netdev - commit b9ef5513c99b breaks ping to ipv6 linklocal addresses on debian buster

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Date:   Sun, 5 Jan 2020 18:26:42 -0700
From:   David Ahern <dsahern@...il.com>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Casey Schaufler <casey@...aufler-ca.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: commit b9ef5513c99b breaks ping to ipv6 linklocal addresses on debian
 buster

I am seeing ping failures to IPv6 linklocal addresses with Debian
buster. Easiest example to reproduce is:

$ ping -c1 -w1 ff02::1%eth1
connect: Invalid argument

$ ping -c1 -w1 ff02::1%eth1
PING ff02::01%eth1(ff02::1%eth1) 56 data bytes
64 bytes from fe80::e0:f9ff:fe0c:37%eth1: icmp_seq=1 ttl=64 time=0.059 ms


git bisect traced the failure to:

commit b9ef5513c99bf9c8bfd9c9e8051b67f52b2dee1e
Author: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Date:   Fri Apr 12 19:59:35 2019 +0900

    smack: Check address length before reading address family


Arguably ping is being stupid since the buster version is not setting
the address family properly (ping on stretch for example does):

$ strace -e connect ping6 -c1 -w1 ff02::1%eth1
connect(4, {sa_family=AF_UNSPEC,
sa_data="\4\1\0\0\0\0\377\2\0\0\0\0\0\0\0\0\0\0\0\0\0\1\3\0\0\0"}, 28) = 0

but the command works fine on kernels prior to this commit, so this is
breakage which goes against the Linux paradigm of "don't break userspace"