[PATCH bpf-next 2/3] net/ipv6: Add helper to return path MTU based on fib result

[David Ahern <dsahern@gmail.com>]

Determine path MTU from a FIB lookup result. Logic is based on
ip6_dst_mtu_forward plus lookup of nexthop exception.

Signed-off-by: David Ahern <dsahern@gmail.com>
---
 include/net/ip6_fib.h   |  6 ++++++
 include/net/ip6_route.h |  3 +++
 net/ipv6/route.c        | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 57 insertions(+)

diff --git a/include/net/ip6_fib.h b/include/net/ip6_fib.h
index cc70f6da8462..7897efe80727 100644
--- a/include/net/ip6_fib.h
+++ b/include/net/ip6_fib.h
@@ -412,6 +412,12 @@ static inline struct net_device *fib6_info_nh_dev(const struct fib6_info *f6i)
 	return f6i->fib6_nh.nh_dev;
 }
 
+static inline
+struct lwtunnel_state *fib6_info_nh_lwt(const struct fib6_info *f6i)
+{
+	return f6i->fib6_nh.nh_lwtstate;
+}
+
 void inet6_rt_notify(int event, struct fib6_info *rt, struct nl_info *info,
 		     unsigned int flags);
 
diff --git a/include/net/ip6_route.h b/include/net/ip6_route.h
index 4cf1ef935ed9..7b9c82de11cc 100644
--- a/include/net/ip6_route.h
+++ b/include/net/ip6_route.h
@@ -300,6 +300,9 @@ static inline unsigned int ip6_dst_mtu_forward(const struct dst_entry *dst)
 	return mtu;
 }
 
+u32 ip6_mtu_from_fib6(struct fib6_info *f6i, struct in6_addr *daddr,
+		      struct in6_addr *saddr);
+
 struct neighbour *ip6_neigh_lookup(const struct in6_addr *gw,
 				   struct net_device *dev, struct sk_buff *skb,
 				   const void *daddr);
diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index cc24ed3bc334..a9b2c8e06404 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -2603,6 +2603,54 @@ static unsigned int ip6_mtu(const struct dst_entry *dst)
 	return mtu - lwtunnel_headroom(dst->lwtstate, mtu);
 }
 
+/* MTU selection:
+ * 1. mtu on route is locked - use it
+ * 2. mtu from nexthop exception
+ * 3. mtu from egress device
+ *
+ * based on ip6_dst_mtu_forward and exception logic of
+ * rt6_find_cached_rt; called with rcu_read_lock
+ */
+u32 ip6_mtu_from_fib6(struct fib6_info *f6i, struct in6_addr *daddr,
+		      struct in6_addr *saddr)
+{
+	struct rt6_exception_bucket *bucket;
+	struct rt6_exception *rt6_ex;
+	struct in6_addr *src_key;
+	struct inet6_dev *idev;
+	u32 mtu = 0;
+
+	if (unlikely(fib6_metric_locked(f6i, RTAX_MTU))) {
+		mtu = f6i->fib6_pmtu;
+		if (mtu)
+			goto out;
+	}
+
+	src_key = NULL;
+#ifdef CONFIG_IPV6_SUBTREES
+	if (f6i->fib6_src.plen)
+		src_key = saddr;
+#endif
+
+	bucket = rcu_dereference(f6i->rt6i_exception_bucket);
+	rt6_ex = __rt6_find_exception_rcu(&bucket, daddr, src_key);
+	if (rt6_ex && !rt6_check_expired(rt6_ex->rt6i))
+		mtu = dst_metric_raw(&rt6_ex->rt6i->dst, RTAX_MTU);
+
+	if (likely(!mtu)) {
+		struct net_device *dev = fib6_info_nh_dev(f6i);
+
+		mtu = IPV6_MIN_MTU;
+		idev = __in6_dev_get(dev);
+		if (idev && idev->cnf.mtu6 > mtu)
+			mtu = idev->cnf.mtu6;
+	}
+
+	mtu = min_t(unsigned int, mtu, IP6_MAX_MTU);
+out:
+	return mtu - lwtunnel_headroom(fib6_info_nh_lwt(f6i), mtu);
+}
+
 struct dst_entry *icmp6_dst_alloc(struct net_device *dev,
 				  struct flowi6 *fl6)
 {
-- 
2.11.0