net: add a noref bit on skb dst

Use low order bit of skb->_skb_dst to tell dst is not refcounted.

Change _skb_dst to _skb_refdst to make sure all uses are catched.

skb_dst() returns the dst, regardless of noref bit set or not, but
with a lockdep check to make sure a noref dst is not given if current
user is not rcu protected.

New skb_dst_set_noref() helper to set an notrefcounted dst on a skb.
(with lockdep check)

skb_dst_drop() drops a reference only if skb dst was refcounted.

skb_dst_force() helper is used to force a refcount on dst, when skb
is queued and not anymore RCU protected.

Use skb_dst_force() in __sk_add_backlog(), __dev_xmit_skb() if
!IFF_XMIT_DST_RELEASE or skb enqueued on qdisc queue, in
sock_queue_rcv_skb(), in __nf_queue().

Use skb_dst_force() in dev_requeue_skb().

Note: dst_use_noref() still dirties dst, we might transform it
later to do one dirtying per jiffies.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 5 insertions, 4 deletions

diff --git a/net/ipv4/ip_options.c b/net/ipv4/ip_options.c
index 4c09a31fd140..3244133c24f6 100644
--- a/net/ipv4/ip_options.c
+++ b/net/ipv4/ip_options.c
@@ -601,6 +601,7 @@ int ip_options_rcv_srr(struct sk_buff *skb)
         unsigned char *optptr = skb_network_header(skb) + opt->srr;
         struct rtable *rt = skb_rtable(skb);
         struct rtable *rt2;
+        unsigned long orefdst;
         int err;
 
         if (!opt->srr)
@@ -624,16 +625,16 @@ int ip_options_rcv_srr(struct sk_buff *skb)
                 }
                 memcpy(&nexthop, &optptr[srrptr-1], 4);
 
-                rt = skb_rtable(skb);
+                orefdst = skb->_skb_refdst;
                 skb_dst_set(skb, NULL);
                 err = ip_route_input(skb, nexthop, iph->saddr, iph->tos, skb->dev);
                 rt2 = skb_rtable(skb);
                 if (err || (rt2->rt_type != RTN_UNICAST && rt2->rt_type != RTN_LOCAL)) {
-                        ip_rt_put(rt2);
-                        skb_dst_set(skb, &rt->u.dst);
+                        skb_dst_drop(skb);
+                        skb->_skb_refdst = orefdst;
                         return -EINVAL;
                 }
-                ip_rt_put(rt);
+                refdst_drop(orefdst);
                 if (rt2->rt_type != RTN_LOCAL)
                         break;
                 /* Superfast 8) loopback forward */