LAPDm, msgb and osmo-bts breakage

[Holger Hans Peter Freyther]

Hi Jolly,

I have finally tracked down the LAPDm breakage we have in osmo-bts. It comes 
from the fact that we get a RSL message from the BSC and then forward the msgb 
that holds the IPA header followed by the RSL message. There is code in both 
lapdm.c and lapd_core.c that tries to extract the RSL_IE_L3_INFO and then 
updates the msgb to be of the length of that TLV and sets the l3h to the start 
of the L3_INFO:


Currently:
         msgb_pull_l2h(msg);
         msg->len = length;
         msg->tail = msg->data + length;

this kind of assumes that before the "msgb_pull_l2h", msg->l2h == msg->data 
but in the above case it does not hold true. You will need to use "msg->l3h + 
length" with this idiom.

Fixes:
I have pushed some fixes to "zecke/lapd-fixes", but I would like to see the
above pattern move to msgb.h, and it would be nice if we could write test 
cases for the other methods in lapdm.c that manipulate the msgb.h (currently 
only data request is tested).


One more thing:
I would be somehow happy if lapdm_rslms_recvmsg would indicate if any messages 
were sent or queued, currently it will return 0 in more than one case.


cheers
	holger