rtnetlink: Restore RTM_NEW/DELLINK notification behavior

The commits referenced below allows userspace to use the NLM_F_ECHO flag
for RTM_NEW/DELLINK operations to receive unicast notifications for the
affected link. Prior to these changes, applications may have relied on
multicast notifications to learn the same information without specifying
the NLM_F_ECHO flag.

For such applications, the mentioned commits changed the behavior for
requests not using NLM_F_ECHO. Multicast notifications are still received,
but now use the portid of the requester and the sequence number of the
request instead of zero values used previously. For the application, this
message may be unexpected and likely handled as a response to the
NLM_F_ACKed request, especially if it uses the same socket to handle
requests and notifications.

To fix existing applications relying on the old notification behavior,
set the portid and sequence number in the notification only if the
request included the NLM_F_ECHO flag. This restores the old behavior
for applications not using it, but allows unicasted notifications for
others.

Fixes: f3a63cce1b4f ("rtnetlink: Honour NLM_F_ECHO flag in rtnl_delete_link")
Fixes: d88e136cab37 ("rtnetlink: Honour NLM_F_ECHO flag in rtnl_newlink_create")
Signed-off-by: Martin Willi <[e-mail]>
Acked-by: Guillaume Nault <[e-mail]>
Acked-by: Hangbin Liu <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

include/linux/rtnetlink.h

···
 struct sk_buff *rtmsg_ifinfo_build_skb(int type, struct net_device *dev,
 				       unsigned change, u32 event,
 				       gfp_t flags, int *new_nsid,
-				       int new_ifindex, u32 portid, u32 seq);
+				       int new_ifindex, u32 portid,
+				       const struct nlmsghdr *nlh);
 void rtmsg_ifinfo_send(struct sk_buff *skb, struct net_device *dev,
 		       gfp_t flags, u32 portid, const struct nlmsghdr *nlh);
 

net/core/dev.c

···
 		    dev->rtnl_link_state == RTNL_LINK_INITIALIZED)
 			skb = rtmsg_ifinfo_build_skb(RTM_DELLINK, dev, ~0U, 0,
 						     GFP_KERNEL, NULL, 0,
-						     portid, nlmsg_seq(nlh));
+						     portid, nlh);
 
 		/*
 		 *	Flush the unicast and multicast chains

net/core/rtnetlink.c

···
 struct sk_buff *rtmsg_ifinfo_build_skb(int type, struct net_device *dev,
 				       unsigned int change,
 				       u32 event, gfp_t flags, int *new_nsid,
-				       int new_ifindex, u32 portid, u32 seq)
+				       int new_ifindex, u32 portid,
+				       const struct nlmsghdr *nlh)
 {
 	struct net *net = dev_net(dev);
 	struct sk_buff *skb;
 	int err = -ENOBUFS;
+	u32 seq = 0;
 
 	skb = nlmsg_new(if_nlmsg_size(dev, 0), flags);
 	if (skb == NULL)
 		goto errout;
+
+	if (nlmsg_report(nlh))
+		seq = nlmsg_seq(nlh);
+	else
+		portid = 0;
 
 	err = rtnl_fill_ifinfo(skb, dev, dev_net(dev),
 			       type, portid, seq, change, 0, 0, event,
···
 		return;
 
 	skb = rtmsg_ifinfo_build_skb(type, dev, change, event, flags, new_nsid,
-				     new_ifindex, portid, nlmsg_seq(nlh));
+				     new_ifindex, portid, nlh);
 	if (skb)
 		rtmsg_ifinfo_send(skb, dev, flags, portid, nlh);
 }