Quoting Eric W. Biederman (ebiederm@xmission.com):
> 
> 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>
> ---
>  include/linux/if_link.h |    1 +
>  net/core/rtnetlink.c    |   35 +++++++++++++++++++++++++++++++++++
>  2 files changed, 36 insertions(+), 0 deletions(-)
> 
> diff --git a/include/linux/if_link.h b/include/linux/if_link.h
> index 422084d..84c3492 100644
> --- a/include/linux/if_link.h
> +++ b/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
>  };
> 
> diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
> index 44f91bb..1b9c32d 100644
> --- a/net/core/rtnetlink.c
> +++ b/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>
> @@ -727,6 +728,7 @@ 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] = {
> @@ -734,12 +736,45 @@ static const struct nla_policy ifla_info_policy[IFLA_INFO_MAX+1] = {
>  	[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);

Thinking...  Ok, I'm not sure this is 100% safe in the target tree, but
the long-term correct way probably isn't yet implemented in the net-
tree.  Eventually you will want to:

	net_ns = NULL;
	rcu_read_lock();
	tsk = find_task_by_pid();  /* or _pidns equiv? */
	nsproxy = task_nsproxy(tsk);
	if (nsproxy)
		net_ns = get_net(nsproxy->net_ns);
	rcu_read_unlock;

What you have here is probably unsafe if tsk is the last task pointing
to it's nsproxy and it does an unshare, bc unshare isn't protected by
task_lock, and you're not rcu_dereferencing tsk->nsproxy (which
task_nsproxy does).  At one point we floated a patch to reuse the same
nsproxy in that case which would prevent you having to worry about it,
but that isn't being done in -mm now so i doubt it's in -net.

> +	}
> +	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;
> -- 
> 1.5.3.rc6.17.g1911
> 
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