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wireshark-dev: Re: [Wireshark-dev] composite tvbuffs

Re: [Wireshark-dev] composite tvbuffs

From: Jeff Morriss <jeff.morriss.ws_at_nospam>
Date: Tue Oct 12 2010 - 19:19:45 GMT
To: John Sullivan <jsethdev@kanargh.force9.co.uk>, Developer support list for Wireshark <wireshark-dev@wireshark.org>

John Sullivan wrote:
> On Tuesday, October 12, 2010, 3:27:09 PM, Jeff Morriss wrote:
>> I think composite tvbuffs probably do work, but their use is hindered by
>> the fact that each tvb that you get (each time a frame is handed to your
>> dissector) is freed/reused after the packet is dissected. That is,
>> Wireshark does _not_ keep the entire capture file in memory. Because of
>> that, there's relatively little need/use for the composite functions and
>> it's why you need to allocate copies of whatever data you want to
>> reassemble.
>
> This. I was digging into the RTMPT dissector again over the weekend,

This? ;-)

> hoping I could move from allocating memory for every reassembled pdu
> to just creating composite tvbs overlaying the original packets. This
> would have been a big win memory-wise, since the reassembled packets
> have to live for the entire capture session (multiple non-contiguous
> chunks based on multiple prior TCP segments), but turns out not to be
> possible for the above reason.
>
> So it looks like it's necessary to either tvb_memdup from the original
> data, then add a tvb_new_real_data to a composite and use that, or
> se_alloc the entire buffer up front, tvb_memcpy into it and use a
> tvb_new_real_data from that which has a lower overhead in terms of
> tvbuff structures.
>
> (I have a concern that neither of those will free up the used memory.
> tvb_memdup uses g_malloc and there is no notification of when this is
> safe to free so AFAICS it is leaked. The se_alloced blocks will of
> course be freed once the capture is closed, but what of the extra tvb
> structures on top of them? I presume they are permanent rather than
> seasonal so ought to be tvb_freed at some point, but again I can't see
> any notification of when that should happen, so it doesn't.)

The tvb structures should be freed whenever they're not used any more.
If you want to g_malloc() the data, you can use tvb_set_free_cb(). From
"tvbuff.h":

* Although you may call tvb_free(), the tvbuff may still be in use
* by other tvbuff's (TVBUFF_SUBSET or TVBUFF_COMPOSITE), so it is not
* safe, unless you know otherwise, to free your guint8* data. If you
* cannot be sure that your TVBUFF_REAL_DATA is not in use by another
* tvbuff, register a callback with tvb_set_free_cb(); when your tvbuff
* is _really_ freed, then your callback will be called, and at that time
* you can free your original data.

An example from packet-data.c:

guint8 *real_data = tvb_memdup(tvb, 0, bytes);
data_tvb = tvb_new_child_real_data(tvb,real_data,bytes,bytes);
                                 tvb_set_free_cb(data_tvb, g_free);
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