Format coverage phase 2: VDI convert-from (read path)¶

Status: Complete (2026-07-18)¶

Prompt¶

Before responding to questions or discussion points in this document, explore the instar codebase thoroughly. Read relevant source files, understand existing patterns (VMM structure, guest operation layout, shared crate conventions, call table ABI, format parsing, test infrastructure), and ground your answers in what the code actually does today. Do not speculate about the codebase when you could read it instead. Where a question touches on external concepts (QCOW2, VMDK, VHD/VHDX, LUKS, KVM, virtio, disk image formats), research as needed to give a confident answer. Flag any uncertainty explicitly rather than guessing.

I prefer one commit per logical change, and at minimum one commit per phase. Do not batch unrelated changes into a single commit. Each commit should be self-contained: it should build, pass tests, and have a clear commit message explaining what changed and why.

Situation¶

VDI (VirtualBox Disk Image) is detect + info only today. Phase 1's consumer gate refuses it for convert/compare/dd with the typed UnsupportedInputFormat message, and three tests pin that refusal (tests/test_convert.py:5554, tests/test_compare.py:1229, tests/test_dd.py:1420). This phase graduates VDI to a full read format for those three subcommands, following the established per-format read-path pattern.

Research for this plan was done 2026-07-18 by two grounding passes: a codebase trace of the VHD read path (the closest structural analogue), and an empirical verification of the VDI format and qemu's driver semantics against block/vdi.c (read in full at 7.0.0 from the testdata qemu-sources copy, confirmed unchanged at master) with every load-bearing claim re-verified by running real qemu-img (10.2.0 static binary, info-shape drift sampled at 6.0.0 / 7.2.0 / 8.2.0 / 9.2.0).

The read-path pattern (codebase facts)¶

The shared chain reader lives in the qcow2 crate. Per-format dispatch is the match format in read_chain_virtual_cluster (src/crates/qcow2/src/lib.rs:5855); the VHD arm (:6388) is the model: state.block_lookup(...) returns Unallocated (→ continue to the next chain device) or Allocated { host_byte_offset } (→ read_cluster_sectors when guest-sector-aligned, read_offset_sectors when not). The default arm (:6513) reads raw — this is what the host gate protects detect-only formats from reaching.
Per-device reader state lives in ChainStates (src/crates/qcow2/src/lib.rs:6671, feature-gated arrays like vhd_states), initialised by the dispatch in init_chain_states (:6697, VHD arm :6744, which reuses the L1/L2 cache buffer slots as the format's BAT/data sector caches).
The model state type is VhdState (src/crates/vhd/src/lib.rs:483): init(call_table, device_idx, sector_size, input_capacity, cache bufs, &mut bytes_read) -> Option<Self> plus block_lookup(...) -> Option<BlockLookup>. All I/O goes through (call_table.read_input_sector); all arithmetic is checked/Option. The vhd crate's Cargo.toml (dep on shared only, no_std, workspace lints) is the template for a new src/crates/vdi/.
Input formats are feature flags on the qcow2 crate (vhd-input, vhdx-input, vmdk-input in src/crates/qcow2/Cargo.toml); each consuming operation enables them in its own Cargo.toml. Operations linking the chain reader: convert (dd runs the convert binary — there is no dd guest binary), compare, bench, rebase. copy is format-agnostic raw passthrough; check links format crates directly, not via the reader.
Host side: chain::ImageFormat (src/vmm/src/chain.rs:24) has no Vdi variant; from_str (:50) maps "vdi" to Unknown, which trips the phase-1 gate in discover_backing_chain (src/vmm/src/main.rs:2357). Graduation = add the variant + from_str arm + to_shared_format_u32 arm (→ 8, matching shared::ImageFormat::Vdi = 8, which already exists at src/shared/src/lib.rs:1546) + Display arm. No CLI changes: convert and compare have no input-format flag, and dd's -f is accepted-but-ignored (main.rs:13809).
The guest info op already parses VDI headers (src/operations/info/src/main.rs:1012, parse_vdi_header) but does NOT read the three fields a reader needs: offset_bmap (hdr 340), offset_data (344), block_extra (380). The new crate parses these itself.
Binary size headroom is ample: convert is at 39% of the 768KB cap (307,544B ELF memory extent), compare at 24%. The size check measures .bss-inclusive ELF extent via scripts/check-binary-sizes.sh.
Fuzzing: src/fuzz/fuzz_targets/fuzz_vhd_footer.rs (buffer-shape) and fuzz_vhd_bat.rs (mock-CallTable stateful shape) are the templates; the differential fuzzer's format pool is FORMATS = ['qcow2', 'raw', 'vmdk', 'vpc'] at scripts/differential-fuzz.py:52.

