094023.3685-1-kuniyu@amazon.co.jp/
====================

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

4 files changed, 692 insertions, 1 deletions

diff --git a/tools/testing/selftests/bpf/network_helpers.c b/tools/testing/selftests/bpf/network_helpers.c
index 12ee40284da0..2060bc122c53 100644
--- a/tools/testing/selftests/bpf/network_helpers.c
+++ b/tools/testing/selftests/bpf/network_helpers.c
@@ -40,7 +40,7 @@ struct ipv6_packet pkt_v6 = {
 	.tcp.doff = 5,
 };
 
-static int settimeo(int fd, int timeout_ms)
+int settimeo(int fd, int timeout_ms)
 {
 	struct timeval timeout = { .tv_sec = 3 };
 
diff --git a/tools/testing/selftests/bpf/network_helpers.h b/tools/testing/selftests/bpf/network_helpers.h
index 7205f8afdba1..5e0d51c07b63 100644
--- a/tools/testing/selftests/bpf/network_helpers.h
+++ b/tools/testing/selftests/bpf/network_helpers.h
@@ -33,6 +33,7 @@ struct ipv6_packet {
 } __packed;
 extern struct ipv6_packet pkt_v6;
 
+int settimeo(int fd, int timeout_ms);
 int start_server(int family, int type, const char *addr, __u16 port,
 		 int timeout_ms);
 int connect_to_fd(int server_fd, int timeout_ms);
diff --git a/tools/testing/selftests/bpf/prog_tests/migrate_reuseport.c b/tools/testing/selftests/bpf/prog_tests/migrate_reuseport.c
new file mode 100644
index 000000000000..0fa3f750567d
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/migrate_reuseport.c
@@ -0,0 +1,555 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Check if we can migrate child sockets.
+ *
+ *   1. call listen() for 4 server sockets.
+ *   2. call connect() for 25 client sockets.
+ *   3. call listen() for 1 server socket. (migration target)
+ *   4. update a map to migrate all child sockets
+ *        to the last server socket (migrate_map[cookie] = 4)
+ *   5. call shutdown() for first 4 server sockets
+ *        and migrate the requests in the accept queue
+ *        to the last server socket.
+ *   6. call listen() for the second server socket.
+ *   7. call shutdown() for the last server
+ *        and migrate the requests in the accept queue
+ *        to the second server socket.
+ *   8. call listen() for the last server.
+ *   9. call shutdown() for the second server
+ *        and migrate the requests in the accept queue
+ *        to the last server socket.
+ *  10. call accept() for the last server socket.
+ *
+ * Author: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
+ */
+
+#include <bpf/bpf.h>
+#include <bpf/libbpf.h>
+
+#include "test_progs.h"
+#include "test_migrate_reuseport.skel.h"
+#include "network_helpers.h"
+
+#define IFINDEX_LO 1
+
+#define NR_SERVERS 5
+#define NR_CLIENTS (NR_SERVERS * 5)
+#define MIGRATED_TO (NR_SERVERS - 1)
+
+/* fastopenq->max_qlen and sk->sk_max_ack_backlog */
+#define QLEN (NR_CLIENTS * 5)
+
+#define MSG "Hello World\0"
+#define MSGLEN 12
+
+static struct migrate_reuseport_test_case {
+	const char *name;
+	__s64 servers[NR_SERVERS];
+	__s64 clients[NR_CLIENTS];
+	struct sockaddr_storage addr;
+	socklen_t addrlen;
+	int family;
+	int state;
+	bool drop_ack;
+	bool expire_synack_timer;
+	bool fastopen;
+	struct bpf_link *link;
+} test_cases[] = {
+	{
+		.name = "IPv4 TCP_ESTABLISHED  inet_csk_listen_stop",
+		.family = AF_INET,
+		.state = BPF_TCP_ESTABLISHED,
+		.drop_ack = false,
+		.expire_synack_timer = false,
+		.fastopen = false,
+	},
+	{
+		.name = "IPv4 TCP_SYN_RECV     inet_csk_listen_stop",
+		.family = AF_INET,
+		.state = BPF_TCP_SYN_RECV,
+		.drop_ack = true,
+		.expire_synack_timer = false,
