copy_mount_options()

["David S. Miller" <davem@redhat.com>]

So the sparc64 user copy bug I fixed recently is pretty
much present on every platform.

Basically, copy_mount_options() requires exact byte granularity
to exception reporting from copy_from_user().  If you don't
do this it can break things like busybox's mount().

Even reporting on a word boundary is illegal.  On sparc64
it was quite poignant because we can report on a 64-byte
boundary for large copies because that is the granularity
of the load/store we use.

Other platforms will need to fix this.  I recommend a two
stage exception handling scheme.  Basically, on the first
exception, you merely note that an exception occurred
and you retry the user copy a byte at a time until you
hit the exact address that fails.  You cannot optimize
this to just check a page at a time, because copy_mount_options
wants all the data to be there.