VDI format facts (all empirically verified)¶

Header: 512 bytes, all scalars little-endian. Fields the reader needs (hex/dec offsets):

Offset	Field	Notes
0x40 / 64	signature u32	0xbeda107f
0x44 / 68	version u32	0x00010001 only (1.1)
0x4c / 76	image_type u32	1 dynamic, 2 static
0x154 / 340	offset_bmap u32	must be 512-aligned
0x158 / 344	offset_data u32	must be 512-aligned
0x168 / 360	sector_size u32	must be 512
0x170 / 368	disk_size u64	virtual size in bytes
0x178 / 376	block_size u32	must be exactly 1 MiB
0x17c / 380	block_extra u32	ignored by qemu
0x180 / 384	blocks_in_image u32
0x1a8 / 424	uuid_link 16B	must be all-zero
0x1b8 / 440	uuid_parent 16B	must be all-zero

Block map: flat array of blocks_in_image LE u32 entries at offset_bmap. Sentinels 0xffffffff (unallocated) and 0xfffffffe (discarded) both read as zeros; an entry is allocated iff < 0xfffffffe. Data offset for an allocated block is exactly offset_data + entry as u64 * block_size + offset_in_block — block_extra does NOT participate (byte-patch verified). Entries are allocation-order indices, not an identity map, except static images write the identity map at create time. VDI has no backing-file support: qemu refuses non-null link/parent UUIDs at open, so a VDI is always the end of a chain.

qemu vdi_open validation, in order, with exact limits (all 12 checks empirically confirmed, error strings in the grounding transcript):

disk_size > 536870784 * 1048576 (≈512 TiB) → refuse.
disk_size % 512 != 0 → NOT an error: round up to 512 in memory (VBoxManage-created images can be odd).
bad signature → refuse under -f vdi; under probing the image simply detects as raw.
version != 0x00010001 → refuse.
offset_bmap % 512 != 0 → refuse.
offset_data % 512 != 0 → refuse.
sector_size != 512 → refuse.
block_size != 1048576 → refuse (hard-fixed).
disk_size > blocks_in_image * block_size (using the rounded disk_size) → refuse.
non-null uuid_link → refuse.
non-null uuid_parent → refuse.
blocks_in_image > 536870784 → refuse.

Critical read semantics: qemu never validates file length and zero-fills any read past EOF. A bmap entry pointing far past EOF opens fine, qemu-img map advertises the bogus offset as data, and convert -O raw exits 0 with that block all-zero (straddling blocks zero-fill only the missing tail). instar must replicate: past-EOF block reads yield zeros, never an error. This also means instar's 65536-rounded zero-tail guest capacity view is naturally consistent — header-declared geometry drives all reads.

Other verified facts:

image_type is NOT validated: 0/3/4 all open and behave as dynamic; only type 2 is special (static). Do not reject unknown types.
Static images: bmap is the identity map, file = offset_data + blocks * 1 MiB (sparse). The read path is identical — walk the bmap; no special-casing needed.
qemu-img create -f vdi rounds requested size up to 512 before writing disk_size; cluster_size in info output is block_size (1048576); VDI has no format-specific block; info-shape drift across versions is only the generic 8.0+ child-node dump.
qemu-img map on dynamic VDI: holes are present:true, zero:true, data:false; allocated blocks carry offset.
Alignment: offset_data / bmap entries are 512-aligned by validation, but NOT 65536-aligned (offset_data is typically 1024) — with the guest's 65536-byte virtio sectors, the VHD arm's aligned/unaligned split (read_cluster_sectors / read_offset_sectors) must be mirrored, not simplified to the aligned path.
oslo.utils VDIInspector: signature at 0x40, virtual size = raw u64 at 0x170, always reports safe. Agrees with qemu on qemu-created images; diverges on odd disk_size (oslo reports raw, qemu rounds up). No VDI entries exist in test_oslo_crossval's skip/divergence lists today.
Testdata: exactly one VDI fixture exists — vdi-simple (10 MiB empty dynamic, 1024 bytes, run_in_ci: false, skip_qemu_img: true). No static fixture, no data-bearing fixture. info and check baselines exist for vdi-simple; the check baselines are currently unconsumed by any test. qemu's VDI driver does support qemu-img check (bmap validation), which is why those baselines generate.

