netlink: add NETLINK_BROADCAST_ERROR socket option

This patch adds NETLINK_BROADCAST_ERROR which is a netlink
socket option that the listener can set to make netlink_broadcast()
return errors in the delivery to the caller. This option is useful
if the caller of netlink_broadcast() do something with the result
of the message delivery, like in ctnetlink where it drops a network
packet if the event delivery failed, this is used to enable reliable
logging and state-synchronization. If this socket option is not set,
netlink_broadcast() only reports ESRCH errors and silently ignore
ENOBUFS errors, which is what most netlink_broadcast() callers
should do.

This socket option is based on a suggestion from Patrick McHardy.
Patrick McHardy can exchange this patch for a beer from me ;).

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>

2 files changed, 24 insertions, 2 deletions

diff --git a/include/linux/netlink.h b/include/linux/netlink.h
index 51b09a1f46c3..1e6bf995435c 100644
--- a/include/linux/netlink.h
+++ b/include/linux/netlink.h
@@ -103,6 +103,7 @@ struct nlmsgerr
 #define NETLINK_ADD_MEMBERSHIP  1
 #define NETLINK_DROP_MEMBERSHIP 2
 #define NETLINK_PKTINFO         3
+#define NETLINK_BROADCAST_ERROR 4
 
 struct nl_pktinfo
 {
diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 6ee69c27f806..ed587be1e1c2 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -85,6 +85,7 @@ struct netlink_sock {
 
 #define NETLINK_KERNEL_SOCKET   0x1
 #define NETLINK_RECV_PKTINFO    0x2
+#define NETLINK_BROADCAST_SEND_ERROR    0x4
 
 static inline struct netlink_sock *nlk_sk(struct sock *sk)
 {
@@ -995,12 +996,15 @@ static inline int do_one_broadcast(struct sock *sk,
                 netlink_overrun(sk);
                 /* Clone failed. Notify ALL listeners. */
                 p->failure = 1;
+                if (nlk->flags & NETLINK_BROADCAST_SEND_ERROR)
+                        p->delivery_failure = 1;
         } else if (sk_filter(sk, p->skb2)) {
                 kfree_skb(p->skb2);
                 p->skb2 = NULL;
         } else if ((val = netlink_broadcast_deliver(sk, p->skb2)) < 0) {
                 netlink_overrun(sk);
-                p->delivery_failure = 1;
+                if (nlk->flags & NETLINK_BROADCAST_SEND_ERROR)
+                        p->delivery_failure = 1;
         } else {
                 p->congested |= val;
                 p->delivered = 1;
@@ -1048,7 +1052,7 @@ int netlink_broadcast(struct sock *ssk, struct sk_buff *skb, u32 pid,
         if (info.skb2)
                 kfree_skb(info.skb2);
 
-        if (info.delivery_failure || info.failure)
+        if (info.delivery_failure)
                 return -ENOBUFS;
 
         if (info.delivered) {
@@ -1163,6 +1167,13 @@ static int netlink_setsockopt(struct socket *sock, int level, int optname,
                 err = 0;
                 break;
         }
+        case NETLINK_BROADCAST_ERROR:
+                if (val)
+                        nlk->flags |= NETLINK_BROADCAST_SEND_ERROR;
+                else
+                        nlk->flags &= ~NETLINK_BROADCAST_SEND_ERROR;
+                err = 0;
+                break;
         default:
                 err = -ENOPROTOOPT;
         }
@@ -1195,6 +1206,16 @@ static int netlink_getsockopt(struct socket *sock, int level, int optname,
                         return -EFAULT;
                 err = 0;
                 break;
+        case NETLINK_BROADCAST_ERROR:
+                if (len < sizeof(int))
+                        return -EINVAL;
+                len = sizeof(int);
+                val = nlk->flags & NETLINK_BROADCAST_SEND_ERROR ? 1 : 0;
+                if (put_user(len, optlen) ||
+                    put_user(val, optval))
+                        return -EFAULT;
+                err = 0;
+                break;
         default:
                 err = -ENOPROTOOPT;
         }