netdev - Re: [PATCH] ipv6: Fix the pmtu path for connected UDP socket

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Date:	Mon, 21 Mar 2016 23:02:20 -0700
From:	Martin KaFai Lau <kafai@...com>
To:	Wei Wang <tracywwnj@...il.com>
CC:	Cong Wang <xiyou.wangcong@...il.com>, Wei Wang <weiwan@...gle.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH] ipv6: Fix the pmtu path for connected UDP socket

On Mon, Mar 21, 2016 at 10:13:41AM -0700, Wei Wang wrote:
> Hey Cong,
>
> This solution probably will not work.
> First of all, if you look into __ip6_rt_update_pmtu(), it creates a
> new dst and this dst does not get passed back to its caller. So unless
> we tweak this function to pass the new dst back, we can only update
> sk->sk_dst_cache inside the function itself.
> Secondly, ip6_update_pmtu is called in multiple places. Not only here.
> I am not sure how many places need to be changed like this. It seems
> not a good thing to do.
>
> Thanks.
> Wei
>
> On Fri, Mar 18, 2016 at 3:09 PM, Cong Wang <xiyou.wangcong@...il.com> wrote:
> > On Fri, Mar 18, 2016 at 2:26 PM, Wei Wang <tracywwnj@...il.com> wrote:
> >> I don't think ip6_sk_update_pmtu() is a good place to put it as all it
> >> does is to call ip6_update_pmtu(). And ip6_update_pmtu() does the
> >> route lookup and call __ip6_rt_update_pmtu.
> >> We can put it in ip6_update_pmtu(). But that still means we need to
> >> pass sk to ip6_update_pmtu() and I don't think it makes any difference
> >> compared to the current fix.
I think Cong Wang is suggesting, in ip6_sk_update_pmtu():
1. call ip6_upate_pmtu() as it is
2. do a dst_check()
3. re-lookup() if it is invalid
4. and then do a ip6_dst_store()/dst_set

The above is exactly what inet6_csk_update_pmtu(), which was also used in the
first patch, is doing.

In term of difference, AFAICT, the current patch is an optimization in the
sense that the update_pmtu() code path does not have to do a dst_check to
discover its sk->sk_dst_cache is invalid, and then do a relookup to find out
that the just created RTF_CACHE clone should be used.  To get this, it may
make more sense to remove all the relookup code together during update_pmtu().
Even if this slow path was to be optimized, should it be put in a
separate patch where net-next is a better candidate?

I think fixing it in __udp6_lib_err() or what Cong Wang is suggesting makes
more sense for a net branch fix.  If there is logic specific to connected-udp,
I would do it in the __udp6_lib_err() instead.  After looking at
udpv6_sendmsg() and how it calls ip6_dst_store(), may also need to be careful
what daddr and saddr should be passed to ip6_dst_store(), or at least a commit
message.  The first patch is essentially passing NULL to daddr and saddr
while the second patch seems passing something else.

> >>
> >
> > Well, your patch touches all the callers of ip6_update_pmtu() , if you just
> > fix ip6_sk_update_pmtu() as I suggested, you only need to change one
> > function, ideally. And the ipv4 code is there, although I am not sure, it
> > looks like we can just mimic the logic here:
> >
> > diff --git a/net/ipv6/route.c b/net/ipv6/route.c
> > index ed44663..b88c2ff 100644
> > --- a/net/ipv6/route.c
> > +++ b/net/ipv6/route.c
> > @@ -1417,8 +1417,28 @@ EXPORT_SYMBOL_GPL(ip6_update_pmtu);
> >
> >  void ip6_sk_update_pmtu(struct sk_buff *skb, struct sock *sk, __be32 mtu)
> >  {
> > -       ip6_update_pmtu(skb, sock_net(sk), mtu,
> > -                       sk->sk_bound_dev_if, sk->sk_mark);
> > +       const struct ipv6hdr *iph = (struct ipv6hdr *) skb->data;
> > +       struct net *net = sock_net(sk);
> > +       struct dst_entry *dst;
> > +       struct flowi6 fl6;
> > +
> > +       bh_lock_sock(sk);
> > +
> > +       memset(&fl6, 0, sizeof(fl6));
> > +       fl6.flowi6_oif = sk->sk_bound_dev_if;
> > +       fl6.flowi6_mark = sk->sk_mark ? : IP6_REPLY_MARK(net, skb->mark);
> > +       fl6.daddr = iph->daddr;
> > +       fl6.saddr = iph->saddr;
> > +       fl6.flowlabel = ip6_flowinfo(iph);
> > +
> > +       dst = ip6_route_output(net, NULL, &fl6);
> > +       if (!dst->error)
> > +               __ip6_rt_update_pmtu(dst, NULL, iph, ntohl(mtu));
> > +
> > +       sk_dst_set(sk, &rt->dst);
> > +       bh_unlock_sock(sk);
> > +
> > +       dst_release(dst);
> >  }
> >
> >
> > Please don't judge me on the code, it could still miss a lot of things,
> > but it can show my idea...