Message-Id: <20251022-work-namespace-nstree-listns-v2-61-71a588572371@kernel.org>
Date: Wed, 22 Oct 2025 18:06:39 +0200
From: Christian Brauner <brauner@...nel.org>
To: linux-fsdevel@...r.kernel.org, Josef Bacik <josef@...icpanda.com>, 
 Jeff Layton <jlayton@...nel.org>
Cc: Jann Horn <jannh@...gle.com>, Mike Yuan <me@...dnzj.com>, 
 Zbigniew Jędrzejewski-Szmek <zbyszek@...waw.pl>, 
 Lennart Poettering <mzxreary@...inter.de>, 
 Daan De Meyer <daan.j.demeyer@...il.com>, Aleksa Sarai <cyphar@...har.com>, 
 Amir Goldstein <amir73il@...il.com>, Tejun Heo <tj@...nel.org>, 
 Johannes Weiner <hannes@...xchg.org>, Thomas Gleixner <tglx@...utronix.de>, 
 Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>, 
 linux-kernel@...r.kernel.org, cgroups@...r.kernel.org, bpf@...r.kernel.org, 
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
 netdev@...r.kernel.org, Arnd Bergmann <arnd@...db.de>, 
 Christian Brauner <brauner@...nel.org>
Subject: [PATCH v2 61/63] selftests/namespaces: tenth inactive namespace
 resurrection test

Test that socket-kept netns can be reopened via file handle.
Verify that a network namespace kept alive by a socket FD can be
reopened using file handles even after the creating process exits.

Signed-off-by: Christian Brauner <brauner@...nel.org>
---
 .../testing/selftests/namespaces/siocgskns_test.c  | 203 +++++++++++++++++++++
 1 file changed, 203 insertions(+)

diff --git a/tools/testing/selftests/namespaces/siocgskns_test.c b/tools/testing/selftests/namespaces/siocgskns_test.c
index 7b3133f3898c..0ea5241f2b6f 100644
--- a/tools/testing/selftests/namespaces/siocgskns_test.c
+++ b/tools/testing/selftests/namespaces/siocgskns_test.c
@@ -781,4 +781,207 @@ TEST(siocgskns_listns_visibility)
 	TH_LOG("Netns %llu correctly disappeared from listns() after socket closed", netns_id);
 }
 
