Re: Proper private_data usage

[Jeff Morriss <jeff.morriss.ws () gmail com>]

On 10/15/13 12:00, Christopher Maynard wrote:
> What is the proper, safe and recommended way to use pinfo->private_data?
>
> If you look at the TCP dissector, you see the following:
>
>          pd_save = pinfo->private_data;
>          TRY {
>              (*dissect_pdu)(next_tvb, pinfo, tree);
>          }
>          CATCH_NONFATAL_ERRORS {
>              /*  Restore the private_data structure in case one of the
>               *  called dissectors modified it (and, due to the exception,
>               *  was unable to restore it).
>               */
>              pinfo->private_data = pd_save;
>              show_exception(tvb, pinfo, tree, EXCEPT_CODE, GET_MESSAGE);
>          }
>          ENDTRY;
>
> I did not check all dissectors, but this is certainly not done for all of
> them.  I guess the question I have is, should it?  Is this the recommended
> approach?

I started adding that back in r34408 (because I was getting a crash 
after a malformed packet exception).  At the time I went through most of 
the dissectors and tried to make the same change (e.g., r34436 but I 
think there were more revisions).  Of course I may have missed some and 
of course that was a long time ago and new dissectors have arrived...

But, yes, I think that when using private_data the restoration is 
necessary: otherwise if you have multiple PDUs in one frame then an 
exception in one of the early PDUs will cause the private_data to be 
wrong when the next PDU is dissected.

This is one of the reasons not to use private_data at all.

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