From: Bernard Pidoux <redacted>
Date: Thu, 19 Jun 2008 18:28:46 +0200

Unfortunately I don't quite understand exactly why a structure in user 
space cannot be changed.

Because there are userland binaries already compiled out there using
the current structure layout.  If we change the kernel, those binaries
stop using a datastructure that matches what the kernel is using.
Such binaries will break.

In generic, because of this, you can never change the layout of a
data structure exported to userland as a kernel interface.