+		.fastopen = true,
+	},
+	{
+		.name = "IPv4 TCP_NEW_SYN_RECV reqsk_timer_handler",
+		.family = AF_INET,
+		.state = BPF_TCP_NEW_SYN_RECV,
+		.drop_ack = true,
+		.expire_synack_timer = true,
+		.fastopen = false,
+	},
+	{
+		.name = "IPv4 TCP_NEW_SYN_RECV inet_csk_complete_hashdance",
+		.family = AF_INET,
+		.state = BPF_TCP_NEW_SYN_RECV,
+		.drop_ack = true,
+		.expire_synack_timer = false,
+		.fastopen = false,
+	},
+	{
+		.name = "IPv6 TCP_ESTABLISHED  inet_csk_listen_stop",
+		.family = AF_INET6,
+		.state = BPF_TCP_ESTABLISHED,
+		.drop_ack = false,
+		.expire_synack_timer = false,
+		.fastopen = false,
+	},
+	{
+		.name = "IPv6 TCP_SYN_RECV     inet_csk_listen_stop",
+		.family = AF_INET6,
+		.state = BPF_TCP_SYN_RECV,
+		.drop_ack = true,
+		.expire_synack_timer = false,
+		.fastopen = true,
+	},
+	{
+		.name = "IPv6 TCP_NEW_SYN_RECV reqsk_timer_handler",
+		.family = AF_INET6,
+		.state = BPF_TCP_NEW_SYN_RECV,
+		.drop_ack = true,
+		.expire_synack_timer = true,
+		.fastopen = false,
+	},
+	{
+		.name = "IPv6 TCP_NEW_SYN_RECV inet_csk_complete_hashdance",
+		.family = AF_INET6,
+		.state = BPF_TCP_NEW_SYN_RECV,
+		.drop_ack = true,
+		.expire_synack_timer = false,
+		.fastopen = false,
+	}
+};
+
+static void init_fds(__s64 fds[], int len)
+{
+	int i;
+
+	for (i = 0; i < len; i++)
+		fds[i] = -1;
+}
+
+static void close_fds(__s64 fds[], int len)
+{
+	int i;
+
+	for (i = 0; i < len; i++) {
+		if (fds[i] != -1) {
+			close(fds[i]);
+			fds[i] = -1;
+		}
+	}
+}
+
+static int setup_fastopen(char *buf, int size, int *saved_len, bool restore)
+{
+	int err = 0, fd, len;
+
+	fd = open("/proc/sys/net/ipv4/tcp_fastopen", O_RDWR);
+	if (!ASSERT_NEQ(fd, -1, "open"))
+		return -1;
+
+	if (restore) {
+		len = write(fd, buf, *saved_len);
+		if (!ASSERT_EQ(len, *saved_len, "write - restore"))
+			err = -1;
+	} else {
+		*saved_len = read(fd, buf, size);
+		if (!ASSERT_GE(*saved_len, 1, "read")) {
+			err = -1;
+			goto close;
+		}
+
+		err = lseek(fd, 0, SEEK_SET);
+		if (!ASSERT_OK(err, "lseek"))
+			goto close;
+
+		/* (TFO_CLIENT_ENABLE | TFO_SERVER_ENABLE |
+		 *  TFO_CLIENT_NO_COOKIE | TFO_SERVER_COOKIE_NOT_REQD)
+		 */
+		len = write(fd, "519", 3);
+		if (!ASSERT_EQ(len, 3, "write - setup"))
+			err = -1;
+	}
+
+close:
+	close(fd);
+
+	return err;
+}
+
+static int drop_ack(struct migrate_reuseport_test_case *test_case,
+		    struct test_migrate_reuseport *skel)
+{
+	if (test_case->family == AF_INET)
+		skel->bss->server_port = ((struct sockaddr_in *)
+					  &test_case->addr)->sin_port;
+	else
+		skel->bss->server_port = ((struct sockaddr_in6 *)
+					  &test_case->addr)->sin6_port;
+
+	test_case->link = bpf_program__attach_xdp(skel->progs.drop_ack,
+						  IFINDEX_LO);
+	if (!ASSERT_OK_PTR(test_case->link, "bpf_program__attach_xdp"))
+		return -1;
+
+	return 0;
+}
+
+static int pass_ack(struct migrate_reuseport_test_case *test_case)
+{
+	int err;
+
+	err = bpf_link__detach(test_case->link);
+	if (!ASSERT_OK(err, "bpf_link__detach"))
+		return -1;
+
+	test_case->link = NULL;
+
+	return 0;
+}
+
+static int start_servers(struct migrate_reuseport_test_case *test_case,
+			 struct test_migrate_reuseport *skel)
+{
+	int i, err, prog_fd, reuseport = 1, qlen = QLEN;
+
+	prog_fd = bpf_program__fd(skel->progs.migrate_reuseport);
+
+	make_sockaddr(test_case->family,
+		      test_case->family == AF_INET ? "127.0.0.1" : "::1", 0,
+		      &test_case->addr, &test_case->addrlen);
+
+	for (i = 0; i < NR_SERVERS; i++) {
+		test_case->servers[i] = socket(test_case->family, SOCK_STREAM,
