rtnetlink: always put IFLA_LINK for links with a link-netnsid

Currently, nla_put_iflink() doesn't put the IFLA_LINK attribute when
iflink == ifindex.

In some cases, a device can be created in a different netns with the
same ifindex as its parent. That device will not dump its IFLA_LINK
attribute, which can confuse some userspace software that expects it.
For example, if the last ifindex created in init_net and foo are both
8, these commands will trigger the issue:

    ip link add parent type dummy                   # ifindex 9
    ip link add link parent netns foo type macvlan  # ifindex 9 in ns foo

So, in case a device puts the IFLA_LINK_NETNSID attribute in a dump,
always put the IFLA_LINK attribute as well.

Thanks to Dan Winship for analyzing the original OpenShift bug down to
the missing netlink attribute.

v2: change Fixes tag, it's been here forever, as Nicolas Dichtel said
    add Nicolas' ack
v3: change Fixes tag
    fix subject typo, spotted by Edward Cree
Analyzed-by: Dan Winship <danw@redhat.com>
Fixes: d8a5ec67 ("[NET]: netlink support for moving devices between network namespaces.")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -1496,14 +1496,15 @@ static int put_master_ifindex(struct sk_buff *skb, struct net_device *dev)
 	return ret;
 }
 
-static int nla_put_iflink(struct sk_buff *skb, const struct net_device *dev)
+static int nla_put_iflink(struct sk_buff *skb, const struct net_device *dev,
+			  bool force)
 {
 	int ifindex = dev_get_iflink(dev);
 
-	if (dev->ifindex == ifindex)
-		return 0;
+	if (force || dev->ifindex != ifindex)
+		return nla_put_u32(skb, IFLA_LINK, ifindex);
 
-	return nla_put_u32(skb, IFLA_LINK, ifindex);
+	return 0;
 }
 
 static noinline_for_stack int nla_put_ifalias(struct sk_buff *skb,
@@ -1520,6 +1521,8 @@ static int rtnl_fill_link_netnsid(struct sk_buff *skb,
 				  const struct net_device *dev,
 				  struct net *src_net)
 {
+	bool put_iflink = false;
+
 	if (dev->rtnl_link_ops && dev->rtnl_link_ops->get_link_net) {
 		struct net *link_net = dev->rtnl_link_ops->get_link_net(dev);
 
@@ -1528,10 +1531,12 @@ static int rtnl_fill_link_netnsid(struct sk_buff *skb,
 
 			if (nla_put_s32(skb, IFLA_LINK_NETNSID, id))
 				return -EMSGSIZE;
+
+			put_iflink = true;
 		}
 	}
 
-	return 0;
+	return nla_put_iflink(skb, dev, put_iflink);
 }
 
 static int rtnl_fill_link_af(struct sk_buff *skb,
@@ -1617,7 +1622,6 @@ static int rtnl_fill_ifinfo(struct sk_buff *skb,
 #ifdef CONFIG_RPS
 	    nla_put_u32(skb, IFLA_NUM_RX_QUEUES, dev->num_rx_queues) ||
 #endif
-	    nla_put_iflink(skb, dev) ||
 	    put_master_ifindex(skb, dev) ||
 	    nla_put_u8(skb, IFLA_CARRIER, netif_carrier_ok(dev)) ||
 	    (dev->qdisc &&