[PATCH net-next v2 2/8] rtnetlink: Add netns_atomic flag in rtnl_link_ops

[Xiao Liang <shaw.leon@xxxxxxxxx>]

Currently these two steps are needed to create a net device with
IFLA_LINK_NETNSID attr:

 1. create and setup the netdev in the link netns with
    rtnl_create_link()
 2. move it to the target netns with dev_change_net_namespace()

This has some side effects, including extra ifindex allocation, ifname
validation and link notifications in link netns.

Add a netns_atomic flag, that if set to true, devices will be created in
the target netns directly.

Signed-off-by: Xiao Liang <shaw.leon@xxxxxxxxx>
---
 include/net/rtnetlink.h | 3 +++
 net/core/rtnetlink.c    | 7 ++++---
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/include/net/rtnetlink.h b/include/net/rtnetlink.h
index b260c0cc9671..7e78f3952774 100644
--- a/include/net/rtnetlink.h
+++ b/include/net/rtnetlink.h
@@ -75,6 +75,8 @@ static inline int rtnl_msg_family(const struct nlmsghdr *nlh)
  *	@srcu: Used internally
  *	@kind: Identifier
  *	@netns_refund: Physical device, move to init_net on netns exit
+ *	@netns_atomic: Device can be created in target netns even when
+ *		       link-netns is different, avoiding netns change.
  *	@maxtype: Highest device specific netlink attribute number
  *	@policy: Netlink policy for device specific attribute validation
  *	@validate: Optional validation function for netlink/changelink parameters
@@ -116,6 +118,7 @@ struct rtnl_link_ops {
 	void			(*setup)(struct net_device *dev);
 
 	bool			netns_refund;
+	bool			netns_atomic;
 	unsigned int		maxtype;
 	const struct nla_policy	*policy;
 	int			(*validate)(struct nlattr *tb[],
diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index 8119f4ad9e5f..b0d1cbb44a03 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -3690,8 +3690,9 @@ static int rtnl_newlink_create(struct sk_buff *skb, struct ifinfomsg *ifm,
 		name_assign_type = NET_NAME_ENUM;
 	}
 
-	dev = rtnl_create_link(link_net ? : tgt_net, ifname,
-			       name_assign_type, ops, tb, extack);
+	dev = rtnl_create_link(!link_net || ops->netns_atomic ?
+			       tgt_net : link_net, ifname, name_assign_type,
+			       ops, tb, extack);
 	if (IS_ERR(dev)) {
 		err = PTR_ERR(dev);
 		goto out;
@@ -3711,7 +3712,7 @@ static int rtnl_newlink_create(struct sk_buff *skb, struct ifinfomsg *ifm,
 	err = rtnl_configure_link(dev, ifm, portid, nlh);
 	if (err < 0)
 		goto out_unregister;
-	if (link_net) {
+	if (link_net && !ops->netns_atomic) {
 		err = dev_change_net_namespace(dev, tgt_net, ifname);
 		if (err < 0)
 			goto out_unregister;
-- 
2.47.0