LayerMount for Windows — engine

LayerMount is a Windows engine for Linux-style overlay filesystems (union mounts) — a writable upper layer, prioritized read-only lowers, copy-up on first write, with optional VHD/VHDX and VSS snapshot sources and .lmnt layer-image packing.

What it isn't: LayerMount is not a filesystem driver. It enforces overlay semantics — copy-up, whiteouts, layer priority, image packing — over real paths in a writable upper directory. Presenting the resulting overlay as a mountable filesystem (at a drive letter or directory) is the job of a separate host adapter that targets a userspace-filesystem driver. Host adapters live in their own repositories and depend on this engine via NuGet.

This repo ships the engine: the native library (LayerMount.dll) and its managed wrapper (LayerMount.NET). The engine is host-agnostic and links only system + CRT libraries (a post-build check enforces this).

Install

.NET 8+ consumers (managed wrapper, recommended)

dotnet add package LayerMount

LayerMount is AnyCPU and transitively pulls in LayerMount.Native, which carries the engine DLL for win-x64 and win-arm64. Windows-only at runtime.

Native C/C++ consumers

dotnet add package LayerMount.Native

LayerMount.h lands under build\native\include\ inside the package; the .def is alongside under build\native\. Either link against the import library yourself or LoadLibrary the DLL at runtime.

Versioning

LayerMount follows Semantic Versioning and ships two version axes:

• LM_VER_MAJOR.LM_VER_MINOR.LM_VER_PATCH (in LayerMount.h, mirrored on the managed LayerMount.GetVersion()) — the SemVer string. Both NuGet packages (LayerMount and LayerMount.Native) always carry the same version and are bumped in lockstep.
• LM_ABI_VERSION — independent of the SemVer string. Only ticks on binary-breaking changes to the public C exports in LayerMount.def; bumping it requires consumers to re-link.

LayerMount is currently in the 0.x series. While on 0.x, minor bumps may carry breaking changes (per the SemVer spec's pre-1.0 guidance) — those are called out explicitly in CHANGELOG.md. The contract stabilizes at 1.0.0.

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Concepts

An overlay is composed of three path roles, borrowed from Linux overlayfs:

• Upper — the writable layer. All new writes land here. Required.
• Lower — read-only layers, prioritized left-to-right (first lower wins for reads). Zero or more.
• Work dir — scratch space used for atomic copy-up. Defaults to <upperParent>\.layermount-work if not supplied.

When a file exists only in a lower layer, reads pass through. The first write promotes (copies up) the file into the upper layer and subsequent access reads/writes the upper copy. Lower layers are never modified.

Two additional layer sources can stand in for a directory path:

• VHD / VHDX — attached at mount, the volume GUID is used as a lower. Requires admin.
• VSS snapshot — a snapshot of a volume taken at mount time, used as a lower. Requires admin.

For the deeper engine architecture (path resolution, whiteouts, copy-up flavors, metadata, caching), see docs/engine/ARCHITECTURE.md.

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Features

• Overlay semantics — upper / lower / work paths with copy-up on first write, file and directory whiteouts, opaque-directory markers.
• Lazy copy-up — metacopy promotes metadata immediately and completes data copy on first read or close, keeping mount-time fast.
• VHD / VHDX layers — attach a virtual disk at mount time and use its volume GUID as a high-priority lower.
• VSS snapshot layers — take a Volume Shadow Copy at mount time and use it as a read-only lower without holding open file handles.
• .lmnt layer images — pack a directory tree into a portable zstd-compressed image with SHA-256 footer; supports differential packs against a base image and multi-image manifests.
• Capability-gated fallbacks — opt out of ADS, reparse points, sparse files, multiple streams, or NTFS ACLs and the engine routes around the missing feature instead of erroring.
• Reparse-point and ADS preservation — both surface on copy-up.
• ACL preservation — DACL on every copy-up; SACL too when the process holds SE_SECURITY_NAME.
• Per-process access tracking — optional access log and JSON-rule-driven gating, keyed by (pid, image path, creation time).
• Diagnostic events — single managed event stream for warnings, copy-ups, whiteouts, and access denials.
• Host-agnostic — links only system + CRT libraries (a post-build check enforces this); zero dependency on any userspace-filesystem driver.
• AOT-compatible managed wrapper — [LibraryImport]-based P/Invoke with trim analysis enabled; ships as AnyCPU with per-RID native via LayerMount.Native.

