- copies a mesage from/to userspace without need of translating
addresses
- the assumption is that the address space is installed, i.e. ldt and
cr3 are loaded correctly
- if a pagefault or a general protection occurs while copying from
userland to kernel (or vice versa) and error is returned which gives
the caller a chance to respond in a proper way
- error happens _only_ because of a wrong user pointer if the function
is used correctly
- if the prerequisites of the function do no hold, the function will
most likely fail as the user address becomes random
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