[PATCH net v2] OVS: Ignore negative headroom value

net_device->ndo_set_rx_headroom (introduced in
871b642adebe300be2e50aa5f65a418510f636ec) says

  "Setting a negtaive value reset the rx headroom
   to the default value".

It seems that the OVS implementation in
3a927bc7cf9d0fbe8f4a8189dd5f8440228f64e7 overlooked this and sets
dev->needed_headroom unconditionally.

This doesn't have an immediate effect, but can mess up later
LL_RESERVED_SPACE calculations, such as done in
net/ipv6/mcast.c:mld_newpack.  For reference, this issue was found
from a skb_panic raised there after the length calculations had given
the wrong result.

Note the other current users of this interface
(drivers/net/tun.c:tun_set_headroom and
drivers/net/veth.c:veth_set_rx_headroom) are both checking this
correctly thus need no modification.

Thanks to Ben for some pointers from the crash dumps!

Cc: Benjamin Poirier <redacted>
Cc: Paolo Abeni <pabeni@redhat.com>
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1361414
Signed-off-by: Ian Wienand <redacted>
---
 net/openvswitch/vport-internal_dev.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/openvswitch/vport-internal_dev.c b/net/openvswitch/vport-internal_dev.c
index 434e04c..95c3614 100644
--- a/net/openvswitch/vport-internal_dev.c
+++ b/net/openvswitch/vport-internal_dev.c

@@ -140,7 +140,7 @@ internal_get_stats(struct net_device *dev, struct rtnl_link_stats64 *stats)
 
 static void internal_set_rx_headroom(struct net_device *dev, int new_hr)
 {
-	dev->needed_headroom = new_hr;
+	dev->needed_headroom = new_hr < 0 ? 0 : new_hr;
 }
 
 static const struct net_device_ops internal_dev_netdev_ops = {

-- 
2.7.4