On Wed, 2016-03-16 at 19:53 -0400, David Miller wrote:
> From: Wei Wang <wei...@google.com>
> Date: Mon, 14 Mar 2016 13:59:47 -0700
>
> > From: Wei Wang <wei...@google.com>
> >
> > When ICMPV6_PKT_TOOBIG message is received by a connected UDP socket,
> > the new mtu value is not properly updated in the dst_entry associated
> > with the socket.
> > This leads to the issue that the mtu value returned by getsockopt(sockfd,
> > IPPROTO_IPV6, IPV6_MTU, ...) is wrong.
> > The fix is to update sk->sk_dst_cache and other corresponding fields
> > when a new routing cache is allocated for the new pmtu in UDP connected
> > socket case.
> >
> > Signed-off-by: Wei Wang <wei...@google.com>
>
> Wait a second:
>
> > if (nrt6) {
> > rt6_do_update_pmtu(nrt6, mtu);
> > -
> > - /* ip6_ins_rt(nrt6) will bump the
> > - * rt6->rt6i_node->fn_sernum
> > - * which will fail the next rt6_check() and
> > - * invalidate the sk->sk_dst_cache.
> > - */
> > + if (sk)
> > + ip6_dst_store(sk, &nrt6->dst, daddr, saddr);
> > ip6_ins_rt(nrt6);
> > }
> > }
>
> I still haven't seen a satisfactory answer as to why the as-designed
> invalidation mechanism using fn_sernum is not working.
>
> If that's broken, then a lot of other things won't work properly
> either.
>
> I've read the ip6_ins_rt() code path several times, and it always
> increments the serial number, and therefore the next dst->check() call
> (which every cached route usage should invoke) should invalide
> this socket's route and lookup the new one.
>
> Why does this not work?
One of the issue is that IPV6_MTU getsockopt() will not check the dst,
but simply use __sk_dst_get() : It will then report old mtu.
Not sure we want to use the full check and then if dst appears to be
obsolete, do another route lookup ?
