Module mmu 

Software MMU: page-granular mapping with per-byte permissions.

This is the process-memory model the QCode emulator’s flat EmulatedSpace map cannot express. A flat addr -> byte map answers “what is here”, but a VM needs to answer “may this access happen at all”, and to answer it without aborting the run — an unmapped read is a fault the guest may legitimately take, not an emulator bug.

Permissions are tracked per byte rather than per page. The cost is one extra byte of bookkeeping per guest byte; the benefit is that sub-page granularity (a redzone between two heap chunks, a partially initialized stack frame) costs nothing extra to express, which is the whole point of a fuzzing-oriented memory model.

Only the RAM space is mapped through here. Register, unique and temporary spaces keep their flat representation: permission-checking a varnode write to RAX is meaningless, and putting it on this path would add a page lookup to the hottest operation in the interpreter.

Modules

perm

Permission bits. MAP is what distinguishes “mapped with no access” from “not mapped at all” — the two produce different faults, and a guest can observe the difference (mprotect(PROT_NONE) succeeds on mapped memory and fails on unmapped memory).

Structs

MemFault

A failed access, with the exact byte that failed rather than the start of the access. A 8-byte read straddling a mapping boundary faults at the boundary, and that address is what a guest fault handler would see in CR2.

Mmu

A sparse, page-granular guest address space.

MmuSnapshot

An opaque point-in-time copy of an Mmu.

PageData

One guest page: its bytes, and one permission bitset per byte.

Enums

FaultKind

Why an access could not be performed.

Constants

PAGE_PERM_OFFSET

Distance from a page’s first data byte to its first permission byte.

PAGE_SIZE

Guest page size. Chosen to match the x86-64 base page so that guest mmap granularity and MMU granularity agree; nothing here depends on the value.

Type Aliases

Perm

A permission bitset, one per guest byte.