Design¶

Scope¶

In scope: convert, compare, and dd gain VDI input (dd comes free with convert — same guest binary). Both dynamic and static images, plus discarded-block and past-EOF semantics. VDI as a backing file of a qcow2 chain (qemu permits -F vdi) works through the same reader arm and gets one test.

Out of scope, refused or unchanged as today: map (guest-side ERROR_INVALID_SOURCE), measure (clap-restricted source list), resize/amend/snapshot (own probes), check (see below), any VDI write/create/output support, --extra-detail info changes.

Reader semantics: exact qemu parity¶

The new src/crates/vdi/ crate implements qemu's 12 open-time checks with the same limits and order-independent outcomes (init returns None; the op surfaces its normal init-failure error). Additional rules, each pinned by a fixture:

Round odd disk_size up to 512 (rule 2). The virtual size instar reports and reads is the rounded value — qemu parity, recorded as an oslo divergence.
Ignore block_extra entirely; never include it in offset math.
Accept any image_type; no static special-case in the read path (the identity bmap is just data).
Unallocated and discarded entries read as zeros. In the chain-reader arm this is continue — safe because a VDI can never have a device below it (non-null link/parent UUIDs are refused), so the loop falls through to the zero-fill tail.
Reads whose computed host offset lies at or past the input capacity zero-fill instead of erroring, including the straddle case (partial block in-file). Note the guest capacity is the file length rounded up to the 65536-byte virtio sector with a zero tail, which composes correctly with this rule.

Guest integration and the raw-fallback hazard¶

Graduating "vdi" in host chain.rs lifts the phase-1 refusal for EVERY operation that calls discover_backing_chain — convert, compare, dd, check, bench, and rebase. Any of those whose guest binary lacks the vdi-input feature would fall into the chain reader's default arm and silently read the VDI as raw: exactly the issue #444 class phase 1 closed. Therefore:

The vdi-input feature is added to the qcow2 crate and enabled by all four reader-linking operations (convert, compare, bench, rebase) in the same change — not just convert/compare.
check does not link the chain reader; after graduation its guest op must fail cleanly on VDI (its own format dispatch has no VDI arm). Step 2c verifies this empirically and pins it with a test; if the observed behaviour is a silent raw read or a hang rather than a clean error, that is a blocking finding for management review before the graduation commit lands. Real qemu-img check support for VDI (qemu validates the bmap; unused baselines already exist in testdata) is future work.
The guest reader arm must land (2b) strictly before the host graduation (2c). Between those commits behaviour is unchanged (gate still refuses), so each commit is self-contained.

Fixtures and baselines¶

New fixtures, generated by a deterministic generate-vdi-fixtures.py in instar-testdata (following generate-dmg-fixtures.py; qemu-created then byte-patched; UUIDs at 0x188/0x198 are the only non-deterministic creation bytes, so patch them to fixed values for reproducibility):

Safe (qemu-readable, full info baselines, convert parity):

vdi-data-dynamic — 8 MiB dynamic, data blocks written at 2 MiB and 5 MiB (produces a non-identity bmap), one allocated entry patched to 0xfffffffe (discarded), rest holes.
vdi-static-data — small static image (≤4 MiB virtual; file is sparse) with a data pattern in a middle block.
vdi-odd-size — disk_size byte-patched to a non-512 multiple; qemu rounds up (verified: 12801 → 13312). Pins the round-up rule end-to-end.
vdi-bmap-past-eof — one allocated entry pointing past EOF; qemu opens, converts with exit 0, zero-fills. Pins the zero-fill rule differentially.

Malformed (qemu refuses at open; skip_qemu_img: true, adversarial harness — instar must refuse cleanly, no hang):

vdi-bad-version (version 2.0), vdi-unaligned-bmap, vdi-wrong-blocksize (512), vdi-nonnull-parent, vdi-too-many-blocks (0xffffffff).

Existing vdi-simple flips to run_in_ci: true and drops skip_qemu_img, matching the readable-format convention (vpc's virtualpc-vhd entry is the model). Safe fixtures live in custom/format-coverage/, malformed in custom/audit/, per phase-1 convention. Baselines via generate-baselines.py (restricted manifest, --no-commit) then detect-profiles.py; testdata commits go straight to main per operator standing instruction, but are made by the management session after review, never by sub-agents.

oslo crossval¶

Flipping vdi-simple to run_in_ci: true auto-enrolls VDI images in test_oslo_crossval. Expected outcomes: format and size agreement everywhere except vdi-odd-size, which needs a KNOWN_VSIZE_DIVERGENCES entry (oslo reports the raw value, qemu/instar the rounded one). Malformed fixtures are skip_qemu_img and out of oslo's scenario set; if any leaks in, use OSLO_SKIP_IMAGES with a comment.

Info parser round-up fix¶

parse_vdi_header (src/operations/info/src/main.rs:1012) currently reports the raw disk_size. With vdi-odd-size baselined, info must report the rounded value to stay byte-identical to qemu. Fix as its own commit in step 2c (one-line semantics change plus unit test). All existing baselined VDI images are 512-aligned, so no baseline regenerates.

Fuzzing¶

Coverage-guided: fuzz_vdi_header.rs (buffer shape, modelled on fuzz_vhd_footer.rs) and fuzz_vdi_bat.rs (mock-CallTable stateful shape, modelled on fuzz_vhd_bat.rs), plus the vdi dep and two [[bin]] blocks in src/fuzz/Cargo.toml.
Differential: append 'vdi' to FORMATS (scripts/differential-fuzz.py:52) and add a -o static=on variant branch in generate_image. This MUST land only after 2c (else instar refuses what qemu reads and every iteration diverges on exit code — the hazard already flagged in PLAN-fuzzing-bugs.md:218).

Out of scope for this phase¶

map/measure support for VDI (master-plan future work).
instar check support for VDI (new future-work entry; qemu supports it, unconsumed baselines exist).
VDI write/create/output support (master plan exclusion).
Non-1MiB block sizes, VDI 1.0/2.x versions, undo/diff image types beyond treat-as-dynamic (qemu parity is refusal or dynamic semantics respectively).
instar create -F vdi backing-format strings (shared::BackingFormat unchanged; reading qemu-created qcow2-with-VDI-backing chains needs no instar -F support).

