Wireshark-dev: Re: [Wireshark-dev] How to call a sub-dissector many times without knowing the e
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From: wsgd <[email protected]>
Date: Sun, 01 Feb 2015 11:01:08 +0100
Hello Guy,

Le 31/01/2015 22:56, Guy Harris a écrit :
On Jan 31, 2015, at 1:46 PM, wsgd <[email protected]> wrote:

I want to call a sub-dissector many times.
I know the total size of my data.
I do NOT know the size to give to the sub-dissector.
The sub-dissector will know (by itself) the size to dissect.

The corresponding scheme from the parent dissector point of view :

|-----------------------------------Packet data (total size is known)--------------------------------|
|--Parent dissector--|--sub-dissector 1 (size is unknown)--|...|--sub-dissector N (size is unknown)--|


The sub-dissector will be called by dissector_try_uint (or call_dissector or eventually dissector_try_heuristic).


Question 1)
Is it possible to call dissector_try_uint (or ...) with more data than needed by the sub-dissector ?
Yes.

Question 2)
How to know how many bytes have been dissected by the sub-dissector ?
Have the sub-dissector be a "new-style" dissector, which is expected to return the number of bytes of data that it dissected.
For now, I will care only about "new-style" dissector.
For "old-style" dissectors, which do not return a value, the dissector-calling mechanism will return tvb_captured_length(), so it appears as if the entire tvbuff handed to it was dissected.

dissector_try_uint(), call_dissector(), etc. return the return value of a new-style dissector or the tvbuff captured length of an old-style dissector.
Looking at packet.h, call_dissector returns the return value of a new-style dissector : ok
Looking at packet.h, dissector_try_uint returns a gboolean, not a size.
So, instead of calling dissector_try_uint, I suppose I can do :
- dissector_get_uint_handle
- call_dissector
I will try this.


Thanks,
Olivier
Note that a new-style dissector can return 0, which is assumed to mean "this isn't a packet for my protocol".

This does *not* work for heuristic dissectors.  Perhaps heuristic dissectors could be changed to return a bytes-dissected value rather than a Boolean, with 0 meaning "I dissected nothing because this isn't a packet for my protocol"; I think there were cases for non-heuristic dissectors where nothing is dissected but the dissection was valid, e.g. an RPC-like protocol where a reply has no data at the RPC layer but has data at the layer of the particular RPC protocol being used, and I think I discovered that when trying to make all dissectors new-style dissectors, but I can't remember what I found.

For a heuristic dissector, however, the dissector has to look at *some* data in order to determine whether the packet is for its protocol or not, so presumably it could never return 0 if the packet is for its protocol.
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