netdev - Re: IPv6 PMTU discovery fails with source-specific routing

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Date:   Wed, 15 May 2019 11:31:44 -0700
From:   Wei Wang <weiwan@...gle.com>
To:     Martin Lau <kafai@...com>
Cc:     Stefano Brivio <sbrivio@...hat.com>,
        David Ahern <dsahern@...il.com>,
        Mikael Magnusson <mikael.kernel@...ts.m7n.se>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: IPv6 PMTU discovery fails with source-specific routing

On Wed, May 15, 2019 at 11:06 AM Martin Lau <kafai@...com> wrote:
>
> On Tue, May 14, 2019 at 12:33:25PM -0700, Wei Wang wrote:
> > I think the bug is because when creating exceptions, src_addr is not
> > always set even though fib6_info is in the subtree. (because of
> > rt6_is_gw_or_nonexthop() check)
> > However, when looking up for exceptions, we always set src_addr to the
> > passed in flow->src_addr if fib6_info is in the subtree. That causes
> > the exception lookup to fail.
> > I will make it consistent.
> > However, I don't quite understand the following logic in ip6_rt_cache_alloc():
> >         if (!rt6_is_gw_or_nonexthop(ort)) {
> >                 if (ort->fib6_dst.plen != 128 &&
> >                     ipv6_addr_equal(&ort->fib6_dst.addr, daddr))
> >                         rt->rt6i_flags |= RTF_ANYCAST;
> > #ifdef CONFIG_IPV6_SUBTREES
> >                 if (rt->rt6i_src.plen && saddr) {
> >                         rt->rt6i_src.addr = *saddr;
> >                         rt->rt6i_src.plen = 128;
> >                 }
> > #endif
> >         }
> > Why do we need to check that the route is not gateway and has next hop
> > for updating rt6i_src? I checked the git history and it seems this
> > part was there from very early on (with some refactor in between)...
> I also failed to understand the RTF_GATEWAY check.  The earliest related
> commit seems to be c440f1609b65 ("ipv6: Do not depend on rt->n in ip6_pol_route().")
>
> How was it working when the exception route was in the tree?
>
When adding all exception route to the main routing tree, because
route cache has dest_addr as /128, the longest prefix match will
always match the /128 route entry.

> >
> >
> > From: Stefano Brivio <sbrivio@...hat.com>
> > Date: Tue, May 14, 2019 at 7:33 AM
> > To: Mikael Magnusson
> > Cc: Wei Wang, David Ahern, Linux Kernel Network Developers, Martin KaFai Lau
> >
> > > On Mon, 13 May 2019 23:12:31 -0700
> > > Wei Wang <weiwan@...gle.com> wrote:
> > >
> > > > Thanks Mikael for reporting this issue. And thanks David for the bisection.
> > > > Let me spend some time to reproduce it and see what is going on.
> > >
> > > Mikael, by the way, once this is sorted out, it would be nice if you
> > > could add your test as a case in tools/testing/selftests/net/pmtu.sh --
> > > you could probably reuse all the setup parts that are already
> > > implemented there.
> > >
> > > --
> > > Stefano