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From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date: Fri, 18 May 2018 13:09:37 -0400
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Subject: Re: WARNING in ip_recv_error
To: David Miller <davem@davemloft.net>
Cc: Eric Dumazet <eric.dumazet@gmail.com>,
        DaeLyong Jeong <threeearcat@gmail.com>,
        Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        Network Development <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Byoungyoung Lee <byoungyoung@purdue.edu>,
        Kyungtae Kim <kt0755@gmail.com>, bammanag@purdue.edu,
        Willem de Bruijn <willemb@google.com>
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On Fri, May 18, 2018 at 11:44 AM, David Miller <davem@davemloft.net> wrote:
> From: Eric Dumazet <eric.dumazet@gmail.com>
> Date: Fri, 18 May 2018 08:30:43 -0700
>
>> We probably need to revert Willem patch (7ce875e5ecb8562fd44040f69bda96c999e38bbc)
>
> Is it really valid to reach ip_recv_err with an ipv6 socket?

I guess the issue is that setsockopt IPV6_ADDRFORM is not an
atomic operation, so that the socket is neither fully ipv4 nor fully
ipv6 by the time it reaches ip_recv_error.

  sk->sk_socket->ops = &inet_dgram_ops;
  < HERE >
  sk->sk_family = PF_INET;

Even calling inet_recv_error to demux would not necessarily help.

Safest would be to look up by skb->protocol, similar to what
ipv6_recv_error does to handle v4-mapped-v6.

Or to make that function safe with PF_INET and swap the order
of the above two operations.

All sound needlessly complicated for this rare socket option, but
I don't have a better idea yet. Dropping on the floor is not nice,
either.