net: add an ioctl to get a socket network namespace

Each socket operates in a network namespace where it has been created,
so if we want to dump and restore a socket, we have to know its network
namespace.

We have a socket_diag to get information about sockets, it doesn't
report sockets which are not bound or connected.

This patch introduces a new socket ioctl, which is called SIOCGSKNS
and used to get a file descriptor for a socket network namespace.

A task must have CAP_NET_ADMIN in a target network namespace to
use this ioctl.

Cc: "David S. Miller" <[e-mail]>
Cc: Eric W. Biederman <[e-mail]>
Signed-off-by: Andrei Vagin <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

fs/nsfs.c

···
 	return ret;
 }
 
-static int open_related_ns(struct ns_common *ns,
+int open_related_ns(struct ns_common *ns,
 		   struct ns_common *(*get_ns)(struct ns_common *ns))
 {
 	struct path path = {};

include/linux/proc_fs.h

···
 	return proc_mkdir_data(name, 0, parent, net);
 }
 
+struct ns_common;
+int open_related_ns(struct ns_common *ns,
+		   struct ns_common *(*get_ns)(struct ns_common *ns));
+
 #endif /* _LINUX_PROC_FS_H */

include/uapi/linux/sockios.h

···
 #define SIOCWANDEV	0x894A		/* get/set netdev parameters	*/
 
 #define SIOCOUTQNSD	0x894B		/* output queue size (not sent only) */
+#define SIOCGSKNS	0x894C		/* get socket network namespace */
 
 /* ARP cache control calls. */
 		    /*  0x8950 - 0x8952  * obsolete calls, don't re-use */

net/socket.c

···
  *	what to do with it - that's up to the protocol still.
  */
 
+static struct ns_common *get_net_ns(struct ns_common *ns)
+{
+	return &get_net(container_of(ns, struct net, ns))->ns;
+}
+
 static long sock_ioctl(struct file *file, unsigned cmd, unsigned long arg)
 {
 	struct socket *sock;
···
 			if (dlci_ioctl_hook)
 				err = dlci_ioctl_hook(cmd, argp);
 			mutex_unlock(&dlci_ioctl_mutex);
+			break;
+		case SIOCGSKNS:
+			err = -EPERM;
+			if (!ns_capable(net->user_ns, CAP_NET_ADMIN))
+				break;
+
+			err = open_related_ns(&net->ns, get_net_ns);
 			break;
 		default:
 			err = sock_do_ioctl(net, sock, cmd, arg);
···
 	case SIOCSIFVLAN:
 	case SIOCADDDLCI:
 	case SIOCDELDLCI:
+	case SIOCGSKNS:
 		return sock_ioctl(file, cmd, arg);
 
 	case SIOCGIFFLAGS: