1 files changed, 44 insertions, 1 deletions

diff --git a/net/rxrpc/ar-peer.c b/net/rxrpc/ar-peer.c
index d399de4a7fe2..ce08b78647ce 100644
--- a/net/rxrpc/ar-peer.c
+++ b/net/rxrpc/ar-peer.c
@@ -19,6 +19,7 @@
 #include <net/sock.h>
 #include <net/af_rxrpc.h>
 #include <net/ip.h>
+#include <net/route.h>
 #include "ar-internal.h"
 
 static LIST_HEAD(rxrpc_peers);
@@ -28,6 +29,47 @@ static DECLARE_WAIT_QUEUE_HEAD(rxrpc_peer_wq);
 static void rxrpc_destroy_peer(struct work_struct *work);
 
 /*
+ * assess the MTU size for the network interface through which this peer is
+ * reached
+ */
+static void rxrpc_assess_MTU_size(struct rxrpc_peer *peer)
+{
+        struct rtable *rt;
+        struct flowi fl;
+        int ret;
+
+        peer->if_mtu = 1500;
+
+        memset(&fl, 0, sizeof(fl));
+
+        switch (peer->srx.transport.family) {
+        case AF_INET:
+                fl.oif = 0;
+                fl.proto = IPPROTO_UDP,
+                fl.nl_u.ip4_u.saddr = 0;
+                fl.nl_u.ip4_u.daddr = peer->srx.transport.sin.sin_addr.s_addr;
+                fl.nl_u.ip4_u.tos = 0;
+                /* assume AFS.CM talking to AFS.FS */
+                fl.uli_u.ports.sport = htons(7001);
+                fl.uli_u.ports.dport = htons(7000);
+                break;
+        default:
+                BUG();
+        }
+
+        ret = ip_route_output_key(&rt, &fl);
+        if (ret < 0) {
+                kleave(" [route err %d]", ret);
+                return;
+        }
+
+        peer->if_mtu = dst_mtu(&rt->u.dst);
+        dst_release(&rt->u.dst);
+
+        kleave(" [if_mtu %u]", peer->if_mtu);
+}
+
+/*
  * allocate a new peer
  */
 static struct rxrpc_peer *rxrpc_alloc_peer(struct sockaddr_rxrpc *srx,
@@ -47,7 +89,8 @@ static struct rxrpc_peer *rxrpc_alloc_peer(struct sockaddr_rxrpc *srx,
                 peer->debug_id = atomic_inc_return(&rxrpc_debug_id);
                 memcpy(&peer->srx, srx, sizeof(*srx));
 
-                peer->mtu = peer->if_mtu = 65535;
+                rxrpc_assess_MTU_size(peer);
+                peer->mtu = peer->if_mtu;
 
                 if (srx->transport.family == AF_INET) {
                         peer->hdrsize = sizeof(struct iphdr);