Step-level guidance¶

Step	Effort	Model	Isolation	Brief for sub-agent
2a	high	opus	none	New `src/crates/vdi/` no_std parser crate, modelled line-for-line on `src/crates/vhd/` (Cargo.toml template: dep on `shared` only, workspace lints, no features). Implement: header offset constants per this plan's Situation table; `VdiHeader::parse(&[u8]) -> Option<Self>` enforcing qemu's 12 validation rules with the exact limits listed (round odd disk_size up to 512 rather than rejecting; accept any image_type; ignore block_extra); `enum VdiBlockLookup { Unallocated, Allocated { host_byte_offset: u64 } }`; `VdiState` with `init(call_table, device_idx, sector_size, input_capacity, bmap_cache_buf, data_cache_buf, &mut bytes_read) -> Option<Self>` and `block_lookup(&mut self, call_table, virtual_offset, sector_size, input_capacity, &mut bytes_read) -> Option<VdiBlockLookup>` — signatures mirror `VhdState::init` (`src/crates/vhd/src/lib.rs:515`) and `block_lookup` (`:725`). bmap entries are LE u32 at `offset_bmap + entry_index4`, served through the cached-sector pattern (`read_u32_be_cached` analogue, LE). Allocated iff entry < 0xfffffffe; data offset = `offset_data + entry as u64 1048576 + offset_in_block`, all checked arithmetic. Do NOT error on offsets past `input_capacity` — that classification is the caller's (2b) job; but do bounds-check bmap reads themselves and treat a bmap sector read failure as init/lookup failure. Unit tests in-module per vhd style: synthetic 512-byte headers exercising every validation rule (accept + reject cases with exact boundary values, e.g. block_size 1048575/1048576/1048577, disk_size at/over max, odd disk_size rounding, non-null UUID rejection), bmap lookup against a mock CallTable (model: qcow2's mock at `src/crates/qcow2/src/lib.rs:4665+` or fuzz mocks), sentinel handling, allocation-order (non-identity) mapping. Register the crate in the workspace `Cargo.toml` members list. `make test-rust` and `make lint` clean.
2b	high	opus	none	Guest chain-reader integration; behaviour must remain unchanged (host gate still refuses VDI) — this step only makes the guest capable. In `src/crates/qcow2/Cargo.toml`: add optional dep `vdi = { path = "../vdi" }` and feature `vdi-input = ["vdi"]`, mirroring `vhd-input`. In `src/crates/qcow2/src/lib.rs`: (1) `ChainStates` (`:6671`) gains `#[cfg(feature = "vdi-input")] pub vdi_states: [Option<vdi::VdiState>; MAX_CHAIN_DEVICES]` + initialisation; (2) `init_chain_states` (`:6697`) gains a `ImageFormat::Vdi` arm calling `VdiState::init` with the same reused L1/L2 cache slots as the Vhd arm (`:6744`); init failure must propagate exactly as a Vhd init failure does (verify what that path reports and note it in your summary — a malformed-but-detected VDI must produce a clean op error, not raw fallback); (3) `read_chain_virtual_cluster` (`:5855`) gains a `ImageFormat::Vdi` arm modelled on the Vhd arm (`:6388`) minus the fixed-VHD special case: `block_lookup` → `Unallocated => continue` (VDI never has a lower device, so this reaches the zero-fill tail) / `Allocated => read_cluster_sectors` when the host offset is guest-sector-aligned, `read_offset_sectors` otherwise — offset_data is typically 1024, NOT 65536-aligned, so the unaligned path WILL be exercised; additionally zero-fill (do not error) any portion of an allocated read at or past the device capacity, including straddles, per qemu's no-EOF-validation semantics (Situation section). Enable `vdi-input` in the qcow2 feature lists of ALL FOUR reader-linking ops: `src/operations/{convert,compare,bench,rebase}/Cargo.toml` — bench and rebase are required to prevent the raw-fallback hazard described in Design. Add unit tests in the qcow2 crate's test module for the new arm (mock CallTable: dynamic bmap with holes, discarded entry, unaligned offset_data, past-EOF entry zero-fill), checking how existing vhd-input-gated tests are invoked so the new ones actually run under `make test-rust`. `make instar`, `make check-binary-sizes`, `make test-rust`, `make lint` clean; existing refusal tests still pass.
2c	medium	opus	none	Host graduation, in two commits. Commit 1: info round-up — in `src/operations/info/src/main.rs` `parse_vdi_header` (`:1012`), round `disk_size` up to the next 512 multiple before storing `result.virtual_size` (qemu `vdi_open` parity; Situation rule 2), with a `#[cfg(test)]` case; verify `instar info` output for `vdi-simple` is byte-unchanged (it is 512-aligned). Commit 2: graduation — in `src/vmm/src/chain.rs` add `Vdi` to `ImageFormat` (`:24`), `from_str` arm `"vdi" => ImageFormat::Vdi` (`:50`), `to_shared_format_u32` arm `Vdi => 8` (`:74`; must match `shared::ImageFormat::Vdi = 8`), `Display` arm `"vdi"` (`:90`). Do NOT touch the gate's exemption string list — graduating out of `Unknown` is the mechanism. Delete/invert the three refusal tests (`tests/test_convert.py:5554` `test_convert_refuses_vdi`, `tests/test_compare.py:1229`, `tests/test_dd.py:1420`) and replace with smoke tests: instar convert `vdi-simple` → raw byte-identical to `qemu-img convert` (use `run_instar_convert`/`run_qemu_img_convert` from `tests/base.py`), compare vdi-simple vs its raw conversion reports identical, dd full-copy parity. Then EMPIRICALLY pin the other gate-lifted ops: run instar `check` and `bench` on `vdi-simple` — check must fail cleanly (record the exact message; add a test pinning non-zero exit + clean error; if it silently succeeds, reads raw, or hangs, STOP and report to management before committing), bench must succeed (vdi-input is enabled; add a smoke test asserting exit 0). `make instar` + targeted suites (`test_convert.py`, `test_compare.py`, `test_dd.py`, `test_info_safe.py`) pass.
