On 16/06/21 15:36, David Perry wrote:
Sorry to drag up an old topic, but I've been thinking about this:
Message: 5
Date: Sat, 29 May 2021 09:32:29 +0200
From: Anders Broman <a.broman58@xxxxxxxxx>
To: Developer support list for Wireshark <wireshark-dev@xxxxxxxxxxxxx>
Subject: Re: [Wireshark-dev] Calling a dissector: Type for data
parameter
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<CAOpyz=zDycm33PXUwtBCTew7gTTEcSLiJ-f8SHW0L-863Q517A@xxxxxxxxxxxxxx>
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Hi,
Yes the method is fragile. At the time of development I think it was
proposed to pass a struct containing a string and the void pointer where
the string could be used as a identifier. But that was voted down.
Regards
Anders
I wasn't around for that discussion so I don't know the reasons, but
how does this sound as a refined approach?:
* Define a `dissector_data_t` that has a `guint32` identifier field,
and a `void *` data field.
* Replace the `void *data` parameter to dissectors with a pointer to a
`dissector_data_t`.
* Either:
* Easy way: maintain a static list of identifiers that map to
expected data types, or
* Have dissector X request an identifier in its registration
function for the type of data it expects, and have dissector Y (which
will call X) request, in its handoff function, the identifier of the
type of data it needs to pass to X.
* Dissectors check for the right identifier in their
`dissector_data_t` parameter and don't try to use it if it's wrong.
Thoughts?
I think what you suggest would be the most straightforward fix.
To avoid breaking backward compatibility and changing thousands of
dissectors at the same time, both of which are highly problematic, it
can be done by adding a new dissector type (like it was done with
"dissector_cb_t", only using a different signature).[1]
Also a giant static list of dissector_data_t type identifiers would be
pretty clunky. I think we could recycle the protocol registration number
for that.
João
[1]https://gitlab.com/wireshark/wireshark/-/blob/master/epan/packet.h#L79
David
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