Re: PMTU discovery broken in Linux for UDP/raw application if the socket is not bound to a device

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On 10/3/18 10:15 PM, Preethi Ramachandra wrote:

Hi,

 

While testing the PMTU discovery for UDP/raw applications, Linux is not
doing PMTU discovery if the UDP server socket is not bound to a device.
 In the scenario we are testing there could be multiple VRF devices
created and an application like UDP/RAW can use a common socket for all
vrf devices. While sending packet IP_PKTINFO socket option can be used
to specify the vrf interface through which packet will be sent out. In
this case, when packet too big icmp6 error message comes back to Linux
on a vrf device, a route lookup is done on default routing-table(0) for
src/dst address which case, the route will not be found and packet is
dropped. If the route lookup happened with proper VRF device (packet’s
incoming index), the route lookup succeeds,  PMTU discovery is successful.

 

This might need a fix, please take a look.

 

*Linux version *

 

Linux  4.8.24

 

*Code flow *

 

Linux code where it expects socket’s bound device in order for PMTU
discovery to happen.

*void ip6_sk_update_pmtu*(struct sk_buff *skb, struct sock *sk, __be32 mtu)

{

                    struct dst_entry *dst;

 

                    ip6_update_pmtu(skb, sock_net(sk), mtu,

                                                            
sk->sk_bound_dev_if, sk->sk_mark, sk->sk_uid);*<<<<< This is the point
where it expects socket’s sk_bound_dev_if to be set. In our testing this
is actually 0, since the socket is not really bound to a vrf device.*

Try this based on top of tree for 4.19-next (whitespace damaged on paste
so you'll need to manually apply and handle differences with 4.8):

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 6c1d817151ca..50b95b48b911 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c

@@ -2360,10 +2360,13 @@ EXPORT_SYMBOL_GPL(ip6_update_pmtu);

 void ip6_sk_update_pmtu(struct sk_buff *skb, struct sock *sk, __be32 mtu)
 {
+       int oif = sk->sk_bound_dev_if;
        struct dst_entry *dst;

-       ip6_update_pmtu(skb, sock_net(sk), mtu,
-                       sk->sk_bound_dev_if, sk->sk_mark, sk->sk_uid);
+       if (!oif && skb->dev)
+               oif = l3mdev_master_ifindex(skb->dev);
+
+       ip6_update_pmtu(skb, sock_net(sk), mtu, oif, sk->sk_mark,

sk->sk_uid);

        dst = __sk_dst_get(sk);
        if (!dst || !dst->obsolete ||