+					       IPPROTO_TCP);
+		if (!ASSERT_NEQ(test_case->servers[i], -1, "socket"))
+			return -1;
+
+		err = setsockopt(test_case->servers[i], SOL_SOCKET,
+				 SO_REUSEPORT, &reuseport, sizeof(reuseport));
+		if (!ASSERT_OK(err, "setsockopt - SO_REUSEPORT"))
+			return -1;
+
+		err = bind(test_case->servers[i],
+			   (struct sockaddr *)&test_case->addr,
+			   test_case->addrlen);
+		if (!ASSERT_OK(err, "bind"))
+			return -1;
+
+		if (i == 0) {
+			err = setsockopt(test_case->servers[i], SOL_SOCKET,
+					 SO_ATTACH_REUSEPORT_EBPF,
+					 &prog_fd, sizeof(prog_fd));
+			if (!ASSERT_OK(err,
+				       "setsockopt - SO_ATTACH_REUSEPORT_EBPF"))
+				return -1;
+
+			err = getsockname(test_case->servers[i],
+					  (struct sockaddr *)&test_case->addr,
+					  &test_case->addrlen);
+			if (!ASSERT_OK(err, "getsockname"))
+				return -1;
+		}
+
+		if (test_case->fastopen) {
+			err = setsockopt(test_case->servers[i],
+					 SOL_TCP, TCP_FASTOPEN,
+					 &qlen, sizeof(qlen));
+			if (!ASSERT_OK(err, "setsockopt - TCP_FASTOPEN"))
+				return -1;
+		}
+
+		/* All requests will be tied to the first four listeners */
+		if (i != MIGRATED_TO) {
+			err = listen(test_case->servers[i], qlen);
+			if (!ASSERT_OK(err, "listen"))
+				return -1;
+		}
+	}
+
+	return 0;
+}
+
+static int start_clients(struct migrate_reuseport_test_case *test_case)
+{
+	char buf[MSGLEN] = MSG;
+	int i, err;
+
+	for (i = 0; i < NR_CLIENTS; i++) {
+		test_case->clients[i] = socket(test_case->family, SOCK_STREAM,
+					       IPPROTO_TCP);
+		if (!ASSERT_NEQ(test_case->clients[i], -1, "socket"))
+			return -1;
+
+		/* The attached XDP program drops only the final ACK, so
+		 * clients will transition to TCP_ESTABLISHED immediately.
+		 */
+		err = settimeo(test_case->clients[i], 100);
+		if (!ASSERT_OK(err, "settimeo"))
+			return -1;
+
+		if (test_case->fastopen) {
+			int fastopen = 1;
+
+			err = setsockopt(test_case->clients[i], IPPROTO_TCP,
+					 TCP_FASTOPEN_CONNECT, &fastopen,
+					 sizeof(fastopen));
+			if (!ASSERT_OK(err,
+				       "setsockopt - TCP_FASTOPEN_CONNECT"))
+				return -1;
+		}
+
+		err = connect(test_case->clients[i],
+			      (struct sockaddr *)&test_case->addr,
+			      test_case->addrlen);
+		if (!ASSERT_OK(err, "connect"))
+			return -1;
+
+		err = write(test_case->clients[i], buf, MSGLEN);
+		if (!ASSERT_EQ(err, MSGLEN, "write"))
+			return -1;
+	}
+
+	return 0;
+}
+
+static int update_maps(struct migrate_reuseport_test_case *test_case,
+		       struct test_migrate_reuseport *skel)
+{
+	int i, err, migrated_to = MIGRATED_TO;
+	int reuseport_map_fd, migrate_map_fd;
+	__u64 value;
+
+	reuseport_map_fd = bpf_map__fd(skel->maps.reuseport_map);
+	migrate_map_fd = bpf_map__fd(skel->maps.migrate_map);
+
+	for (i = 0; i < NR_SERVERS; i++) {
+		value = (__u64)test_case->servers[i];
+		err = bpf_map_update_elem(reuseport_map_fd, &i, &value,
+					  BPF_NOEXIST);
+		if (!ASSERT_OK(err, "bpf_map_update_elem - reuseport_map"))
+			return -1;
+
+		err = bpf_map_lookup_elem(reuseport_map_fd, &i, &value);
+		if (!ASSERT_OK(err, "bpf_map_lookup_elem - reuseport_map"))
+			return -1;
+
+		err = bpf_map_update_elem(migrate_map_fd, &value, &migrated_to,
+					  BPF_NOEXIST);
+		if (!ASSERT_OK(err, "bpf_map_update_elem - migrate_map"))
+			return -1;
+	}
+
+	return 0;
+}
+
+static int migrate_dance(struct migrate_reuseport_test_case *test_case)
+{
+	int i, err;
+
+	/* Migrate TCP_ESTABLISHED and TCP_SYN_RECV requests
+	 * to the last listener based on eBPF.
