[PATCH v4 0/3] m68k: Improved switch stack handling

[Michael Schmitz <schmitzmic@gmail.com>]

m68k version of Eric's patch series 'alpha/ptrace: Improved
switch_stack handling'.

Registers d6, d7, a3-a6 are not saved on the stack by default
on every syscall entry by the m68k kernel. A separate switch
stack frame is pushed to save those registers as needed.
This leaves the majority of syscalls with only a subset of
registers on the stack, and access to unsaved registers in
those would expose or modify random stack addresses.  

Patch 1 and 2 add a switch stack for all syscalls that were
found to need one to allow ptrace access to all registers
outside of syscall entry/exit tracing, as well as kernel
worker threads. This ought to protect against accidents.

Patch 3 adds safety checks and debug output to m68k get_reg()
and put_reg() functions. Any unsafe register access during
process tracing will be prevented and reported. 

Suggestions for optimizations or improvements welcome!

Cheers,

   Michael
   
Link: https://lore.kernel.org/r/<87pmwlek8d.fsf_-_@disp2133>