Dwfl Callbacks, how would I pass a ByteBuffer to find_elf (possibly through user_data).

[Nurdin <npremji@redhat.com>]

The following depends on the patch in bug:
<http://sourceware.org/bugzilla/show_bug.cgi?id=4513>
Specifically the changes to Dwfl.cxx

The problem I am trying to solve is to get the Dwfl_Callback find_elf:

dwfl_frysk_proc_find_elf (Dwfl_Module *mod __attribute__ ((unused)),
              void **userdata __attribute__ ((unused)),
              const char *module_name, Dwarf_Addr base,
              char **file_name, Elf **elfp)

 to use frysk to read a processes memory rather than reading 
/proc/pid/mem. For the vdso case.

In order to do this I must somehow pass the memory from frysk to the 
callback.
What the callback does currently is store the pid in the Dwfl_Module 
name. (as "[vdso pid]")
and then lookup the /proc/pid/mem. It uses a callback with the signature:

read_proc_memory (void *arg, void *data, GElf_Addr address,
          size_t minread, size_t maxread)

So if I can change the void *arg to refer to a byte buffer rather than a 
file descriptor and just read from the byte buffer I should be one step 
closer to getting procedure names for a stack trace using elfutils.

I think the best way to do this is to pass the byte buffer to the 
callback through the void **userdata field, but I'm unsure as to how to 
set that field.

I have thought about having a member in the Dwfl class to hold the byte 
buffer but since neither the read_proc_memory or 
dwfl_frysk_proc_find_elf callback isn't linked to a java method I can't 
use the "this" keyword.

I've also thought about wrapping the Dwfl_Callback in a 
Frysk_Dwfl_Callback struct which has a bytebuffer as well, but I can't 
reference the callback structure in read_proc_memory or 
dwfl_frysk_proc_find_elf.