29. The protection module

29.1. Introduction

.intro: This is the generic design of the Protection Module. The protection module provides protection services to other parts of the MPS. It is expected that different operating systems will have different implementations of this module.

.readership: Any MPS developer.

29.2. Interface

void ProtSetup(void)

.if.setup: ProtSetup() will be called exactly once (per process). It will be called as part of the initialization of the first space that is created. It should arrange for the setup and initialization of any datastructures or services that are necessary in order to implement the protection module. (On UNIX it expected that it will install a signal handler, on Windows it will do nothing)

void ProtSet(Addr base, Addr limit, AccessSet mode)

.if.set: ProtSet() should set the protection of the memory between base and limit, including base, but not including limit (ie the half-open interval [base,limit)) to that specified by mode. The mode parameter should have the AccessWRITE bit set if write accesses to the page are to be forbidden, and should have the AccessREAD bit set if read accesses to the page are to be forbidden. A request to forbid read accesses (that is, AccessREAD is set) may also forbid write accesses, but read accesses will not be forbidden unless AccessREAD is set.

void ProtSync(Space space)

.if.sync: ProtSync() is called to ensure that the actual protection of each segment (as determined by the OS) is in accordance with the segments’s pm field.

struct MutatorFaultContextStruct *MutatorFaultContext

.if.context-type: This abstract type is implemented by the protection module (impl.c.prot*). It represents the continuation of the mutator which is restored after a mutator fault has been handled. The functions ProtCanStepInstruction() (.if.canstep below) and ProtStepInstruction() (.if.step below) inspect and manipulate the context.

Bool ProtCanStepInstruction(MutatorFaultContext context)

.if.canstep: Examines the context to determine whether the protection module can single-step the instruction which is causing the fault. Should return TRUE if and only if the instruction can be single-stepped (that is, ProtStepInstruction() can be called).

Bool Res ProtStepInstruction(MutatorFaultContext context)

.if.step: Single-steps the instruction which is causing the fault. This function should only be called if ProtCanStepInstruction() applied to the context returned TRUE. It should return ResUNIMPL if the instruction cannot be single-stepped. It should return ResOK if the instruction is single-stepped.

The mutator context will be updated by the emulation/execution of the instruction such that resuming the mutator will not cause the instruction which was causing the fault to be executed.