and size of a userland buffer. The kernel shall not copyout more
than ifr_buflen bytes to ifr_buf. For future ioctls that use
ifr_buf and ifr_buflen instead of ifr_data, the kernel can return
a larger struct in the future than when the ioctl is introduced,
without breaking ABI compatibility, provided that the size, order,
and semantics of the fields at the front of the struct does not
change.
