# TC010 48-bit statfs Large Provider

Status: **RESOLVED - qualified sparse vnode provider validated**

Resolved: 2026-08-09

Baseline: `cd0e985b5ac54a4b7acb7042422329ad1729fb3e`

## Original Problem

TC010 requires a real EROFS mount whose 48-bit superblock block count has a
nonzero high word. A nonzero `blocks_hi` declares at least `2^32` filesystem
blocks, so a 4 KiB filesystem needs a provider larger than 16 TiB. Earlier
vnode-md work used only small regular images and correctly could not claim PASS.

This affects:

- 48-bit superblock block-count decoding;
- primary-device media-size validation;
- FreeBSD `statfs(2)` and `df(1)` 64-bit totals;
- the union selector between `rb.blocks_hi` and `rb.rootnid_2b`.

## Exact Trigger

1. `EROFS_FEATURE_INCOMPAT_48BIT` is set.
2. `rootnid_8b` is nonzero, selecting `rb.blocks_hi`.
3. `blocks_hi` is nonzero.
4. Provider media size is at least `total_blocks << blkszbits`.
5. Superblock CRC32C, root inode, and normal mount checks pass.

A short provider fails with `ENXIO`; that negative guard cannot replace the
positive statfs test. TC150's `rootnid_8b == 0` fallback and multidevice totals
also select different production paths.

## Earlier Attempts

1. Structured metadata patching and CRC32C recomputation succeeded.
2. The normal small vnode provider failed media-size validation as expected.
3. Static source comparison confirmed Linux's `48BIT && rootnid_8b` selector,
   but static arithmetic was not counted as dynamic coverage.
4. No earlier run had attached and mounted a qualified 16 TiB provider, so the
   issue remained SHELVED.

## Resolution

The isolated FreeBSD 15.0-RELEASE-p8 guest uses UFS2, which supports a sparse
regular file large enough for vnode md without allocating 16 TiB physically:

```text
truncate -s 17592193667072 TC010-provider.raw
stat: size=17592193667072 allocated_blocks=15232 block_size=32768
mdconfig -a -t vnode -f TC010-provider.raw: md0
diskinfo mediasize: 17592193667072 bytes
```

The structured fixture encoded:

```text
blocks_lo=1861
blocks_hi=1
rootnid_8b=36
total_blocks=4294969157
required_provider_length=17592193667072
```

Prefix SHA256:
`d3ffadc6fc9e73f85a8a5973b4ac5ee2a2da57b4e8baeccd259fa4325a2146cf`.

The exact prefix was copied to the sparse provider, then extended with zeros.
Mount succeeded with KLD SHA256
`8349c97c7ced253fff32a9e706f29313f309cb63a270e4e39a4501426f2b95c6`.
`df` reported:

```text
Filesystem  Type  1024-blocks        Used Avail Capacity
/dev/md0    erofs 17179876628 17179876628     0   100%
```

`17179876628 == 4294969157 * 4`; no 32-bit wrap occurred. `f_bfree` and
available blocks were zero, so Used correctly equaled total. The control file
matched, dmesg had no new fault, and the mount, md unit, sparse provider, and
KLD were cleaned up.

## Related Coverage

- TC017 separately repeated the small-provider `ENXIO` guard and qualified
  sparse-provider mount, proving the nonzero high-word parse dynamically.
- TC018 separately read exact bytes from physical offset
  `17592191561728`, above 16 TiB; it did not substitute a large `df` result for
  high-address data I/O.

No repo22 kernel source change was made during this resolution.
