# Phase C+13 fix.diff — targeted excerpt
# (full tree has many uncommitted prior-phase changes; this file
# documents only the C+13 delta against the pre-C+13 state.)

## crates/xenia-kernel/src/exports.rs

### 1. New helper `is_disc_prefix` (~30 LOC, inserted near `STATUS_OBJECT_NAME_COLLISION`, ~line 1282)

```rust
/// Phase C+13 — does `raw_path` start with a prefix that aliases the
/// (read-only) game disc? Used to scope the synth-empty fallback in
/// `open_vfs_file`: missing disc files report `STATUS_OBJECT_NAME_NOT_FOUND`
/// (matching canary's `NtCreateFile_entry` for game-data lookups), while
/// missing writable-partition paths keep the legacy zero-byte synth.
///
/// Mirrors the disc-mapped subset of `crate::path::DEVICE_PREFIXES`:
///  - `game:\` — canary's symbolic-link alias for the disc
///    (xenia-canary/src/xenia/kernel/kernel_state.cc registrations).
///  - `d:\` / `D:\` — drive-letter alias for the disc.
///  - `\Device\Cdrom0\` — NT device path for the disc.
///
/// Compares case-insensitively to match canary's path resolver.
fn is_disc_prefix(raw_path: &str) -> bool {
    let lowered = raw_path.trim_start().to_ascii_lowercase();
    const DISC_PREFIXES: &[&str] = &[
        "game:\\",
        "game:/",
        "d:\\",
        "d:/",
        "\\device\\cdrom0\\",
        "\\device\\cdrom0/",
    ];
    DISC_PREFIXES.iter().any(|p| lowered.starts_with(p))
}
```

### 2. `open_vfs_file` — capture raw path (after the `path` initialization, ~line 1314)

```rust
    // Phase C+13 — recover the raw (un-stripped) path so we can tell a
    // disc-aliased prefix (`game:\`, `d:\`, `\Device\Cdrom0\`) apart from a
    // writable-partition prefix (`\Device\Harddisk0\…`, `\??\`, raw "no
    // prefix" cases). The synth-empty fallback below covers both today but
    // canary's `NtCreateFile_entry` (xboxkrnl_io.cc:83-110) returns the
    // VFS lookup status verbatim, which is `STATUS_OBJECT_NAME_NOT_FOUND`
    // for any disc path that isn't in the ISO. Scoping the synth to
    // non-disc prefixes makes us match canary's behaviour for missing
    // game-data files (e.g. `game:\dat\files.tbl` at Phase C+13 idx 103862).
    let raw_path = crate::path::object_attributes_raw_name(mem, obj_attrs_ptr)
        .unwrap_or_default();
```

### 3. `open_vfs_file` — short-circuit disc paths in the `Err(e)` synth-empty branch (~line 1413)

```rust
        Err(e) => {
            // Phase C+13 — scope the synth-empty fallback to non-disc
            // prefixes only. Canary's `NtCreateFile_entry` returns the VFS
            // result verbatim (xboxkrnl_io.cc:83-110); for a missing disc
            // file like `game:\dat\files.tbl` that's
            // `STATUS_OBJECT_NAME_NOT_FOUND`. Sylpheed handles NOT_FOUND
            // cleanly (next event in canary's trace at idx 103862 is
            // `RtlNtStatusToDosError(0xc0000034) -> 2`, then the boot
            // validator continues), so the synth was masking the
            // correct branch.
            //
            // Synth-empty is still kept for writable system partitions
            // (`\Device\Harddisk0\…`, `\Device\Mass*`, `\??\`, raw paths)
            // because those aren't backed by the disc — Canary mounts
            // them on host directories
            // ([xenia_main.cc:612-651](xenia-canary/src/xenia/app/xenia_main.cc));
            // ours skips the host mount for those and falls back to the
            // legacy stub to avoid regressing audit-006 / audit-018
            // disc-validation probes. `cache:/` was already routed to
            // `open_cache_file` upstream of this branch (AUDIT-038).
            if is_disc_prefix(&raw_path) {
                if handle_out != 0 {
                    mem.write_u32(handle_out, 0);
                }
                write_io_status_block(
                    mem,
                    io_status_block,
                    STATUS_OBJECT_NAME_NOT_FOUND as u32,
                    0,
                );
                tracing::info!(
                    "Disc path missing: raw={:?} norm={:?} err={} -> NOT_FOUND",
                    raw_path,
                    path,
                    e
                );
                return STATUS_OBJECT_NAME_NOT_FOUND;
            }
            // … existing synth-empty branch unchanged …
        }
```

### 4. Tests — 4 new tests inserted before `cache_resolve_strips_path_traversal` (~line 7838)

- `is_disc_prefix_recognises_disc_aliases` — unit test for the prefix classifier; covers `game:\`, `D:\`, `\Device\Cdrom0\`, and several non-disc prefixes that MUST return false.
- `nt_create_file_game_prefix_missing_returns_not_found` — primary fix test (idx 103862 in canary's cold trace). Asserts `STATUS_OBJECT_NAME_NOT_FOUND`, null handle, and IOSB.status records NOT_FOUND.
- `nt_create_file_cdrom_prefix_missing_returns_not_found` — `\Device\Cdrom0\` alias variant.
- `nt_create_file_non_disc_prefix_missing_still_synthesizes` — regression guard: a missing `\Device\Harddisk0\Partition1\sys.bin` still gets a zero-byte synth (preserves audit-006 / audit-018 behaviour).

## Summary

Total: ~30 LOC engine code + ~95 LOC tests = ~125 LOC. Kernel tests 177 → 181.
Stable digest shifts: `ad4f74ee324fdedb0bfdd4cc4c6468e9` → `e1dfcb1559f987b35012a7f2dc6d93f5`.
Phase B `image_loaded_sha256` unchanged: `ea8d160e…`.
Main cold-vs-cold matched prefix: **103862 → 104574 (+712)**.