Problem with 32-bit negative return values

[Mike Mason <mmlnx@us.ibm.com>]

I'm trying to use the return value from sock_recvmsg() to determine how 
many bytes are received.  It ignores negative return values (errors).

probe kernel.function("sock_recvmsg").return
{
         p = pid()
         execname[p] = execname()
         user[p] = uid()
         sk_pid[p] ++
         sk_rx[p] <<< $return > 0 ? $return : 0

         printf("RECV %d\n", $return)
}

On x86, the negative values are correctly interpreted and printed:

RECV 1368
RECV -11
RECV 1368
RECV -11
RECV 1368
RECV -11
RECV 1368
RECV -11

But on x86_64, the negative values are interpreted and printed as large 
positive values:

RECV 1440
RECV 4294967285
RECV 1440
RECV 4294967285
RECV 1440
RECV 4294967285

This completely messes up the sk_rx aggregation.  It looks like 32-bit 
negative integers are not being correctly cast to 64-bit negative integers. 
  Is this a bug or am I doing something wrong?

- Mike