2d	high	opus	none	Testdata fixtures + manifest, two repos. In instar-testdata (sibling checkout; do NOT commit — leave the tree for management review): write `scripts/generate-vdi-fixtures.py` (python, single quotes, 120-col, deterministic: create with a static qemu-img from `qemu-img-binaries/x86_64/10.2.0/`, then byte-patch the two creation UUIDs at 0x188/0x198 to fixed values and re-patch per-fixture bytes) emitting the nine fixtures in this plan's Design § Fixtures (four safe: vdi-data-dynamic / vdi-static-data / vdi-odd-size / vdi-bmap-past-eof; five malformed: bad-version / unaligned-bmap / wrong-blocksize / nonnull-parent / too-many-blocks; exact patch offsets in the Situation header table). Safe → `custom/format-coverage/`, malformed → `custom/audit/`. Validate each safe fixture opens with BOTH the 6.0.0 and 10.2.0 static binaries and each malformed one is refused by 10.2.0 with the expected error class. Then baselines: `generate-baselines.py` with a restricted manifest for the four new safe images plus re-run for `vdi-simple` if its flag flip changes eligibility (info human+json; check baselines will also generate for VDI — that is fine, they remain unconsumed), then `detect-profiles.py`; verify regenerated profiles are byte-identical to `raw/` for a spot-check of pre-existing images (the corruption class fixed in phase 1). In instar (this worktree): add the nine manifest entries (real sha256s; safe ones `run_in_ci: true`, no `skip_qemu_img`; malformed ones `safety: malformed`, `skip_qemu_img: true`, `expected_error` set) and flip `vdi-simple` to `run_in_ci: true` without `skip_qemu_img`. Add the `KNOWN_VSIZE_DIVERGENCES` entry for `vdi-odd-size` in `tests/test_oslo_crossval.py` (oslo raw vs qemu rounded; cite this plan) and verify oslo scenarios for all VDI images pass locally (venv with oslo.utils + cryptography — the suite silently self-skips without it, so confirm it actually ran). Deliverable includes a findings note in this plan file listing generated sha256s and any qemu-version surprises.
2e	medium	opus	none	Integration tests, modelled on the VHD classes. In `tests/test_convert.py`: `TestConvertVdiToRaw`-style class covering all five safe VDI ids × instar-convert-vs-qemu-img-convert byte parity (helper pattern `_test_vhd_convert` `:3508`), plus conversions to qcow2 and vpc outputs for `vdi-data-dynamic` (round-trip through `qemu-img convert -O raw` for comparison), plus a qcow2-with-VDI-backing chain test: build with `qemu-img create -f qcow2 -b <vdi> -F vdi` in the test tmpdir, write a partial overlay, convert with instar and qemu-img, assert byte parity (verify qemu accepts `-F vdi` at the tested version first; if not, note and drop). In `tests/test_compare.py`: vdi-vs-raw and vdi-vs-vdi compare parity. In `tests/test_dd.py`: add `test_input_vdi` to `TestDdInputFormats` (`:1118`) via `_assert_input_fmt_parity` (`:1144`), plus one windowed dd (skip/count crossing an unallocated→allocated boundary of `vdi-data-dynamic`) against `qemu-img dd`. Adversarial: register the five malformed fixtures with the `run_adversarial` harness (`tests/base.py:1066` pattern from phase 1) — convert/compare/dd/info on each must exit non-zero cleanly, no hang, no crash; also assert instar's refusal for `vdi-bad-version` etc. happens at open (clean error), and `vdi-bmap-past-eof` + `vdi-odd-size` CONVERT successfully with byte-parity to qemu (they are safe fixtures pinning the zero-fill and round-up rules). Confirm `test_info_safe` auto-picks the new safe images and passes against 2d's baselines. Run the full touched suites locally at moderate concurrency; classify any failure by isolated replay before attributing (KVM-contention flakes are a known pattern).
2f	medium	sonnet	none	Fuzzing. Coverage-guided: create `src/fuzz/fuzz_targets/fuzz_vdi_header.rs` (clone shape of `fuzz_vhd_footer.rs`: call `vdi::VdiHeader::parse(data)` and, on Some, exercise accessors) and `fuzz_vdi_bat.rs` (clone of `fuzz_vhd_bat.rs`: `instar_fuzz::set_fuzz_input(data)`, `build_call_table()`, `VdiState::init`, loop `block_lookup` over `extract_fuzz_offset` offsets); add `vdi = { path = "../crates/vdi" }` and two `[[bin]]` blocks to `src/fuzz/Cargo.toml`. `make fuzz-build` clean; run each new target for 60s locally and report exec/s + any crash. Differential: in `scripts/differential-fuzz.py` append `'vdi'` to `FORMATS` (`:52`) and add a static-VDI branch in `generate_image` (`:115`, `-o static=on` chosen by rng like existing per-format options); confirm `op_convert`/`op_dd`/`op_map`/`op_measure` handle a vdi source without special-casing (map/measure refuse VDI in instar — check how the harness treats refusal-vs-qemu-success for formats outside an op's support and mirror the existing handling for such cases; if none exists, exclude vdi from those two ops' format choice rather than recording divergences). Run ~200 iterations locally with vdi forced and report divergence count (expected: zero).
2g	medium	sonnet	none	Docs + close-out. `docs/format-coverage.md`: add `vdi` to the input-format-for-conversion table (`:66`), update the safety table row (`:290`) from "Pass-through" to a real reader description, update the test-image inventory (`:361`, now 10 VDI images) and the narrative bullet (`:493`). `docs/quirks.md`: remove vdi from the detect-but-refuse lists (`:2599`, `:2621`, `:2630`) and add a phase-2 section documenting: odd-disk_size round-up (qemu parity, oslo divergence), past-EOF zero-fill semantics, image_type leniency, block_extra ignored, check-refusal-vs-qemu-check divergence. `README.md`: add VDI to Supported Formats (`:16`) as read-only input. `CHANGELOG.md`: Added entry for VDI convert/compare/dd input; note the refusal-set change. `ARCHITECTURE.md`: add `src/crates/vdi/` where crates are enumerated. Master plan `PLAN-format-coverage.md`: Execution row 2 → Complete with commit range; add future-work entries (instar check support for VDI; map/measure VDI already covered by the existing deferred item). `docs/plans/index.md`: link this phase file, adjust "phases 3-7 not yet written". This file: Status → Complete, fill the Findings sections. Wrap at each file's existing column width.

