DTrace - capturing two userspace strack frames on top of system call

I want to capture the (userland) stack trace on top  of the write(2)
system call.  I seem to have some difficulty switching from kernel to
user mode here.  For every write system call I want to print the
calling userlevel frames.  I can't care whether they are individually
printed or counted.

Here is what I think should do it:
syscall::write*:entry /arg1/ { @traces[ustack()] = count(); }

However, I get one error each for each write call:
dtrace: error on enabled probe ID 2 (ID 56902:
  syscall:freebsd:write:entry): invalid address (0x0) in action #2

This gives the same error:
syscall::write*:entry /arg1/ { ustack(); }


%%

If I use system stackframes it works, but of course it doesn't print
the calling frames:

syscall::write*:entry /arg1/ { @traces[stack()] = count(); }

dtrace: script 'stack-to-write.dtrace' matched 3 probes
dtrace: buffer size lowered to 2m
dtrace: aggregation size lowered to 2m
dtrace: pid 11790 has exited


              kernel`handle_el0_sync+0x40
              136

%%

Is what I am trying to do even possible? Can I mix kernel and
userlevel space like this? 

Any other ideas? I could brute-force it with LD_PRELOAD overloading
of write(2), but dtrace would be more elegant.

Thanks in advance
    Martin
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Martin Cracauer <cracauer@cons.org>   http://www.cons.org/cracauer/