+/*
+ * Test that socket-kept netns can be reopened via file handle.
+ * Verify that a network namespace kept alive by a socket FD can be
+ * reopened using file handles even after the creating process exits.
+ */
+TEST(siocgskns_file_handle)
+{
+	int sock_fd, netns_fd, reopened_fd;
+	int ipc_sockets[2];
+	pid_t pid;
+	int status;
+	struct stat st1, st2;
+	ino_t netns_ino;
+	__u64 netns_id;
+	struct file_handle *handle;
+	struct nsfs_file_handle *nsfs_fh;
+	int ret;
+
+	/* Allocate file_handle structure for nsfs */
+	handle = malloc(sizeof(struct file_handle) + sizeof(struct nsfs_file_handle));
+	ASSERT_NE(handle, NULL);
+	handle->handle_bytes = sizeof(struct nsfs_file_handle);
+	handle->handle_type = FILEID_NSFS;
+
+	EXPECT_EQ(socketpair(AF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0, ipc_sockets), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/* Child: create new netns and socket */
+		close(ipc_sockets[0]);
+
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		sock_fd = socket(AF_INET, SOCK_DGRAM, 0);
+		if (sock_fd < 0) {
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		/* Send socket FD to parent via SCM_RIGHTS */
+		struct msghdr msg = {0};
+		struct iovec iov = {0};
+		char buf[1] = {'X'};
+		char cmsg_buf[CMSG_SPACE(sizeof(int))];
+
+		iov.iov_base = buf;
+		iov.iov_len = 1;
+		msg.msg_iov = &iov;
+		msg.msg_iovlen = 1;
+		msg.msg_control = cmsg_buf;
+		msg.msg_controllen = sizeof(cmsg_buf);
+
+		struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+		cmsg->cmsg_level = SOL_SOCKET;
+		cmsg->cmsg_type = SCM_RIGHTS;
+		cmsg->cmsg_len = CMSG_LEN(sizeof(int));
+		memcpy(CMSG_DATA(cmsg), &sock_fd, sizeof(int));
+
+		if (sendmsg(ipc_sockets[1], &msg, 0) < 0) {
+			close(sock_fd);
+			close(ipc_sockets[1]);
+			exit(1);
+		}
+
+		close(sock_fd);
+		close(ipc_sockets[1]);
+		exit(0);
+	}
+
+	/* Parent: receive socket FD */
+	close(ipc_sockets[1]);
+
+	struct msghdr msg = {0};
+	struct iovec iov = {0};
+	char buf[1];
+	char cmsg_buf[CMSG_SPACE(sizeof(int))];
+
+	iov.iov_base = buf;
+	iov.iov_len = 1;
+	msg.msg_iov = &iov;
+	msg.msg_iovlen = 1;
+	msg.msg_control = cmsg_buf;
+	msg.msg_controllen = sizeof(cmsg_buf);
+
+	ssize_t n = recvmsg(ipc_sockets[0], &msg, 0);
+	close(ipc_sockets[0]);
+	ASSERT_EQ(n, 1);
+
+	struct cmsghdr *cmsg = CMSG_FIRSTHDR(&msg);
+	ASSERT_NE(cmsg, NULL);
+	memcpy(&sock_fd, CMSG_DATA(cmsg), sizeof(int));
+
+	/* Wait for child to exit */
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	if (WEXITSTATUS(status) != 0) {
+		free(handle);
+		SKIP(close(sock_fd); return, "Child failed to create namespace");
+	}
+
+	/* Get network namespace from socket */
+	netns_fd = ioctl(sock_fd, SIOCGSKNS);
+	if (netns_fd < 0) {
+		free(handle);
+		close(sock_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "SIOCGSKNS not supported");
+		ASSERT_GE(netns_fd, 0);
+	}
+
+	ASSERT_EQ(fstat(netns_fd, &st1), 0);
+	netns_ino = st1.st_ino;
+
+	/* Get namespace ID */
+	ret = ioctl(netns_fd, NS_GET_ID, &netns_id);
+	if (ret < 0) {
+		free(handle);
+		close(sock_fd);
+		close(netns_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "NS_GET_ID not supported");
+		ASSERT_EQ(ret, 0);
+	}
+
+	/* Construct file handle from namespace ID */
+	nsfs_fh = (struct nsfs_file_handle *)handle->f_handle;
+	nsfs_fh->ns_id = netns_id;
+	nsfs_fh->ns_type = 0;  /* Type field not needed for reopening */
+	nsfs_fh->ns_inum = 0;  /* Inum field not needed for reopening */
+
+	TH_LOG("Constructed file handle for netns %lu (id=%llu)", netns_ino, netns_id);
+
+	/* Reopen namespace using file handle (while socket still keeps it alive) */
+	reopened_fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	if (reopened_fd < 0) {
+		free(handle);
+		close(sock_fd);
+		if (errno == EOPNOTSUPP || errno == ENOSYS || errno == EBADF)
+			SKIP(return, "open_by_handle_at with FD_NSFS_ROOT not supported");
+		TH_LOG("open_by_handle_at failed: %s", strerror(errno));
+		ASSERT_GE(reopened_fd, 0);
+	}
+
+	/* Verify it's the same namespace */
+	ASSERT_EQ(fstat(reopened_fd, &st2), 0);
+	ASSERT_EQ(st1.st_ino, st2.st_ino);
+	ASSERT_EQ(st1.st_dev, st2.st_dev);
+
+	TH_LOG("Successfully reopened netns %lu via file handle", netns_ino);
+
+	close(reopened_fd);
+
+	/* Close the netns FD */
+	close(netns_fd);
+
+	/* Try to reopen via file handle - should fail since namespace is now inactive */
+	reopened_fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	if (reopened_fd >= 0) {
+		/* This might succeed if there's still a reference somewhere */
+		TH_LOG("Warning: reopened netns after socket closed (fd=%d)", reopened_fd);
+		close(reopened_fd);
+	} else {
+		TH_LOG("Correctly failed to reopen inactive netns: %s", strerror(errno));
+	}
+
+	/* Get network namespace from socket */
+	netns_fd = ioctl(sock_fd, SIOCGSKNS);
+	if (netns_fd < 0) {
+		free(handle);
+		close(sock_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "SIOCGSKNS not supported");
+		ASSERT_GE(netns_fd, 0);
+	}
+
+	/* Reopen namespace using file handle (while socket still keeps it alive) */
+	reopened_fd = open_by_handle_at(FD_NSFS_ROOT, handle, O_RDONLY);
+	if (reopened_fd < 0) {
+		free(handle);
+		close(sock_fd);
+		if (errno == EOPNOTSUPP || errno == ENOSYS || errno == EBADF)
+			SKIP(return, "open_by_handle_at with FD_NSFS_ROOT not supported");
+		TH_LOG("open_by_handle_at failed: %s", strerror(errno));
+		ASSERT_GE(reopened_fd, 0);
+	}
+
+	/* Verify it's the same namespace */
+	ASSERT_EQ(fstat(reopened_fd, &st2), 0);
+	ASSERT_EQ(st1.st_ino, st2.st_ino);
+	ASSERT_EQ(st1.st_dev, st2.st_dev);
+
+	TH_LOG("Successfully reopened netns %lu via file handle", netns_ino);
+
+	/* Close socket - namespace should become inactive */
+	close(sock_fd);
+	free(handle);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3

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