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

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Date:   Wed, 15 May 2019 21:54:47 +0000
From:   Martin Lau <kafai@...com>
To:     Wei Wang <weiwan@...gle.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 11:31:44AM -0700, Wei Wang wrote:
> 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.
Got it.  Thanks for the explanation.

> 
> > >
> > >
> > > 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