netdev - Re: Sending short raw packets using sendmsg() broke

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Message-ID: <CAF=yD-JOQZZnU6XE9FhEVbxstKmsB-ds58e5v678SiO24VV0+g@mail.gmail.com>
Date:	Fri, 26 Feb 2016 12:33:13 -0500
From:	Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:	Alan Cox <alan@...ux.intel.com>
Cc:	David Miller <davem@...emloft.net>,
	Heikki Hannikainen <hessu@....iki.fi>,
	Network Development <netdev@...r.kernel.org>,
	Willem de Bruijn <willemb@...gle.com>
Subject: Re: Sending short raw packets using sendmsg() broke

On Fri, Feb 26, 2016 at 9:44 AM, Alan Cox <alan@...ux.intel.com> wrote:
> On Thu, 2016-02-25 at 15:26 -0500, David Miller wrote:
>> From: Heikki Hannikainen <hessu@....iki.fi>
>> Date: Thu, 25 Feb 2016 21:36:07 +0200 (EET)
>>
>> > Commit 9c7077622dd9174 added a check, ll_header_truncated(), which
>> > requires that a packet transmitted using sendmsg() with PF_PACKET,
>> > SOCK_RAW must be longer than dev->hard_header_len.
>>
>> Fixed by:
>>
>> commit 880621c2605b82eb5af91a2c94223df6f5a3fb64
>> Author: Martin Blumenstingl <martin.blumenstingl@...glemail.com>
>> Date:   Sun Nov 22 17:46:09 2015 +0100
>>
>>     packet: Allow packets with only a header (but no payload)
>
> The AX.25 case the header is variable length so this still doesn't fix
> the regression as far as I can see.

Right. The simplest, if hacky, fix is to add something along the lines of

  static unsigned short netdev_min_hard_header_len(struct net_device *dev)
  {
      if (unlikely(dev->type ==ARPHDR_AX25))
        return AX25_KISS_HEADER_LEN;
      else
        return dev->hard_header_len;
  }

Depending on how the variable encoding scheme works, a basic min
length check may still produce buggy headers that confuse the stack or
driver. I need to read up on AX25. If so, then extending header_ops
with an optional validate() function is a more generic approach of
checking header sanity.

Assuming that validate() is not needed, I can code up the above and
send it for review if no one objects. A third option is to add an explicit
min_hard_header_len to net_device and use that in ll_header_truncated.