From mboxrd@z Thu Jan  1 00:00:00 1970
From: Vladis Dronov <vdronov@redhat.com>
Subject: Re: [PATCH] net: arp, ipv6: handle special case of tap device
Date: Wed, 3 Oct 2018 09:25:12 -0400 (EDT)
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Cc: kuznet@ms2.inr.ac.ru, yoshfuji@linux-ipv6.org,
        netdev@vger.kernel.org, syzkaller@googlegroups.com,
        linux-kernel@vger.kernel.org
To: David Miller <davem@davemloft.net>
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In-Reply-To: <20181002.222540.1948082296309410659.davem@davemloft.net>
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> From: "David Miller" <davem@davemloft.net>
> Sent: Wednesday, October 3, 2018 7:25:40 AM
>
> > Probably, this functionality is already used by some userspace, so I
> > believe, I cannot just remove TUNSETLINK ioctl.
> 
> Who uses it, how do they use it, and what do they sanely expect about
> the link layer header properties of a tap device which has had ->type
> changed?

Honestly, I have no idea. I was working on a syzkaller bug report and
found that the reason for that bug was TUNSETLINK ioctl.

This ioctl was introduced 13 years ago with ff4cc3ac93e1d and it says:

> For use with
> wireless and other networking types it should be possible to set the
> ARP type via an ioctl.

, that's all. Let me try to prepare a patch that sets dev->addr_len and
dev->broadcast according to dev->type. I would probably need an advise
what other fields must be adjusted too.

Otherwise, we can just to remove the TUNSETLINK ioctl.

Best regards,
Vladis Dronov | Red Hat, Inc. | Product Security Engineer