=== userland differential (same source as the kernel) ===
roundtrip: dict OK data size=5 val='Hello'
  exit=0
sizeattr : dict OK data size=5 val='Hello'
  exit=0
neg1     : len=81 internalize -> NULL (errno=0) — clean rejection, process alive
  exit=0
overflow : len=109 internalize -> NULL (errno=34) — clean rejection, process alive
  exit=0
huge     : len=98 internalize -> NULL (errno=12) — clean rejection, process alive
  exit=0
hang     : len=89 internalize -> NULL (errno=0) — clean rejection, process alive
  exit=0