ipv6: yet another new IPV6_MTU_DISCOVER option IPV6_PMTUDISC_OMIT

This option has the same semantic as IP_PMTUDISC_OMIT for IPv4 which
got recently introduced. It doesn't honor the path mtu discovered by the
host but in contrary to IPV6_PMTUDISC_INTERFACE allows the generation of
fragments if the packet size exceeds the MTU of the outgoing interface
MTU.

Fixes: 93b36cf3425b9b ("ipv6: support IPV6_PMTU_INTERFACE on sockets")
Cc: Florian Weimer <fweimer@redhat.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/ip6_route.h

@@ -171,7 +171,14 @@ static inline int ip6_skb_dst_mtu(struct sk_buff *skb)
 
 static inline bool ip6_sk_accept_pmtu(const struct sock *sk)
 {
-	return inet6_sk(sk)->pmtudisc != IPV6_PMTUDISC_INTERFACE;
+	return inet6_sk(sk)->pmtudisc != IPV6_PMTUDISC_INTERFACE &&
+	       inet6_sk(sk)->pmtudisc != IPV6_PMTUDISC_OMIT;
+}
+
+static inline bool ip6_sk_local_df(const struct sock *sk)
+{
+	return inet6_sk(sk)->pmtudisc < IPV6_PMTUDISC_DO ||
+	       inet6_sk(sk)->pmtudisc == IPV6_PMTUDISC_OMIT;
 }
 
 static inline struct in6_addr *rt6_nexthop(struct rt6_info *rt)

include/uapi/linux/in6.h

@@ -185,6 +185,10 @@ struct in6_flowlabel_req {
  * also see comments on IP_PMTUDISC_INTERFACE
  */
 #define IPV6_PMTUDISC_INTERFACE		4
+/* weaker version of IPV6_PMTUDISC_INTERFACE, which allows packets to
+ * get fragmented if they exceed the interface mtu
+ */
+#define IPV6_PMTUDISC_OMIT		5
 
 /* Flowlabel */
 #define IPV6_FLOWLABEL_MGR	32

net/ipv6/ip6_output.c

@@ -1234,8 +1234,10 @@ int ip6_append_data(struct sock *sk, int getfrag(void *from, char *to,
 			      sizeof(struct frag_hdr) : 0) +
 			     rt->rt6i_nfheader_len;
 
-		maxnonfragsize = (np->pmtudisc >= IPV6_PMTUDISC_DO) ?
-				 mtu : sizeof(struct ipv6hdr) + IPV6_MAXPLEN;
+		if (ip6_sk_local_df(sk))
+			maxnonfragsize = sizeof(struct ipv6hdr) + IPV6_MAXPLEN;
+		else
+			maxnonfragsize = mtu;
 
 		/* dontfrag active */
 		if ((cork->length + length > mtu - headersize) && dontfrag &&
@@ -1543,8 +1545,7 @@ int ip6_push_pending_frames(struct sock *sk)
 	}
 
 	/* Allow local fragmentation. */
-	if (np->pmtudisc < IPV6_PMTUDISC_DO)
-		skb->local_df = 1;
+	skb->local_df = ip6_sk_local_df(sk);
 
 	*final_dst = fl6->daddr;
 	__skb_pull(skb, skb_network_header_len(skb));

net/ipv6/ipv6_sockglue.c

@@ -722,7 +722,7 @@ done:
 	case IPV6_MTU_DISCOVER:
 		if (optlen < sizeof(int))
 			goto e_inval;
-		if (val < IPV6_PMTUDISC_DONT || val > IPV6_PMTUDISC_INTERFACE)
+		if (val < IPV6_PMTUDISC_DONT || val > IPV6_PMTUDISC_OMIT)
 			goto e_inval;
 		np->pmtudisc = val;
 		retv = 0;