On Mon, 23 Jan 2017, Marcin Nowakowski wrote: > With certain EVA configurations it is possible for the kernel address > space to overlap user address space, which allows the user to set > watchpoints on kernel addresses via ptrace. > > If a watchpoint is set in the watch exception handling code (after > exception level has been cleared) then the system will hang in an > infinite loop when hitting a watchpoint while trying to process it. > > To prevent that simply disallow placing any watchpoints at addresses > above start of kernel that overlap userspace. This can be severely crippling for user debugging. Is there no better way? Can't for example the low-level exception handling entry/exit code be moved out of the way of the EVA overlap range and then all watchpoints masked for the duration of kernel mode execution? This would be quite expensive, however it could only be executed if a task flag indicates watchpoints are being used. Alternatively perhaps we could clobber CP0.EntryHi.ASID, at least temporarily; that would be cheaper. Overall I think this situation is asking for a watchpoint flag to be added to inhibit hits in the kernel mode in hardware; for completeness this probably actually ought to be a field to cover the kernel, supervisor and user modes separately -- either a plain bitmask for arbitrary control or an encoded value similar to CP0.Status.KSU which would indicate the most privileged mode to accept a watchpoint in. I had a recollection of such a facility already being available for JTAG debugging, but I can't track it down in the specification, so perhaps it was for another architecture and it would be completely new for ours. Maciej