Quick start

using LayerMount;

// Two-layer overlay: writable upper + one read-only lower.
var config = new LayerMountConfig
{
    UpperPath   = @"C:\overlays\my-app\upper",
    WorkDirPath = @"C:\overlays\my-app\work",
    LowerPaths  = new[] { @"C:\overlays\my-app\base" },
};

using var mount = LayerMount.Create(config);

// Diagnostic events: warnings, copy-up, whiteouts, denials.
mount.Event += (_, e) => Console.WriteLine($"[{e.Kind}] {e.Path}");

// Path resolution: tells you which layer a relative path resolves to,
// and surfaces whiteouts / opaque-dir markers.
var resolved = mount.ResolvePath(@"bin\app.exe");
Console.WriteLine($"{resolved.FullPath}  (origin: {resolved.Origin})");

// Pack the upper layer into a portable, zstd-compressed image.
mount.Images.Pack(
    sourceDir:    config.UpperPath,
    outputPath:   @"C:\images\my-app-0.1.lmnt",
    description:  "first build");

For mount-driven I/O (OpenFile / CreateFile / Read / Write), the engine takes Win32-style access masks and create options because it's designed to be wired straight to a userspace-filesystem driver's I/O requests. The managed unit tests under src/LayerMount.NET.Tests/ are the most practical reference for the I/O patterns.

API surface

The managed wrapper lives in the LayerMount namespace; the native C ABI is src/LayerMount.dll/public/LayerMount.h + LayerMount.def. Top-level entry points:

Type	Purpose
LayerMount	Top-level overlay handle. Static factories Create(LayerMountConfig) and CreateTransient(workDir); instance methods for path/file ops, stats, volume info; Event for diagnostics.
LayerMountConfig	Construction parameters: UpperPath, WorkDirPath, LowerPaths, Capabilities, process-tracking knobs.
LayerMountFile	Handle returned by OpenFile / CreateFile. Read / Write accept managed Span<byte> and an originator PID.
LayerMount.Vhd	Create / Open VHD/VHDX layer files; ListLayers against a manifest directory.
LayerMount.Vss	CreateSnapshot / DeleteSnapshot / ListSnapshots / Cleanup. Admin required.
LayerMount.Images	Pack, PackDifferential, Unpack, Validate, GetMetadata; CreateManifest / GetManifest for multi-image bundles.
LayerMount.ProcessTracker	Per-process access log and rule-based gating (config-gated).

The native ABI exports the same surface as C functions; consumers who want to bypass the managed wrapper can LoadLibrary LayerMount.dll and call directly against LayerMount.h.

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Building from source

msbuild LayerMount.sln /p:Configuration=Release /p:Platform=x64
# Optional: cross-compile the native engine for arm64 as well
msbuild LayerMount.sln /p:Configuration=Release /p:Platform=ARM64

Outputs land in <Platform>\<Configuration>\ (e.g. x64\Release\ or ARM64\Release\). Debug builds work too (/p:Configuration=Debug). The native engine builds on both x64 and ARM64; test projects and the managed wrapper are x64 for dotnet test.

• LayerMount.dll (native engine)
• LayerMount.NET.dll (managed wrapper around the native engine)
• LayerMount.Tests.dll, LayerMount.AbiTests.dll (native test DLLs)
• LayerMount.NET.Tests.dll, LayerMount.TestShared.dll (managed wrapper tests + shared test helpers)

Tests:

# Native: VS Test Explorer / vstest.console.exe x64\<Config>\LayerMount.Tests.dll
dotnet test src/LayerMount.NET.Tests   # managed wrapper unit tests

Adapter-specific build prerequisites, exit codes, troubleshooting, and host-spawning E2E tests live with the adapter projects, not in this repo.

Releases

Releases are cut by a manually-triggered GitHub Actions workflow (release.yml) that runs Versionize over the conventional commits since the previous tag, bumps version.props and CHANGELOG.md, builds + packs both NuGet packages (LayerMount and LayerMount.Native) for win-x64 and win-arm64, pushes them to nuget.org, and creates a GitHub Release with the .nupkg / .snupkg files attached.

See CONTRIBUTING.md for the full release procedure.

License

MIT. Bundled third-party components are listed in THIRD_PARTY_NOTICES.md.