Re: IPv6 PMTU discovery fails with source-specific routing

[Martin Lau <kafai@fb.com>]

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?

> 
> 
> From: Stefano Brivio <sbrivio@redhat.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@google.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