netdev - Re: [PATCH net v2 3/3] packet: fix tpacket

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Date:	Tue, 10 Nov 2015 18:24:25 -0500
From:	Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:	Daniel Borkmann <daniel@...earbox.net>
Cc:	Willem de Bruijn <willemdebruijn.kernel@...il.com>,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Tobias Klauser <tklauser@...tanz.ch>,
	Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH net v2 3/3] packet: fix tpacket_snd max frame and vlan handling

On Tue, Nov 10, 2015 at 6:12 PM, Daniel Borkmann <daniel@...earbox.net> wrote:
> On 11/10/2015 11:52 PM, Willem de Bruijn wrote:
>>>
>>>          if (sock->type == SOCK_DGRAM) {
>>> -               err = dev_hard_header(skb, dev, ntohs(proto), addr,
>>> -                               NULL, tp_len);
>>> +               /* In DGRAM sockets, we expect struct sockaddr_ll was
>>> filled
>>> +                * via struct msghdr, so we have dest mac and
>>> skb->protocol.
>>> +                * Otherwise there's not too much useful things we can do
>>> in
>>> +                * this flush run.
>>> +                */
>>> +               err = dev_hard_header(skb, dev, ntohs(skb->protocol),
>>> addr,
>>> +                                     NULL, tp_len);
>>
>>
>> This change is not really necessary.
>
>
> Sure agreed, I found it helpful though. Don't mind removing it.
>
>>>                  if (unlikely(err < 0))
>>>                          return -EINVAL;
>>> -       } else if (dev->hard_header_len) {
>>
>>
>> Why remove the check on hard_header_len?
>
>
> Hmm, the patch doesn't remove the check (it's moved further below).
>
>>> -               if (ll_header_truncated(dev, tp_len))
>>> -                       return -EINVAL;
>>> +       } else {
>>> +               /* If skb->protocol is still 0, try to infer/guess it.
>>> Might
>>> +                * not be fully reliable in the sense that a user could
>>> still
>>> +                * change/race data afterwards, but on the other hand the
>>> proto
>>
>>
>> The race goes away when probing it after the copy in skb_store_bits.
>> Then it is also certain that tp_len is long enough to hold the entire
>> link layer header.
>
>
> The skb_store_bits() is only done in case we do have a dev->hard_header_len
> or in case where we run into a possible situation where we have the
> additional
> 4 bytes on a full frame. In that case we need to check them properly, which
> requires copying, otherwise we don't copy any header.

I assumed that hard_header_len has to be non-zero if there
is a link layer header to probe. If we only intend to implement
probing in the case of Ethernet, then it certainly holds.

>
>>> +                * can be set arbitrarily anyways. We only need to take
>>> care
>>> +                * in case of extra large VLAN frames.
>>> +                */
>>> +               if (!skb->protocol && tp_len >= ETH_HLEN)
>>> +                       skb->protocol = ((struct ethhdr *)data)->h_proto;
>>
>>
>> Packet sockets are not restricted to link layer of type Ethernet.
>>
>> There are a few other points in this file that also cast mac header
>> to eth_hdr(skb).
>
>
> Ok, the set doesn't address this assumption which we have elsewhere, too.
> Do you suggest to also check on dev->type for these cases?

Yes. If I'm right, then the other cases have to be fixed, too. One of the
eth_hdr(skb) calls was introduced by patch 09effa67a18d, where the
deleted code shows how it is safely restricted to ethernet:

  -       if (dev->type == ARPHRD_ETHER) {
  -               skb->protocol = eth_type_trans(skb, dev);
  -               if (skb->protocol == htons(ETH_P_8021Q))

>
> Thanks,
> Daniel
> --
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