Implement file-backed mmap (MAP_PRIVATE)

[claude]

Overview

Implement file-backed mmap(fd, offset, length, MAP_PRIVATE, ...) in the kernel.

Motivation

This is a prerequisite for dynamic linking (#199). The dynamic linker (ld-musl-riscv64.so.1) maps shared libraries into memory using file-backed mmap with MAP_PRIVATE. Without this, ld.so cannot load shared libraries.

Current State

Solaya currently supports anonymous mmap (no file descriptor). File-backed mmap is not yet implemented.

Requirements

• mmap(fd, offset, length, PROT_READ|PROT_EXEC, MAP_PRIVATE, ...) — map a file region into virtual memory read-only / executable
• mmap(fd, offset, length, PROT_READ|PROT_WRITE, MAP_PRIVATE, ...) — map a file region with copy-on-write semantics
• Copy-on-write: writes to MAP_PRIVATE file-backed mappings must not affect the underlying file
• munmap for these regions (already needed for anonymous mmap)

Implementation Notes

• MAP_PRIVATE with file: initially map pages as read-only backed by file content; on write fault, copy the page (CoW)
• Or: eagerly copy the file content into anonymous pages on mapping (simpler, correct for MAP_PRIVATE)
• The simpler eager-copy approach is acceptable for the dynamic linker use case

Relevant Files

• kernel/src/syscalls/mm_ops.rs — mmap syscall implementation
• kernel/src/memory/ — page allocator, page tables
• kernel/src/processes/process.rs — process memory map

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Created by Claude Code