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>

4 files changed, 12 insertions, 11 deletions

diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
index f3d339f728b0..d65e9215bcd7 100644
--- a/net/ipv4/icmp.c
+++ b/net/ipv4/icmp.c
@@ -587,20 +587,20 @@ void icmp_send(struct sk_buff *skb_in, int type, int code, __be32 info)
                         err = __ip_route_output_key(net, &rt2, &fl);
                 else {
                         struct flowi fl2 = {};
-                        struct dst_entry *odst;
+                        unsigned long orefdst;
 
                         fl2.fl4_dst = fl.fl4_src;
                         if (ip_route_output_key(net, &rt2, &fl2))
                                 goto relookup_failed;
 
                         /* Ugh! */
-                        odst = skb_dst(skb_in);
+                        orefdst = skb_in->_skb_refdst; /* save old refdst */
                         err = ip_route_input(skb_in, fl.fl4_dst, fl.fl4_src,
                                              RT_TOS(tos), rt2->u.dst.dev);
 
                         dst_release(&rt2->u.dst);
                         rt2 = skb_rtable(skb_in);
-                        skb_dst_set(skb_in, odst);
+                        skb_in->_skb_refdst = orefdst; /* restore old refdst */
                 }
 
                 if (err)
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 */
diff --git a/net/ipv4/netfilter.c b/net/ipv4/netfilter.c
index 82fb43c5c59e..07de855e2175 100644
--- a/net/ipv4/netfilter.c
+++ b/net/ipv4/netfilter.c
@@ -17,7 +17,7 @@ int ip_route_me_harder(struct sk_buff *skb, unsigned addr_type)
         const struct iphdr *iph = ip_hdr(skb);
         struct rtable *rt;
         struct flowi fl = {};
-        struct dst_entry *odst;
+        unsigned long orefdst;
         unsigned int hh_len;
         unsigned int type;
 
@@ -51,14 +51,14 @@ int ip_route_me_harder(struct sk_buff *skb, unsigned addr_type)
                 if (ip_route_output_key(net, &rt, &fl) != 0)
                         return -1;
 
-                odst = skb_dst(skb);
+                orefdst = skb->_skb_refdst;
                 if (ip_route_input(skb, iph->daddr, iph->saddr,
                                    RT_TOS(iph->tos), rt->u.dst.dev) != 0) {
                         dst_release(&rt->u.dst);
                         return -1;
                 }
                 dst_release(&rt->u.dst);
-                dst_release(odst);
+                refdst_drop(orefdst);
         }
 
         if (skb_dst(skb)->error)
diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index dea3f9264250..705eccfb4769 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -3033,7 +3033,7 @@ int ip_rt_dump(struct sk_buff *skb,  struct netlink_callback *cb)
                                 continue;
                         if (rt_is_expired(rt))
                                 continue;
-                        skb_dst_set(skb, dst_clone(&rt->u.dst));
+                        skb_dst_set_noref(skb, &rt->u.dst);
                         if (rt_fill_info(net, skb, NETLINK_CB(cb->skb).pid,
                                          cb->nlh->nlmsg_seq, RTM_NEWROUTE,
                                          1, NLM_F_MULTI) <= 0) {