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
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