[PATCH net-next] arp: flush arp cache on IFF_NOARP change

["Timo Teräs" <timo.teras@iki.fi>]

IFF_NOARP affects what kind of neighbor entries are created
(nud NOARP or nud INCOMPLETE). If the flag changes, flush the arp
cache to refresh all entries.

Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Cc: Patrick McHardy <kaber@trash.net>
---
Original patch sent some years ago and the review is at:
 http://patchwork.ozlabs.org/patch/46806/

This is updated per Patrick's suggestion to use priv_flags as the
place to notify that NOARP flag changed.

 include/uapi/linux/if.h | 1 +
 net/core/dev.c          | 5 +++++
 net/ipv4/arp.c          | 4 ++++
 3 files changed, 10 insertions(+)

diff --git a/include/uapi/linux/if.h b/include/uapi/linux/if.h
index 1ec407b..ace4aec 100644
--- a/include/uapi/linux/if.h
+++ b/include/uapi/linux/if.h
@@ -83,6 +83,7 @@
 #define IFF_SUPP_NOFCS	0x80000		/* device supports sending custom FCS */
 #define IFF_LIVE_ADDR_CHANGE 0x100000	/* device supports hardware address
 					 * change when it's running */
+#define IFF_NOARP_CHANGED 0x200000	/* IFF_NOARP changed state recently */
 
 
 #define IF_GET_IFACE	0x0001		/* for querying only */
diff --git a/net/core/dev.c b/net/core/dev.c
index 18e9730..7135fe1 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4691,6 +4691,11 @@ void __dev_notify_flags(struct net_device *dev, unsigned int old_flags)
 {
 	unsigned int changes = dev->flags ^ old_flags;
 
+	if (changes & IFF_NOARP)
+		dev->priv_flags |= IFF_NOARP_CHANGED;
+	else
+		dev->priv_flags &= ~IFF_NOARP_CHANGED;
+
 	if (changes & IFF_UP) {
 		if (dev->flags & IFF_UP)
 			call_netdevice_notifiers(NETDEV_UP, dev);
diff --git a/net/ipv4/arp.c b/net/ipv4/arp.c
index 247ec19..375b2f2 100644
--- a/net/ipv4/arp.c
+++ b/net/ipv4/arp.c
@@ -1241,6 +1241,10 @@ static int arp_netdev_event(struct notifier_block *this, unsigned long event,
 		neigh_changeaddr(&arp_tbl, dev);
 		rt_cache_flush(dev_net(dev));
 		break;
+	case NETDEV_CHANGE:
+		if (dev->priv_flags & IFF_NOARP_CHANGED)
+			neigh_changeaddr(&arp_tbl, dev);
+		break;
 	default:
 		break;
 	}
-- 
1.8.2.3