netlink: change nlmsg_notify() return value logic

This patch changes the return value of nlmsg_notify() as follows:

If NETLINK_BROADCAST_ERROR is set by any of the listeners and
an error in the delivery happened, return the broadcast error;
else if there are no listeners apart from the socket that
requested a change with the echo flag, return the result of the
unicast notification. Thus, with this patch, the unicast
notification is handled in the same way of a broadcast listener
that has set the NETLINK_BROADCAST_ERROR socket flag.

This patch is useful in case that the caller of nlmsg_notify()
wants to know the result of the delivery of a netlink notification
(including the broadcast delivery) and take any action in case
that the delivery failed. For example, ctnetlink can drop packets
if the event delivery failed to provide reliable logging and
state-synchronization at the cost of dropping packets.

This patch also modifies the rtnetlink code to ignore the return
value of rtnl_notify() in all callers. The function rtnl_notify()
(before this patch) returned the error of the unicast notification
which makes rtnl_set_sk_err() reports errors to all listeners. This
is not of any help since the origin of the change (the socket that
requested the echoing) notices the ENOBUFS error if the notification
fails and should resync itself.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 5 insertions, 4 deletions

diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index 790dd205bb5d..d78030f88bd0 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -455,8 +455,8 @@ int rtnl_unicast(struct sk_buff *skb, struct net *net, u32 pid)
         return nlmsg_unicast(rtnl, skb, pid);
 }
 
-int rtnl_notify(struct sk_buff *skb, struct net *net, u32 pid, u32 group,
-                struct nlmsghdr *nlh, gfp_t flags)
+void rtnl_notify(struct sk_buff *skb, struct net *net, u32 pid, u32 group,
+                 struct nlmsghdr *nlh, gfp_t flags)
 {
         struct sock *rtnl = net->rtnl;
         int report = 0;
@@ -464,7 +464,7 @@ int rtnl_notify(struct sk_buff *skb, struct net *net, u32 pid, u32 group,
         if (nlh)
                 report = nlmsg_report(nlh);
 
-        return nlmsg_notify(rtnl, skb, pid, group, report, flags);
+        nlmsg_notify(rtnl, skb, pid, group, report, flags);
 }
 
 void rtnl_set_sk_err(struct net *net, u32 group, int error)
@@ -1246,7 +1246,8 @@ void rtmsg_ifinfo(int type, struct net_device *dev, unsigned change)
                 kfree_skb(skb);
                 goto errout;
         }
-        err = rtnl_notify(skb, net, 0, RTNLGRP_LINK, NULL, GFP_KERNEL);
+        rtnl_notify(skb, net, 0, RTNLGRP_LINK, NULL, GFP_KERNEL);
+        return;
 errout:
         if (err < 0)
                 rtnl_set_sk_err(net, RTNLGRP_LINK, err);