+	 */
+	for (i = 0; i < MIGRATED_TO; i++) {
+		err = shutdown(test_case->servers[i], SHUT_RDWR);
+		if (!ASSERT_OK(err, "shutdown"))
+			return -1;
+	}
+
+	/* No dance for TCP_NEW_SYN_RECV to migrate based on eBPF */
+	if (test_case->state == BPF_TCP_NEW_SYN_RECV)
+		return 0;
+
+	/* Note that we use the second listener instead of the
+	 * first one here.
+	 *
+	 * The fist listener is bind()ed with port 0 and,
+	 * SOCK_BINDPORT_LOCK is not set to sk_userlocks, so
+	 * calling listen() again will bind() the first listener
+	 * on a new ephemeral port and detach it from the existing
+	 * reuseport group.  (See: __inet_bind(), tcp_set_state())
+	 *
+	 * OTOH, the second one is bind()ed with a specific port,
+	 * and SOCK_BINDPORT_LOCK is set. Thus, re-listen() will
+	 * resurrect the listener on the existing reuseport group.
+	 */
+	err = listen(test_case->servers[1], QLEN);
+	if (!ASSERT_OK(err, "listen"))
+		return -1;
+
+	/* Migrate from the last listener to the second one.
+	 *
+	 * All listeners were detached out of the reuseport_map,
+	 * so migration will be done by kernel random pick from here.
+	 */
+	err = shutdown(test_case->servers[MIGRATED_TO], SHUT_RDWR);
+	if (!ASSERT_OK(err, "shutdown"))
+		return -1;
+
+	/* Back to the existing reuseport group */
+	err = listen(test_case->servers[MIGRATED_TO], QLEN);
+	if (!ASSERT_OK(err, "listen"))
+		return -1;
+
+	/* Migrate back to the last one from the second one */
+	err = shutdown(test_case->servers[1], SHUT_RDWR);
+	if (!ASSERT_OK(err, "shutdown"))
+		return -1;
+
+	return 0;
+}
+
+static void count_requests(struct migrate_reuseport_test_case *test_case,
+			   struct test_migrate_reuseport *skel)
+{
+	struct sockaddr_storage addr;
+	socklen_t len = sizeof(addr);
+	int err, cnt = 0, client;
+	char buf[MSGLEN];
+
+	err = settimeo(test_case->servers[MIGRATED_TO], 4000);
+	if (!ASSERT_OK(err, "settimeo"))
+		goto out;
+
+	for (; cnt < NR_CLIENTS; cnt++) {
+		client = accept(test_case->servers[MIGRATED_TO],
+				(struct sockaddr *)&addr, &len);
+		if (!ASSERT_NEQ(client, -1, "accept"))
+			goto out;
+
+		memset(buf, 0, MSGLEN);
+		read(client, &buf, MSGLEN);
+		close(client);
+
+		if (!ASSERT_STREQ(buf, MSG, "read"))
+			goto out;
+	}
+
+out:
+	ASSERT_EQ(cnt, NR_CLIENTS, "count in userspace");
+
+	switch (test_case->state) {
+	case BPF_TCP_ESTABLISHED:
+		cnt = skel->bss->migrated_at_close;
+		break;
+	case BPF_TCP_SYN_RECV:
+		cnt = skel->bss->migrated_at_close_fastopen;
+		break;
+	case BPF_TCP_NEW_SYN_RECV:
+		if (test_case->expire_synack_timer)
+			cnt = skel->bss->migrated_at_send_synack;
+		else
+			cnt = skel->bss->migrated_at_recv_ack;
+		break;
+	default:
+		cnt = 0;
+	}
+
+	ASSERT_EQ(cnt, NR_CLIENTS, "count in BPF prog");
+}
+
+static void run_test(struct migrate_reuseport_test_case *test_case,
+		     struct test_migrate_reuseport *skel)
+{
+	int err, saved_len;
+	char buf[16];
+
