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

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Message-ID: <CAEA6p_Cs7ExpRtTHeTXFFwLEF27zs6_fFOMVN7cgWUuA3=M1rA@mail.gmail.com>
Date:   Tue, 14 May 2019 12:33:25 -0700
From:   Wei Wang <weiwan@...gle.com>
To:     Stefano Brivio <sbrivio@...hat.com>,
        David Ahern <dsahern@...il.com>,
        Martin KaFai Lau <kafai@...com>
Cc:     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

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)...


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