neigh: Allow for user space users of the neighbour table

Currently it is possible to do just about everything with the arp table
from user space except treat an entry like you are using it.  To that end
implement and a flag NTF_USE that when set in a netwlink update request
treats the neighbour table entry like the kernel does on the output path.

This allows user space applications to share the kernel's arp cache.

Signed-off-by: Eric Biederman <ebiederm@aristanetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 6 insertions, 1 deletions

diff --git a/include/linux/neighbour.h b/include/linux/neighbour.h
index 8730d5dae1bc..12c9de138451 100644
--- a/include/linux/neighbour.h
+++ b/include/linux/neighbour.h
@@ -31,6 +31,7 @@ enum
  *      Neighbor Cache Entry Flags
  */
 
+#define NTF_USE         0x01
 #define NTF_PROXY       0x08    /* == ATF_PUBL */
 #define NTF_ROUTER      0x80
 
diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index 417b6d739fb7..a1cbce7fdae5 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -1654,7 +1654,11 @@ static int neigh_add(struct sk_buff *skb, struct nlmsghdr *nlh, void *arg)
                                 flags &= ~NEIGH_UPDATE_F_OVERRIDE;
                 }
 
-                err = neigh_update(neigh, lladdr, ndm->ndm_state, flags);
+                if (ndm->ndm_flags & NTF_USE) {
+                        neigh_event_send(neigh, NULL);
+                        err = 0;
+                } else
+                        err = neigh_update(neigh, lladdr, ndm->ndm_state, flags);
                 neigh_release(neigh);
                 goto out_dev_put;
         }