Re: 6lowpan raw socket problems

[Simon Vincent <simon.vincent@xsilon.com>]

On 18/09/14 17:30, Alexander Aring wrote:
> On Thu, Sep 18, 2014 at 04:54:53PM +0100, Simon Vincent wrote:
>> It looks like in 6lowpan_iphc.c lowpan_header_compress the original ipv6
>> header is removed and the compressed header is attached.
>>
>> These four lines are responsible.
>>      skb_pull(skb, sizeof(struct ipv6hdr));
>>      skb_reset_transport_header(skb);
>>      memcpy(skb_push(skb, hc06_ptr - head), head, hc06_ptr - head);
>>      skb_reset_network_header(skb);
>>
>> I don't think we can do this as the skb is being used in other parts of the
>> ip stack. Hence when the ipv6 header is read elsewhere the addresses become
>> corrupt as they have been overwritten by the 6lowpan compressed header.
>>
>> Any ideas on how to fix this?
>>
> yes, but I don't believe that this makes trouble. <--- or only makes
> trouble by replacing data, see below.
>
> It's called by a callback of header_ops [0].
>
> This is for generating the mac header with address information from
> neighbor discovery cache (mainly destination address) and source
> addresse (mainly netdev->dev_addr).
>
> Another example of this function is ethernet. [1]
>
> On [1] you will se that the ethernet header will created there.
>
> - Get data from skb for ethhdr (ethernet header)
>    struct ethhdr *eth = (struct ethhdr *)skb_push(skb, ETH_HLEN);
>
> - memcpy(eth->h_source, saddr, ETH_ALEN); <-- source address
>
> - memcpy(eth->h_dest, daddr, ETH_ALEN); <-- destination address.
>
>
> So they using the callback there to manipulate the skb here.
>
> Another idea is that, maybe we can ADD data but not REPLACING existing
> data with that. I don't know right now.
I think what is done in ethernet is fine as they are adding data. But in 
the lowpan_header_compress we are doing a skb_pull to remove the 
existing ipv6 header before adding on the new 6lowpan header, so we are 
replacing the existing data.
>
> But I moved this handling out of the create callback of header_ops. This
> also fix the issue by running wireshark&co on a lowpan interface.
>
> What I did is only to save the address information in the reserved room
> of skb in this callback. [2]
>
> Then running replacing header in the xmit callback of lowpan device. It
> looks very different now! I have also splitted the lowpan implementation
> in three files "main.c, tx.c, rx.c". Please note that. This code is part
> of the rework and I want to fix the wireshark&co issue there.
>
> I don't have a solution for this right now which applies on current
> mainline, sorry! But the xmit callback is 100% secure by replacing skb
> header data.
>
> If you like you can try to apply it on mainline. What I said is that
> this also fix the IPv6 capturing on a lowpan interface. I want also say
> to you that I added a comment there "TODO ask david or marc if this run
> into trouble", because I am not sure if the reserved skb room can be
> overwritten sometimes, then we stuck into the same issue. (Worked on my
> side currently), but I also don't do much raw socket RPL messages. ;-)
>
> What I know is that we can't use skb->cb here to save the information,
> this would be overwritten by traffic control.
>
> - Alex
>
> [0] http://lxr.free-electrons.com/source/include/linux/netdevice.h#L255
> [1] http://lxr.free-electrons.com/source/net/ethernet/eth.c#L78
> [2] https://github.com/linux-wpan/linux-wpan-next/blob/7ccf5a24a19a72b857bbcbc76342f4838dd85d6b/net/ieee802154/6lowpan/tx.c#L59
> [3] https://github.com/linux-wpan/linux-wpan-next/blob/7ccf5a24a19a72b857bbcbc76342f4838dd85d6b/net/ieee802154/6lowpan/tx.c#L261