> I agree that for at least put_user() using asm goto would be an even
> better option.  get_user() on the other hand, will be much messier to
> deal with, since asm goto statements can't have outputs, plus it
> zeroes the output register on fault.

get_user() is the much more interesting one for me. A read from
a poisoned user address that generates a machine check is something
that can be recovered (kill the process).  A write to user space doesn't
even generate a machine check.

-Tony