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Wireshark-bugs: [Wireshark-bugs] [Bug 5575] Patch to fix memory leaks/errors in Lua plugin

Date: Sun, 6 Nov 2011 01:43:11 -0700 (PDT)
https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=5575

--- Comment #12 from Robert G. Jakabosky <bobby@xxxxxxxxxxxxxxx> 2011-11-06 01:43:07 PST ---
(In reply to comment #11)
> (In reply to comment #6)
> > Created an attachment (id=7371)
 --> (https://bugs.wireshark.org/bugzilla/attachment.cgi?id=7371) [details]
[details]
> > Fix memory errors in Lua dissectors.
> 
> I just had a look at this patch.  Do we really need to introduce the TvbRange
> cleanup routines?  I would like to change the g_malloc in new_TvbRange() to
> ep_alloc instead of introducing this.  What do you think?

As-is ep_alloc can't be used for the userdata values that are pushed into the
Lua state, since the memory may be de-allocated before the userdata value is
GC'ed.  Even if a full GC is done at the end of each frame (this would be very
slow), there is no guarantee that all the userdata values had been GC'ed.

When I created the first patch for the memory errors, I didn't fully understand
the reason for the clear_outstanding_* logic, and had just used the same logic
for the TvbRange values to fix the access after free memory errors.

The clear_outstanding_* logic could be changed to invalid the userdata values
in the Lua state before they are GC'ed.  Basically the userdata values
(Tvb,TvbRange,Pinfo,TreeInfo,etc...) can be nulled at the end of a frame.  This
would allow ep_alloc() to be used for those datastructures.  This can only be
done for values that the Lua dissector shouldn't be allowed to keep references
to after each frame.  To do this a Lua table would be stored in the Lua
registry that will hold a reference to all frame invalided userdata values. 
Those userdata values would be nulled at the end of a frame.  If a Lua
dissector tries to access one of those values after it had been invalided, a
Lua error can be raised.

> Why did you add col_clear before col_add_str?  I don't think they are needed.

I think those are remnants from when I was updating the patches to the latest
svn revision. Those memory errors had been fixed by other people in revisions
34618 & 35795.  They can be removed.

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