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

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Date:   Sat, 6 Oct 2018 21:28:57 -0600
From:   David Ahern <dsahern@...il.com>
To:     Preethi Ramachandra <preethir@...iper.net>
Cc:     Reji Thomas <rejithomas@...iper.net>,
        Yogesh Ankolekar <ayogesh@...iper.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: PMTU discovery broken in Linux for UDP/raw application if the
 socket is not bound to a device

The correct mailing list is netdev@...r.kernel.org (added)

non-text emails will be rejected.


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 ||