ipv6: fix the outgoing interface selection order in udpv6_sendmsg()

1.When no interface is specified in an IPV6_PKTINFO ancillary data
  item, the interface specified in an IPV6_PKTINFO sticky optionis 
  is used.

RFC3542:
6.7.  Summary of Outgoing Interface Selection

   This document and [RFC-3493] specify various methods that affect the
   selection of the packet's outgoing interface.  This subsection
   summarizes the ordering among those in order to ensure deterministic
   behavior.

   For a given outgoing packet on a given socket, the outgoing interface
   is determined in the following order:

   1. if an interface is specified in an IPV6_PKTINFO ancillary data
      item, the interface is used.

   2. otherwise, if an interface is specified in an IPV6_PKTINFO sticky
      option, the interface is used.

Signed-off-by: Yang Hongyang <yanghy@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 3 insertions, 0 deletions

diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index 38390dd19636..84b1a296eecb 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -793,6 +793,9 @@ do_udp_sendmsg:
         if (!fl.oif)
                 fl.oif = sk->sk_bound_dev_if;
 
+        if (!fl.oif)
+                fl.oif = np->sticky_pktinfo.ipi6_ifindex;
+
         if (msg->msg_controllen) {
                 opt = &opt_space;
                 memset(opt, 0, sizeof(struct ipv6_txoptions));