+	skel->bss->migrated_at_close = 0;
+	skel->bss->migrated_at_close_fastopen = 0;
+	skel->bss->migrated_at_send_synack = 0;
+	skel->bss->migrated_at_recv_ack = 0;
+
+	init_fds(test_case->servers, NR_SERVERS);
+	init_fds(test_case->clients, NR_CLIENTS);
+
+	if (test_case->fastopen) {
+		memset(buf, 0, sizeof(buf));
+
+		err = setup_fastopen(buf, sizeof(buf), &saved_len, false);
+		if (!ASSERT_OK(err, "setup_fastopen - setup"))
+			return;
+	}
+
+	err = start_servers(test_case, skel);
+	if (!ASSERT_OK(err, "start_servers"))
+		goto close_servers;
+
+	if (test_case->drop_ack) {
+		/* Drop the final ACK of the 3-way handshake and stick the
+		 * in-flight requests on TCP_SYN_RECV or TCP_NEW_SYN_RECV.
+		 */
+		err = drop_ack(test_case, skel);
+		if (!ASSERT_OK(err, "drop_ack"))
+			goto close_servers;
+	}
+
+	/* Tie requests to the first four listners */
+	err = start_clients(test_case);
+	if (!ASSERT_OK(err, "start_clients"))
+		goto close_clients;
+
+	err = listen(test_case->servers[MIGRATED_TO], QLEN);
+	if (!ASSERT_OK(err, "listen"))
+		goto close_clients;
+
+	err = update_maps(test_case, skel);
+	if (!ASSERT_OK(err, "fill_maps"))
+		goto close_clients;
+
+	/* Migrate the requests in the accept queue only.
+	 * TCP_NEW_SYN_RECV requests are not migrated at this point.
+	 */
+	err = migrate_dance(test_case);
+	if (!ASSERT_OK(err, "migrate_dance"))
+		goto close_clients;
+
+	if (test_case->expire_synack_timer) {
+		/* Wait for SYN+ACK timers to expire so that
+		 * reqsk_timer_handler() migrates TCP_NEW_SYN_RECV requests.
+		 */
+		sleep(1);
+	}
+
+	if (test_case->link) {
+		/* Resume 3WHS and migrate TCP_NEW_SYN_RECV requests */
+		err = pass_ack(test_case);
+		if (!ASSERT_OK(err, "pass_ack"))
+			goto close_clients;
+	}
+
+	count_requests(test_case, skel);
+
+close_clients:
+	close_fds(test_case->clients, NR_CLIENTS);
+
+	if (test_case->link) {
+		err = pass_ack(test_case);
+		ASSERT_OK(err, "pass_ack - clean up");
+	}
+
+close_servers:
+	close_fds(test_case->servers, NR_SERVERS);
+
+	if (test_case->fastopen) {
+		err = setup_fastopen(buf, sizeof(buf), &saved_len, true);
+		ASSERT_OK(err, "setup_fastopen - restore");
+	}
+}
+
+void test_migrate_reuseport(void)
+{
+	struct test_migrate_reuseport *skel;
+	int i;
+
+	skel = test_migrate_reuseport__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "open_and_load"))
+		return;
+
+	for (i = 0; i < ARRAY_SIZE(test_cases); i++) {
+		test__start_subtest(test_cases[i].name);
+		run_test(&test_cases[i], skel);
+	}
+
+	test_migrate_reuseport__destroy(skel);
+}
diff --git a/tools/testing/selftests/bpf/progs/test_migrate_reuseport.c b/tools/testing/selftests/bpf/progs/test_migrate_reuseport.c
new file mode 100644
index 000000000000..27df571abf5b
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_migrate_reuseport.c
@@ -0,0 +1,135 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Check if we can migrate child sockets.
