Re: [PATCH net v2 2/2] selftests/bpf: check epoll readiness after reuseport migration

[Kuniyuki Iwashima <kuniyu@google.com>]

From: Zhenzhong Wu <jt26wzz@gmail.com>
Date: Sun, 19 Apr 2026 02:13:33 +0800
> After migrate_dance() moves established children to the target
> listener, add it to an epoll set and verify that epoll_wait(..., 0)
> reports it ready before accept().
> 
> This adds epoll coverage for the TCP_ESTABLISHED reuseport migration
> case in migrate_reuseport.
> 
> Keep the check limited to TCP_ESTABLISHED cases. TCP_SYN_RECV and
> TCP_NEW_SYN_RECV still depend on asynchronous handshake completion,
> so a zero-timeout epoll_wait() would race there.
> 
> Signed-off-by: Zhenzhong Wu <jt26wzz@gmail.com>
> ---
>  .../bpf/prog_tests/migrate_reuseport.c        | 32 ++++++++++++++++++-
>  1 file changed, 31 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/migrate_reuseport.c b/tools/testing/selftests/bpf/prog_tests/migrate_reuseport.c
> index 653b0a20f..580a53424 100644
> --- a/tools/testing/selftests/bpf/prog_tests/migrate_reuseport.c
> +++ b/tools/testing/selftests/bpf/prog_tests/migrate_reuseport.c
> @@ -18,13 +18,16 @@
>   *   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.
> + *  10. for TCP_ESTABLISHED cases, call epoll_wait(..., 0)
> + *        for the last server socket.
> + *  11. call accept() for the last server socket.
>   *
>   * Author: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
>   */
>  
>  #include <bpf/bpf.h>
>  #include <bpf/libbpf.h>
> +#include <sys/epoll.h>
>  
>  #include "test_progs.h"
>  #include "test_migrate_reuseport.skel.h"
> @@ -522,6 +525,33 @@ static void run_test(struct migrate_reuseport_test_case *test_case,
>  			goto close_clients;
>  	}
>  
> +	/* Only TCP_ESTABLISHED has already-migrated accept-queue entries
> +	 * here.  Later states still depend on follow-up handshake work.
> +	 */
> +	if (test_case->state == BPF_TCP_ESTABLISHED) {
> +		struct epoll_event ev = {
> +			.events = EPOLLIN,
> +		};
> +		int epfd;
> +		int nfds;
> +
> +		epfd = epoll_create1(EPOLL_CLOEXEC);
> +		if (!ASSERT_NEQ(epfd, -1, "epoll_create1"))
> +			goto close_clients;
> +
> +		ev.data.fd = test_case->servers[MIGRATED_TO];
> +		if (!ASSERT_OK(epoll_ctl(epfd, EPOLL_CTL_ADD,
> +					 test_case->servers[MIGRATED_TO], &ev),
> +			       "epoll_ctl"))
> +			goto close_epfd;
> +
> +		nfds = epoll_wait(epfd, &ev, 1, 0);
> +		ASSERT_EQ(nfds, 1, "epoll_wait");

Thanks for the update, but the test passes without patch 1.

I think it would be best to test just after shutdown()
where migration happens.

Also, TCP_SYN_RECV should be covered in the same way.

---8<---
diff --git a/tools/testing/selftests/bpf/prog_tests/migrate_reuseport.c b/tools/testing/selftests/bpf/prog_tests/migrate_reuseport.c
index 580a534249a7..66fea936649e 100644
--- a/tools/testing/selftests/bpf/prog_tests/migrate_reuseport.c
+++ b/tools/testing/selftests/bpf/prog_tests/migrate_reuseport.c
@@ -353,8 +353,29 @@ static int update_maps(struct migrate_reuseport_test_case *test_case,
 
 static int migrate_dance(struct migrate_reuseport_test_case *test_case)
 {
+	struct epoll_event ev = {
+		.events = EPOLLIN,
+	};
+	int epoll, nfds;
 	int i, err;
 
+	if (test_case->state != BPF_TCP_NEW_SYN_RECV) {
+		epoll = epoll_create1(0);
+		if (!ASSERT_NEQ(epoll, -1, "epoll_create1"))
+			return -1;
+
+		ev.data.fd = test_case->servers[MIGRATED_TO];
+		if (!ASSERT_OK(epoll_ctl(epoll, EPOLL_CTL_ADD,
+					 test_case->servers[MIGRATED_TO], &ev),
+			       "epoll_ctl")) {
+			goto close_epoll;
+		}
+
+		nfds = epoll_wait(epoll, &ev, 1, 0);
+		if (!ASSERT_EQ(nfds, 0, "epoll_wait 1"))
+			goto close_epoll;
+	}
+
 	/* Migrate TCP_ESTABLISHED and TCP_SYN_RECV requests
 	 * to the last listener based on eBPF.
 	 */
@@ -368,6 +389,15 @@ static int migrate_dance(struct migrate_reuseport_test_case *test_case)
 	if (test_case->state == BPF_TCP_NEW_SYN_RECV)
 		return 0;
 
+	nfds = epoll_wait(epoll, &ev, 1, 0);
+	if (!ASSERT_EQ(nfds, 1, "epoll_wait 2")) {
+close_epoll:
+		close(epoll);
+		return -1;
+	}
+
+	close(epoll);
+
 	/* Note that we use the second listener instead of the
 	 * first one here.
 	 *
@@ -525,33 +555,6 @@ static void run_test(struct migrate_reuseport_test_case *test_case,
 			goto close_clients;
 	}
 
-	/* Only TCP_ESTABLISHED has already-migrated accept-queue entries
-	 * here.  Later states still depend on follow-up handshake work.
-	 */
-	if (test_case->state == BPF_TCP_ESTABLISHED) {
-		struct epoll_event ev = {
-			.events = EPOLLIN,
-		};
-		int epfd;
-		int nfds;
-
-		epfd = epoll_create1(EPOLL_CLOEXEC);
-		if (!ASSERT_NEQ(epfd, -1, "epoll_create1"))
-			goto close_clients;
-
-		ev.data.fd = test_case->servers[MIGRATED_TO];
-		if (!ASSERT_OK(epoll_ctl(epfd, EPOLL_CTL_ADD,
-					 test_case->servers[MIGRATED_TO], &ev),
-			       "epoll_ctl"))
-			goto close_epfd;
-
-		nfds = epoll_wait(epfd, &ev, 1, 0);
-		ASSERT_EQ(nfds, 1, "epoll_wait");
-
-close_epfd:
-		close(epfd);
-	}
-
 	count_requests(test_case, skel);
 
 close_clients:
---8<---