netdev - Re: [PATCH] net-ipv6: fix excessive RTF_ADDRCONF flag on ::1/128 local route (and others)

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Date:   Sun, 1 Sep 2019 19:55:27 +0200
From:   Maciej Żenczykowski <zenczykowski@...il.com>
To:     Maciej Żenczykowski <maze@...gle.com>,
        "David S . Miller" <davem@...emloft.net>
Cc:     Linux NetDev <netdev@...r.kernel.org>,
        David Ahern <dsahern@...il.com>
Subject: Re: [PATCH] net-ipv6: fix excessive RTF_ADDRCONF flag on ::1/128
 local route (and others)

Some background:

This was found due to bad interactions with one of the few remaining
Android common kernel networking patches.
(The one that makes it possible for RA's to create routes in interface
specific tables)

The cleanup portion of it scours all tables and deletes all relevant
ADDRCONF routes, which in 5.2-rc1+ now includes ::1/128 and thus
terribly breaks things (in the Android Kernel Networking tests).

However, it *is* a userspace visible change in behaviour (since it's
visible via the above /proc file),
so one could argue for the above patch (or something similar).

The Android patch *could* also probably be adjusted to handle this
case (and thus prevent the breakage).

It's not immediately clear to me what is the better approach as I'm
not immediately certain what RTF_ADDRCONF truly means.
However the in kernel header file comment does explicitly mention this
being used to flag routes derived from RA's, and very clearly ::1/128
is not RA generated, so I *think* the correct fix is to return to the
old way the kernel used to do things and not flag with ADDRCONF...

Opinions?