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

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Message-ID: <f818d389-66e3-e952-4466-fc047cf15c21@gmail.com>
Date:   Wed, 15 May 2019 09:43:32 -0600
From:   David Ahern <dsahern@...il.com>
To:     Wei Wang <weiwan@...gle.com>, Stefano Brivio <sbrivio@...hat.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

On 5/14/19 1:33 PM, 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 can not make sense of that check either.