[NET]: netlink support for moving devices between network namespaces.

The simplest thing to implement is moving network devices between
namespaces.  However with the same attribute IFLA_NET_NS_PID we can
easily implement creating devices in the destination network
namespace as well.  However that is a little bit trickier so this
patch sticks to what is simple and easy.

A pid is used to identify a process that happens to be a member
of the network namespace we want to move the network device to.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/if_link.h

@@ -78,6 +78,7 @@ enum
 	IFLA_LINKMODE,
 	IFLA_LINKINFO,
 #define IFLA_LINKINFO IFLA_LINKINFO
+	IFLA_NET_NS_PID,
 	__IFLA_MAX
 };
 

net/core/rtnetlink.c

@@ -35,6 +35,7 @@
 #include <linux/security.h>
 #include <linux/mutex.h>
 #include <linux/if_addr.h>
+#include <linux/nsproxy.h>
 
 #include <asm/uaccess.h>
 #include <asm/system.h>
@@ -727,19 +728,53 @@ const struct nla_policy ifla_policy[IFLA_MAX+1] = {
 	[IFLA_WEIGHT]		= { .type = NLA_U32 },
 	[IFLA_OPERSTATE]	= { .type = NLA_U8 },
 	[IFLA_LINKMODE]		= { .type = NLA_U8 },
+	[IFLA_NET_NS_PID]	= { .type = NLA_U32 },
 };
 
 static const struct nla_policy ifla_info_policy[IFLA_INFO_MAX+1] = {
 	[IFLA_INFO_KIND]	= { .type = NLA_STRING },
 	[IFLA_INFO_DATA]	= { .type = NLA_NESTED },
 };
 
+static struct net *get_net_ns_by_pid(pid_t pid)
+{
+	struct task_struct *tsk;
+	struct net *net;
+
+	/* Lookup the network namespace */
+	net = ERR_PTR(-ESRCH);
+	rcu_read_lock();
+	tsk = find_task_by_pid(pid);
+	if (tsk) {
+		task_lock(tsk);
+		if (tsk->nsproxy)
+			net = get_net(tsk->nsproxy->net_ns);
+		task_unlock(tsk);
+	}
+	rcu_read_unlock();
+	return net;
+}
+
 static int do_setlink(struct net_device *dev, struct ifinfomsg *ifm,
 		      struct nlattr **tb, char *ifname, int modified)
 {
 	int send_addr_notify = 0;
 	int err;
 
+	if (tb[IFLA_NET_NS_PID]) {
+		struct net *net;
+		net = get_net_ns_by_pid(nla_get_u32(tb[IFLA_NET_NS_PID]));
+		if (IS_ERR(net)) {
+			err = PTR_ERR(net);
+			goto errout;
+		}
+		err = dev_change_net_namespace(dev, net, ifname);
+		put_net(net);
+		if (err)
+			goto errout;
+		modified = 1;
+	}
+
 	if (tb[IFLA_MAP]) {
 		struct rtnl_link_ifmap *u_map;
 		struct ifmap k_map;