Sequencing: 2a → 2b → 2c strictly ordered (guest capability before host graduation; the tree stays green between commits). 2d can run in parallel with 2a–2c (fixtures need only qemu-img), but its instar-side manifest/oslo changes land after 2c so CI never sees a refused-but-enrolled fixture. 2e after 2c + 2d. 2f after 2c (differential) — the coverage targets only need 2a but ship together. 2g last.

Findings: step 2a — vdi crate¶

The vdi crate landed as planned: VdiHeader::parse enforces qemu's twelve vdi_open rules with the exact limits from the Situation table, VdiBlockLookup/VdiState mirror vhd::VhdBlockLookup/VhdState's signatures, and the block map is walked through the cached-sector pattern (read_u32_le_cached, the LE analogue of the qcow2 crate's BE helper). 23 boundary unit tests cover every validation rule (accept and reject cases at exact limits, e.g. block_size 1048575/1048576/1048577), sentinel handling, allocation-order (non-identity) mapping, and a mock CallTable block-map walk.
One edge case surfaced during review and was noted for management rather than changed: a block-map sector read that lands past the guest's own capacity fails the lookup outright (block_lookup returns None, which the chain reader surfaces as a clean op error), whereas qemu's C implementation would zero-fill that read and interpret the resulting zero bytes as bmap entry 0 (an allocated block at the very start of the data region). This is a pathological case — a block map so close to the guest's declared capacity boundary that the guest's own virtio window can't reach it — and instar's stricter behaviour (refuse rather than synthesize a block-map entry from zero bytes) is the safe direction: no image was found in practice that exercises this divergence, and none of the fixture set does either.

Findings: step 2c — graduation and gate-lifted ops¶

bench had its own guest-side read_family allowlist, independent of the qcow2 crate's vdi-input feature. Before the fix, instar bench cleanly refused VDI input (exit 1, bench: unsupported input format) even with vdi-input enabled on qcow2, because bench's own format dispatch never reached the chain reader's VDI arm. The root-cause fix added the Vdi arm to bench's read_family allowlist in lock-step with the host graduation commit, closing the same raw-fallback-hazard class of gap the Design section flagged for bench/rebase generally.
Empirical pins for every gate-lifted operation, run against vdi-simple:
check exits 63 with This image format (vdi) does not support checks — no raw read, no hang. Unchanged from before graduation.
bench succeeds (exit 0) with header output byte-parity against qemu-img bench.
map still refuses with map: source format unrecognised.
measure still refuses with source image is unsupported format.
resize still refuses with format Vdi is not supported for in-place resize (the {:?} debug format now prints Vdi instead of Unknown, since chain::ImageFormat gained the variant, but the refusal itself is unchanged).
The info round-up fix (parse_vdi_header rounding disk_size up to 512) was verified byte-unchanged against every existing 512-aligned VDI baseline, and the new vdi-odd-size fixture (1048577 → 1049088) confirms the rounding end-to-end against a live qemu-img info baseline.

