Re: 6lowpan raw socket problems

[Alexander Aring <alex.aring@gmail.com>]

On Fri, Sep 19, 2014 at 12:27:54PM +0100, Simon Vincent wrote:
> 
> On 19/09/14 12:08, Alexander Aring wrote:
> >On Thu, Sep 18, 2014 at 04:19:11PM +0200, Alexander Aring wrote:
> >>On Thu, Sep 18, 2014 at 03:02:17PM +0100, Simon Vincent wrote:
> >>>I have created a small test program that shows this problem. It looks like a
> >>>race condition as sometimes the addresses are not corrupt.
> >>>
> >>Mhh maybe some used after freed and then we copy somewhere garbage sometimes.
> >>Don't know right now.
> >>
> >>>It looks like if the RAW socket gets the packet before the packet hits the
> >>>6lowpan layer the addresses are fine. If the packet hits the 6lowpan layer
> >>>before the RAW socket gets the packet then the addresses are corrupt.
> >>>
> >>>The test program can be found here.
> >>>https://github.com/xsilon/sockdebug
> >>>
> >>>I will continue debugging!
> >>>
> >>ok, good luck.
> >>
> >I gave this now a try, how can I see the issue now?
> >
> >I see on output:
> >
> >recv_raw_icmp[fe80:0:41:c863:cdab:ffff:bbaa:aaaa%lowpan0->?]
> >
> >this address doesn't exist in my network.
> >
> >I can also upload wpan wireshark logs and lowpan wireshark logs, if you
> >like.
> >
> >In sockdebug I changed also "const char* src_string =" to one of my
> >lowpan addresses. Simon are you still here to debug this issue with me?
> >:-)
> Yes this is the same error I am seeing. I find that sometimes the recv
> address is correct but mostly you get the corrupt address as the ipv6 header
> has been overwritten by our compressed 6lowpan header.
> 
> If you comment out the 6lowpan header compression function it solves the
> problem.

okay, then I dig now into the issue why the address is garbage.

> 
> I am trying to understand how the network stack handles skbs. As it is a
> multicast packet it will be sent out on  802.15.4, raw socket and any other
> interfaces you have but it looks like in this case the interfaces all get a
> skb pointing to the same data. Therefore when we replace the ipv6 header
> with a compressed version everyone else still thinks there is a normal ipv6
> header still there and therefore gets corrupt data. Should each interface
> get a copy of the data? E.g. the ethernet, wifi, 802.15.4 and raw socket all
> get a copy of the skb data not a clone?
> 
> Maybe normally each interface will get a copy of the skb so they can attach
> their own mac header but in the case of the RAW socket they don't bother
> doing a copy as they don't need to add a header for the socket. But then we
> come along and destroy the ipv6 header!!
> 
> Just a theory!
> 

okay, there exists a lot of there. I know what you saying because the
data buffer is shared there exist race conditions because some other skb
has in the next step a 6LoWPAN header, if I understand that correctly.

- Alex