# CLAUDE.md

## Project overview

Rust USB/IP server library and CLI tool. Two workspace crates: `lib/` (library) and `cli/` (binary). Linux only.

## Security model

Host is trusted, client (in VM) is untrusted. All USB handling in userspace to minimize kernel attack surface. The host kernel's vhci_hcd driver is downstream of responses — don't assume it handles malformed data gracefully.

## Building

```bash
nix build              # Nix
nix develop -c cargo build  # Cargo via nix devShell
```

## Testing

```bash
nix develop -c cargo test -p usbip-rs
```

## Fuzzing

Fuzz targets exercise host-side codepaths against untrusted client input.

### libFuzzer (cargo-fuzz)

Targets are in `lib/fuzz-cargo/` and use `cargo fuzz`.

```bash
nix run .#fuzz-cargo                              # List targets
nix run .#fuzz-cargo -- fuzz_urb_hid              # Single process
nix run .#fuzz-cargo -- fuzz_urb_hid --fork=8     # Parallel (overnight)
nix run .#fuzz-clean-cargo -- fuzz_urb_hid        # Prune fixed artifacts
```

Crash artifacts: `lib/fuzz-cargo/artifacts/<target>/`.

### AFL++ with cargo-afl

Targets are in `lib/fuzz-afl/` and use `cargo-afl` with LLVM plugins for persistent mode, CmpLog, and IJON. AFL++ runs as the primary fuzzer with CmpLog auto-enabled (`-c0`); a SymCC-instrumented binary is also built for manual concolic execution.

```bash
nix run .#fuzz-afl                                # List targets
nix run .#fuzz-afl -- fuzz_urb_hid                # Run AFL++ (persistent + CmpLog)
nix run .#fuzz-afl -- fuzz_urb_hid --jobs=8       # Parallel (main + 7 secondaries)
nix run .#fuzz-clean-afl -- fuzz_urb_hid          # Prune fixed crashes
```

The Nix package for `cargo-afl` is built in this repo from `rust-fuzz/afl.rs` with a patch adding `CARGO_AFL_DIR` / `CARGO_AFL_LLVM_DIR` env var overrides (see `nix/cargo-afl-env-paths.patch`). AFL++ is built with LLVM 22 plugins to match rust-nightly's LLVM version.

Background processes (secondary AFL++ instances, SymCC companion) run as systemd transient units in a slice. Ctrl+c or exit cleans up everything; `systemctl --user stop fuzz_afl_<target>.slice` for manual stop.

Crashes: `lib/fuzz-afl/output/<target>/*/crashes/`. Corpora are separate between engines; copy manually if desired.

### Shared harness

All fuzz target logic lives in `lib/src/fuzz_harness.rs`. Response validation is in `lib/src/fuzz_helpers.rs`.

### Fixing fuzzer crashes

1. **Priority**: Protect the host process and host kernel from untrusted client gaining code execution or privilege escalation. DoS is not a concern — the client would only be DoSing its own service.
2. **Check reachability**: Determine whether the crashing state can be reached by a normal, well-behaved client (check the Linux kernel USB/IP source at `../linux/drivers/usb/usbip/` and `../linux/tools/usb/usbip/`). If not reachable by a well-behaved client, return an error rather than continuing to process garbage.
3. **No `unsafe`** in parsing or sanitization paths.
4. **Validate at the boundary**: Check constraints immediately after deserialization, not deep in business logic.
5. **Update fuzz assertions**: Tighten the invariant assertions in `lib/src/fuzz_helpers.rs` whenever you add or change a constraint — the fuzzer can only find violations it can check.
6. We are using our own nusb fork, and API compatibility is not a concern. If an issue is better fixed in nusb, stop and explain the situation.

## Key architecture

- `handle_urb_loop()` — main URB dispatch loop, generic over async transport
- `handler()` — full connection handler (negotiation + URB loop), takes `UsbIpServer`
- `UsbInterfaceHandler` trait — implement for new device types
- Protocol parsing in `usbip_protocol.rs`, device model in `device.rs`
- `MockSocket` in `util::mock` — used by both tests and fuzz targets

## Conventions

- No backwards compatibility concerns for the library API
- `pub use` re-exports from `lib.rs` — e.g. `mock` module is available as `usbip_rs::mock::MockSocket`
- Edition 2024