+ *
+ *   1. If reuse_md->migrating_sk is NULL (SYN packet),
+ *        return SK_PASS without selecting a listener.
+ *   2. If reuse_md->migrating_sk is not NULL (socket migration),
+ *        select a listener (reuseport_map[migrate_map[cookie]])
+ *
+ * Author: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
+ */
+
+#include <stddef.h>
+#include <string.h>
+#include <linux/bpf.h>
+#include <linux/if_ether.h>
+#include <linux/ip.h>
+#include <linux/ipv6.h>
+#include <linux/tcp.h>
+#include <linux/in.h>
+#include <bpf/bpf_endian.h>
+#include <bpf/bpf_helpers.h>
+
+struct {
+	__uint(type, BPF_MAP_TYPE_REUSEPORT_SOCKARRAY);
+	__uint(max_entries, 256);
+	__type(key, int);
+	__type(value, __u64);
+} reuseport_map SEC(".maps");
+
+struct {
+	__uint(type, BPF_MAP_TYPE_HASH);
+	__uint(max_entries, 256);
+	__type(key, __u64);
+	__type(value, int);
+} migrate_map SEC(".maps");
+
+int migrated_at_close = 0;
+int migrated_at_close_fastopen = 0;
+int migrated_at_send_synack = 0;
+int migrated_at_recv_ack = 0;
+__be16 server_port;
+
+SEC("xdp")
+int drop_ack(struct xdp_md *xdp)
+{
+	void *data_end = (void *)(long)xdp->data_end;
+	void *data = (void *)(long)xdp->data;
+	struct ethhdr *eth = data;
+	struct tcphdr *tcp = NULL;
+
+	if (eth + 1 > data_end)
+		goto pass;
+
+	switch (bpf_ntohs(eth->h_proto)) {
+	case ETH_P_IP: {
+		struct iphdr *ip = (struct iphdr *)(eth + 1);
+
+		if (ip + 1 > data_end)
+			goto pass;
+
+		if (ip->protocol != IPPROTO_TCP)
+			goto pass;
+
+		tcp = (struct tcphdr *)((void *)ip + ip->ihl * 4);
+		break;
+	}
+	case ETH_P_IPV6: {
+		struct ipv6hdr *ipv6 = (struct ipv6hdr *)(eth + 1);
+
+		if (ipv6 + 1 > data_end)
+			goto pass;
+
+		if (ipv6->nexthdr != IPPROTO_TCP)
+			goto pass;
+
+		tcp = (struct tcphdr *)(ipv6 + 1);
+		break;
+	}
+	default:
+		goto pass;
+	}
+
+	if (tcp + 1 > data_end)
+		goto pass;
+
+	if (tcp->dest != server_port)
+		goto pass;
+
+	if (!tcp->syn && tcp->ack)
+		return XDP_DROP;
+
+pass:
+	return XDP_PASS;
+}
+
+SEC("sk_reuseport/migrate")
+int migrate_reuseport(struct sk_reuseport_md *reuse_md)
+{
+	int *key, flags = 0, state, err;
+	__u64 cookie;
+
+	if (!reuse_md->migrating_sk)
+		return SK_PASS;
+
+	state = reuse_md->migrating_sk->state;
+	cookie = bpf_get_socket_cookie(reuse_md->sk);
+
+	key = bpf_map_lookup_elem(&migrate_map, &cookie);
+	if (!key)
+		return SK_DROP;
+
+	err = bpf_sk_select_reuseport(reuse_md, &reuseport_map, key, flags);
+	if (err)
+		return SK_PASS;
+
+	switch (state) {
+	case BPF_TCP_ESTABLISHED:
+		__sync_fetch_and_add(&migrated_at_close, 1);
+		break;
+	case BPF_TCP_SYN_RECV:
+		__sync_fetch_and_add(&migrated_at_close_fastopen, 1);
+		break;
+	case BPF_TCP_NEW_SYN_RECV:
+		if (!reuse_md->len)
+			__sync_fetch_and_add(&migrated_at_send_synack, 1);
+		else
+			__sync_fetch_and_add(&migrated_at_recv_ack, 1);
+		break;
+	}
+
+	return SK_PASS;
+}
+
+char _license[] SEC("license") = "GPL";