On 18/09/14 18:09, Alexander Aring wrote:
On Thu, Sep 18, 2014 at 06:03:36PM +0100, Simon Vincent wrote:
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.
Yep, then you could try to test what I did there. Simple save the
address information in reserved skb data and then replacing data in
lowpan_xmit callback. This also solves the capture on lowpan interfaces.
lowpan_xmit should be save to replacing the header.
I am not 100% sure it is safe to replace the header at any point. How
can you guarantee that the skb has been read by the user application RAW
socket by this point? The user might not read the socket for
seconds/minutes/hours.
Also who should free the skb?
Is is worth clarifying this with the netdev mailing list?
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