netdev - Bridged passthru MACVLAN breaks IPv6 multicast?

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Message-ID: <CAGnHSEk-gxY3jr-2k8+NSB0uf9H94SDQyxJFVM1LH3A+Bs+5MA@mail.gmail.com>
Date:   Mon, 23 Aug 2021 02:07:47 +0800
From:   Tom Yan <tom.ty89@...il.com>
To:     netdev@...r.kernel.org
Subject: Bridged passthru MACVLAN breaks IPv6 multicast?

Hi,

Normally when a NIC is (directly) enslaved as a bridge port, the NIC
itself does not need to have a IPv6 link-local address configured on
it for IPv6 multicast / NDP to work properly (instead the address can
simply be configured on the bridge like IPv4 addresses).

Yet it appears that if the bridge port is instead a passthru mode
MACVLAN, IPv6 multicast traffics from (the link/"side" of) it cannot
reach the host (as in, cannot even be captured with tcpdump) unless
either the MACVLAN or its underlying link has a/the[1] IPv6 link-local
address configured.

Is it an expected behavior? Or is it a bug?

[1]: In my configuration, the bridge, the bridged passthru MACVLAN and
its underlying link have the same MAC address and hence (at least by
default) their IPv6 link-local addresses are identical.

Regards,
Tom