docs: changelog and known issues for the remaining conformance errors

Conformance on tank, conformance/run.sh --no-fetch (2026-09-26): 597 of
697 ok, 6 our-errors (4 corrupt objects HDF5 2.0 reads through a bug, the
Blosc2 and ZFP filters), 2 mismatches (the known h5py big-endian VL bug).
Closes the known-issues entries for metadata cache images,
cve-2024-32624, cve-2020-10810/10812, and unfiltered chunks of the wrong
size; the N-Bit / 64-bit scale-offset entry is recorded as not our bug.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

CHANGELOG.md

@@ -2,6 +2,60 @@
 
 ## Unreleased
 
+### Remaining conformance errors (2026-09-26)
+Conformance on tank, `conformance/run.sh --no-fetch`: 597 of 697 files
+ok (575 before). Of the 6 our-errors left, 4 are corrupt data HDF5 2.0
+reads only through a bug (listed in `CONFORMANCE.md`), 2 are the Blosc2 and
+ZFP filters; the 2 mismatches are the known h5py big-endian VL bug.
+- **Metadata cache images are read.** A file written with a metadata cache
+  image keeps its metadata cache entries in an image block the superblock
+  extension points at, and libhdf5 reads them in place of the file's own
+  bytes; in `h5clear_mdc_image.h5` the root group exists only there, and
+  every reader failed with `InvalidObjectHeaderVersion(0)`. `File`,
+  `MmapFile` and `LazyFile` (and `h5rs`) now apply the image at open
+  (`clawhdf5_format::superblock_ext`), with libhdf5's checks. A file whose
+  image libhdf5 cannot load opens in libhdf5 but nothing in it can be read;
+  `File::open` refuses it.
+- **The superblock extension is decoded at open, as libhdf5 does:** a File
+  Space Info or Metadata Cache Image message libhdf5 cannot decode makes the
+  open fail (`cve-2020-10810`, `cve-2020-10812` were opened).
+  `FormatError::InvalidSuperblockExtension`, `InvalidCacheImage`.
+- **Dataset storage libhdf5 refuses at open is refused at open**
+  (`FormatError::InvalidDatasetStorage`, `data_read::check_dataset_storage`):
+  an element count times element size that overflows (`cve-2024-32624`
+  `/Dset_OBJREF` opened and reported its shape), contiguous storage past the
+  end of the file, compact data of the wrong size. An empty contiguous
+  dataset at a defined address, which clawhdf5 up to v2.7.0 wrote, still
+  opens.
+- **Wrong or missing data fixed:**
+  - a simple dataspace of rank 0 holds one element (it held 0;
+    `cve-2020-18494`), and contiguous storage larger than the dataset reads
+    (`cve-2024-32623`, `cve-2025-2309`; libhdf5 ignores the excess);
+  - scale-offset: the packed codes start at byte 21 whatever size the chunk
+    records for `minval`, a chunk with `minbits` 0 and a fill value is all
+    fill values, full-width `minbits` stores the elements as they are
+    (these decoded differently from libhdf5); E-scale is refused, as in
+    libhdf5; codes past the end of the chunk stay an error
+    (`cve-2025-2308`, where HDF5 2.0 reads past its buffer);
+  - shuffle uses its own parameter as the element size, as libhdf5 does
+    (`cve-2025-44905`);
+  - an unfiltered chunk the index records at other than the chunk's size is
+    refused (it read with zeros for the missing bytes; `cve-2025-44904`),
+    as is a chunk B-tree key with a non-zero element offset.
+- **Refused as libhdf5 refuses them:** a v1 group with an empty link name
+  fails its listing (`FormatError::InvalidLinkName`; lookups still work,
+  `cve-2021-46244`); dataspaces with more than 32 dimensions, a rank on a
+  scalar or null dataspace, or a dimension over its maximum
+  (`FormatError::InvalidDataspace`).
+- `ObjectHeader::object_class` classifies a header as libhdf5 does (a
+  dataset needs a datatype *and* a dataspace).
+- Conformance harness: user-defined links were listed as objects by the
+  reference, unopenable objects were not deduplicated, nested array types
+  were hashed wrong (`tarray3.h5`), and the attributes of objects h5py
+  cannot open were compared; all fixed. `CONFORMANCE.md` lists the corrupt
+  objects HDF5 2.0 reads through a bug (`bad_nbit_parms_walk.h5` among
+  them: libhdf5's own test now requires that read to fail).
+
 ### Concurrent reads (2026-09-26)
 - **Full reads of chunked datasets scale with threads again when rayon's
   pool has one thread.** Each full read handed its chunks to rayon to