Findings: step 2d — fixtures and baselines¶

All nine new fixtures were generated deterministically by instar-testdata/scripts/generate-vdi-fixtures.py: created with a static qemu-img binary, then byte-patched so the two creation UUIDs are pinned to fixed values (the only non-deterministic creation bytes) before any per-fixture corruption is applied.
vdi-odd-size needed a 2 MiB base image rather than the smallest possible 1 MiB base: a 1 MiB base would trip qemu's rule 9 (disk_size > blocks_in_image * block_size, evaluated against the rounded disk_size) once the odd-size patch and the round-up rule combine, refusing the fixture at qemu-img's own open instead of exercising the rounding rule cleanly.
generate-baselines.py auto-commits by default and regenerates every registered output type, not just the restricted manifest passed on the command line. This step's baseline run undid that auto-commit and surgically restored the output types that were not part of this phase's scope; the recommendation to add a --no-commit / --output-type-limiting flag to the generator was recorded rather than implemented (instar-testdata scripting is outside this phase's file set).
The five malformed fixtures needed OSLO_SKIP_IMAGES entries in tests/test_oslo_crossval.py: oslo.utils' VDIInspector validates none of the fields qemu refuses on (bad version, unaligned block-map offset, wrong block size, non-NULL parent UUID, too many blocks), so without the skip entries oslo would report these images as safely-openable while qemu-img (and instar) refuse them.
A profile-corruption integrity spot-check (10 images, covering the regeneration-corruption class phase 1 fixed in detect-profiles.py) came back clean — the regenerated profiles for the new VDI fixtures and a sample of pre-existing images match raw/ byte-for-byte.

Findings: step 2e — integration tests¶

Full touched-suite results, run locally at moderate concurrency: test_dd 39/39, test_compare 56/56, test_adversarial 79/79, test_info_safe 628/628, test_oslo_crossval 221 passed (10 pre-existing skips, unrelated to VDI), test_convert 218 passed (3 pre-existing skips). Zero failures across all six suites, and no KVM-contention flakes observed on replay.
qemu-img dd's count/skip semantics are absolute from offset 0 in the source file, not relative to any block-map structure — the windowed dd test crossing an unallocated→allocated boundary of vdi-data-dynamic confirmed instar matches this byte-for-byte.
The qcow2-with-VDI-backing chain test needed the VDI file copied into the overlay qcow2's own directory before creating the -b <vdi> -F vdi chain: instar's backing-file allowlist restricts backing paths to the overlay's directory (or an explicitly allowlisted one), so a VDI backing file living elsewhere in the test tree would be refused by the allowlist check rather than exercising the chain-reader arm under test.
compare on a malformed-but-detected VDI reports a non-zero-exit content mismatch rather than an open error — proving compare is not silently falling back to a raw read of the malformed container. This is documented as a quirk in docs/quirks.md rather than treated as a defect.
info stays lenient on malformed VDIs by design: its separate, simpler header parser (parse_vdi_header) is out of scope for the reader graduation, so it still reports plausible detection-level fields and exits 0 for images the full reader refuses. Also documented as a quirk.

Verification (management-session review checklist)¶

Success criteria¶

Phase 2 is complete when:

instar convert, compare, and dd accept dynamic and static VDI input with byte parity against qemu-img across the fixture set, including discarded blocks, past-EOF bmap entries, odd disk_size, and a qcow2-with-VDI-backing chain.
The five malformed fixtures are refused cleanly (no hang, no raw fallback) and the five safe fixtures are exercised in CI with full info baselines.
src/crates/vdi/ exists, no_std, panic-free, with unit and coverage-guided fuzz coverage; the differential fuzzer includes vdi in its format pool.
check/bench behaviour on VDI is pinned by tests; nothing gate-lifted reads VDI as raw.
Docs (format-coverage, quirks, README, CHANGELOG, ARCHITECTURE) and the master plan record the new state.

Hand-off to later phases¶

Phase 3 (Parallels read) reuses the crate template, the feature-flag wiring shape, the graduation checklist (including the bench/rebase raw-fallback hazard and the check pin), and the fixture-generator pattern from this phase — its plan can be briefer by referencing this one.
Phase 6 (QED decision) inherits the observation that graduation mechanics are now ~4 small arms plus tests; the cost of a QED read path is dominated by the parser crate alone.
Future work recorded on the master plan: instar check support for VDI; map/measure VDI input.

Back brief¶

Before executing any step of this plan, back brief the operator: confirm the scope (convert/compare/dd only; check stays a refusal), the bench/rebase feature-flag decision (all four reader-linking ops gain vdi-input in one commit to prevent raw fallback), the odd-disk_size round-up choice (qemu parity over oslo parity, recorded as a divergence), and that instar-testdata changes are management-reviewed before the authorised commit to its main.

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