selftests/namespaces: first inactive namespace resurrection test

Test basic SIOCGSKNS functionality. Create a socket and verify SIOCGSKNS
returns the correct network namespace.

Link: https://patch.msgid.link/20251029-work-namespace-nstree-listns-v4-55-2e6f823ebdc0@kernel.org
Tested-by: syzbot@syzkaller.appspotmail.com
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

tools/testing/selftests/namespaces/.gitignore

@@ -4,3 +4,4 @@ init_ino_test
 ns_active_ref_test
 listns_test
 listns_permissions_test
+siocgskns_test

tools/testing/selftests/namespaces/Makefile

@@ -2,11 +2,18 @@
 CFLAGS += -Wall -O0 -g $(KHDR_INCLUDES) $(TOOLS_INCLUDES)
 LDLIBS += -lcap
 
-TEST_GEN_PROGS := nsid_test file_handle_test init_ino_test ns_active_ref_test listns_test listns_permissions_test
+TEST_GEN_PROGS := nsid_test \
+		  file_handle_test \
+		  init_ino_test \
+		  ns_active_ref_test \
+		  listns_test \
+		  listns_permissions_test \
+		  siocgskns_test
 
 include ../lib.mk
 
 $(OUTPUT)/ns_active_ref_test: ../filesystems/utils.c
 $(OUTPUT)/listns_test: ../filesystems/utils.c
 $(OUTPUT)/listns_permissions_test: ../filesystems/utils.c
+$(OUTPUT)/siocgskns_test: ../filesystems/utils.c
 

tools/testing/selftests/namespaces/siocgskns_test.c

@@ -0,0 +1,72 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/ioctl.h>
+#include <sys/socket.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include <linux/if.h>
+#include <linux/sockios.h>
+#include <linux/nsfs.h>
+#include <arpa/inet.h>
+#include "../kselftest_harness.h"
+#include "../filesystems/utils.h"
+#include "wrappers.h"
+
+#ifndef SIOCGSKNS
+#define SIOCGSKNS 0x894C
+#endif
+
+#ifndef FD_NSFS_ROOT
+#define FD_NSFS_ROOT -10003
+#endif
+
+#ifndef FILEID_NSFS
+#define FILEID_NSFS 0xf1
+#endif
+
+/*
+ * Test basic SIOCGSKNS functionality.
+ * Create a socket and verify SIOCGSKNS returns the correct network namespace.
+ */
+TEST(siocgskns_basic)
+{
+	int sock_fd, netns_fd, current_netns_fd;
+	struct stat st1, st2;
+
+	/* Create a TCP socket */
+	sock_fd = socket(AF_INET, SOCK_STREAM, 0);
+	ASSERT_GE(sock_fd, 0);
+
+	/* Use SIOCGSKNS to get network namespace */
+	netns_fd = ioctl(sock_fd, SIOCGSKNS);
+	if (netns_fd < 0) {
+		close(sock_fd);
+		if (errno == ENOTTY || errno == EINVAL)
+			SKIP(return, "SIOCGSKNS not supported");
+		ASSERT_GE(netns_fd, 0);
+	}
+
+	/* Get current network namespace */
+	current_netns_fd = open("/proc/self/ns/net", O_RDONLY);
+	ASSERT_GE(current_netns_fd, 0);
+
+	/* Verify they match */
+	ASSERT_EQ(fstat(netns_fd, &st1), 0);
+	ASSERT_EQ(fstat(current_netns_fd, &st2), 0);
+	ASSERT_EQ(st1.st_ino, st2.st_ino);
+
+	close(sock_fd);
+	close(netns_fd);
+	close(current_netns_fd);
+}
+
+TEST_HARNESS_MAIN