docs/known-issues.md

@@ -202,9 +202,25 @@ fill-value item that did is fixed).
     its datatype message stores (12 with 4-byte offsets), and the global
     heap is read with libhdf5's header padding
     (`crates/clawhdf5/tests/vl_offset4_interop.rs`).
-  - Metadata cache images are not supported.
+  - Metadata cache images are not supported. **Fixed 2026-09-26:** the
+    image is applied at open, as libhdf5 loads it over the file's metadata
+    (`clawhdf5_format::superblock_ext`); `h5clear_mdc_image.h5` reads
+    (`crates/clawhdf5/tests/metadata_cache_image.rs`). A file whose image
+    libhdf5 cannot load (`cve-2025-6269-*`, `cve-2025-6516`) opens in
+    libhdf5 with nothing readable in it; `File::open` refuses it.
   - x87 long double and binary128 are refused.
   - N-Bit on 64-bit scale-offset data and some N-Bit parameter layouts fail.
+    **Not our bug (checked 2026-09-26):** both are corrupt files HDF5 2.0
+    reads only by reading past a buffer. `cve-2025-2308`'s
+    `/Scale_offset_long_long_data_le` has scale-offset codes that run past
+    the end of the chunk (libhdf5's develop branch refuses it, "Buffer too
+    short"); `bad_nbit_parms_walk.h5` has an N-Bit parameter list one value
+    short (libhdf5's own `test_filter_bad_params` in `test/dsets.c` now
+    requires that read to fail). We refuse both; `CONFORMANCE.md` lists them
+    under *Known not-our-bug*. Scale-offset did decode three cases
+    differently from libhdf5 (codes after a `minval` of recorded size other
+    than 8, `minbits` 0 with a fill value, full-width `minbits`): fixed
+    2026-09-26.
 - **Filters:** blosc, blosc2, bitshuffle, bzip2, LZF and zfp are not
   implemented. **Fixed 2026-09-26** for LZF (default-on `lzf` feature),
   bitshuffle, bzip2 and Blosc 1 (`bitshuffle`, `bzip2`, `blosc`, or
@@ -219,7 +235,11 @@ fill-value item that did is fixed).
   read with zeros for the missing bytes** (any filter; found reviewing the plugin
   filters). **Fixed 2026-09-26:** it is an error naming the chunk. A corrupt
   chunk must never read as zeros. Unfiltered chunks are read at their stored
-  size and are not checked this way.
+  size and are not checked this way. **Fixed 2026-09-26** for unfiltered
+  chunks too: in a dataset without filters a chunk the index records at
+  other than the chunk's size is refused, as libhdf5's develop branch
+  refuses it (`cve-2025-44904`, where HDF5 2.0 fills the rest from its
+  buffer).
 - **Crash:** a hostile Blosc chunk (frame size below its header) panicked in
   builds with overflow checks. **Fixed 2026-09-26**; the new decoders are
   fuzzed in the unit tests.
@@ -232,13 +252,18 @@ fill-value item that did is fixed).
   refused. 17 of the 18 now
   fail as in libhdf5 (conformance on tank, `conformance/run.sh --no-fetch`,
   2026-09-26: 571 of 697 ok). Still read where libhdf5 refuses:
-  - `cve-2024-32624.h5` `/Dset_OBJREF`: a dataspace whose storage size
+  - ~~`cve-2024-32624.h5` `/Dset_OBJREF`: a dataspace whose storage size
     overflows 64 bits. `File::dataset` and `shape()` succeed (libhdf5
-    refuses at open); reading the values fails.
+    refuses at open); reading the values fails.~~ **Fixed 2026-09-26:**
-  - `cve-2020-10810.h5`, `cve-2020-10812.h5` (whole files libhdf5 cannot
+    `File::dataset` (and `MmapFile`, `LazyFile`) refuse it at open
+    (`FormatError::InvalidDatasetStorage`), as they do contiguous storage
+    past the end of the file.
+  - ~~`cve-2020-10810.h5`, `cve-2020-10812.h5` (whole files libhdf5 cannot
     open, not among the 18): libhdf5 decodes the superblock extension's File
     Space Info and metadata-cache-image messages at open and refuses these
-    files; we do not decode those messages at open.
+    files; we do not decode those messages at open.~~ **Fixed 2026-09-26:**
+    the superblock extension is decoded at open with libhdf5's checks, and
+    both files are refused.
   - Deliberately not refused, because clawhdf5 up to v2.7.0 wrote them: a
     float sign bit position outside the type, and a size-0 string type.
   - Not refused because current libhdf5 reads it though HDF5 2.0.0