The debugger has fallen behind and has become a maintenance burden. It was
the only user of the old kernel US layout system, which is good to get rid
of. The debugger didn't work with graphical output and was likely to
conflict with the new keyboard system if used, which no longer triggered it.
The kernel symbol code was removed to simplify the kernel.
The kernel debugger was an useful debugging feature, but it needs to be done
in a better way before it can be added back.
Enable the NX bit on x86_64 and set if not PROT_EXEC and enable the write
protection mode (CR0.WP) that disables the default behavior where the kernel
is able to write to read-only memory. Fix kernel broken assumptions it can
access read-only memory and take care to never set PROT_KWRITE on user-space
pages unless PROT_WRITE is also set, otherwise user-space will be able to
write to read-only memory.
This achieves X^W in the whole system except for the core kernel itself as
it is currently don't know the purpose of pages when identity mapping the
first 4 MiB.
Jonas 'Sortie' Termansen