[INET]: Add IP(V6)_PMTUDISC_RPOBE

Add IP(V6)_PMTUDISC_PROBE value for IP(V6)_MTU_DISCOVER.  This option forces
us not to fragment, but does not make use of the kernel path MTU discovery.
That is, it allows for user-mode MTU probing (or, packetization-layer path
MTU discovery).  This is particularly useful for diagnostic utilities, like
traceroute/tracepath.

Signed-off-by: John Heffner <jheffner@psc.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 16 insertions, 6 deletions

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 34606eff8a05..534650cad3a8 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -189,6 +189,14 @@ static inline int ip_finish_output2(struct sk_buff *skb)
         return -EINVAL;
 }
 
+static inline int ip_skb_dst_mtu(struct sk_buff *skb)
+{
+        struct inet_sock *inet = skb->sk ? inet_sk(skb->sk) : NULL;
+
+        return (inet && inet->pmtudisc == IP_PMTUDISC_PROBE) ?
+               skb->dst->dev->mtu : dst_mtu(skb->dst);
+}
+
 static inline int ip_finish_output(struct sk_buff *skb)
 {
 #if defined(CONFIG_NETFILTER) && defined(CONFIG_XFRM)
@@ -198,7 +206,7 @@ static inline int ip_finish_output(struct sk_buff *skb)
                 return dst_output(skb);
         }
 #endif
-        if (skb->len > dst_mtu(skb->dst) && !skb_is_gso(skb))
+        if (skb->len > ip_skb_dst_mtu(skb) && !skb_is_gso(skb))
                 return ip_fragment(skb, ip_finish_output2);
         else
                 return ip_finish_output2(skb);
@@ -422,7 +430,7 @@ int ip_fragment(struct sk_buff *skb, int (*output)(struct sk_buff*))
         if (unlikely((iph->frag_off & htons(IP_DF)) && !skb->local_df)) {
                 IP_INC_STATS(IPSTATS_MIB_FRAGFAILS);
                 icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
-                          htonl(dst_mtu(&rt->u.dst)));
+                          htonl(ip_skb_dst_mtu(skb)));
                 kfree_skb(skb);
                 return -EMSGSIZE;
         }
@@ -787,7 +795,9 @@ int ip_append_data(struct sock *sk,
                         inet->cork.addr = ipc->addr;
                 }
                 dst_hold(&rt->u.dst);
-                inet->cork.fragsize = mtu = dst_mtu(rt->u.dst.path);
+                inet->cork.fragsize = mtu = inet->pmtudisc == IP_PMTUDISC_PROBE ?
+                                            rt->u.dst.dev->mtu :
+                                            dst_mtu(rt->u.dst.path);
                 inet->cork.rt = rt;
                 inet->cork.length = 0;
                 sk->sk_sndmsg_page = NULL;
@@ -1203,13 +1213,13 @@ int ip_push_pending_frames(struct sock *sk)
          * to fragment the frame generated here. No matter, what transforms
          * how transforms change size of the packet, it will come out.
          */
-        if (inet->pmtudisc != IP_PMTUDISC_DO)
+        if (inet->pmtudisc < IP_PMTUDISC_DO)
                 skb->local_df = 1;
 
         /* DF bit is set when we want to see DF on outgoing frames.
          * If local_df is set too, we still allow to fragment this frame
          * locally. */
-        if (inet->pmtudisc == IP_PMTUDISC_DO ||
+        if (inet->pmtudisc >= IP_PMTUDISC_DO ||
             (skb->len <= dst_mtu(&rt->u.dst) &&
              ip_dont_fragment(sk, &rt->u.dst)))
                 df = htons(IP_DF);
diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
index c199d2311731..4d544573f48a 100644
--- a/net/ipv4/ip_sockglue.c
+++ b/net/ipv4/ip_sockglue.c
@@ -542,7 +542,7 @@ static int do_ip_setsockopt(struct sock *sk, int level,
                 inet->hdrincl = val ? 1 : 0;
                 break;
         case IP_MTU_DISCOVER:
-                if (val<0 || val>2)
+                if (val<0 || val>3)
                         goto e_inval;
                 inet->pmtudisc = val;
                 break;