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Flax / orbax — Checkpoint Metadata Integrity Gap

Summary

orbax.checkpoint.PyTreeCheckpointer.metadata() reads structure information (shape/dtype) from _METADATA JSON. restore() reads actual tensor values from OCDBT binary files. These are separate subsystems with no cross-validation.

A checkpoint directory can contain an unchanged _METADATA while the OCDBT tensor data files contain different values. metadata() returns the original structure; restore() loads the different values without an integrity warning.

Reproduction

pip install "jax[cpu]>=0.4.0" orbax-checkpoint flax

python3 create_flax_orbax.py artifacts/
# → creates baseline_ckpt, replacement_ckpt, tampered_ckpt

python3 inspect_metadata.py artifacts/
# → METADATA_CONTAINS_VALUE=False
# → METADATA_CHECKSUM_FIELD=False
# → INTEGRITY_GAP_CONFIRMED=True

python3 reproduce_restore.py artifacts/
# → RESTORED_BASELINE_VALUE=1.0
# → RESTORED_TAMPERED_VALUE=999.0
# → OUTPUT_FLIP_CONFIRMED=True
# → WARNING_EMITTED_METADATA_PATH=False
# → METADATA_INTEGRITY_GAP_CONFIRMED=True

Key Findings

Check	Result
`metadata()` contains tensor values	False
`metadata()` contains checksums	False
`metadata()` detects data replacement	False
`restore()` loads replacement values	True
Integrity warning emitted on restore	False
OUTPUT_FLIP_CONFIRMED	True (1.0 → 999.0)

Files

File	Purpose
`create_flax_orbax.py`	Create baseline / replacement / tampered checkpoints
`inspect_metadata.py`	Demonstrate metadata() cannot detect data replacement
`reproduce_restore.py`	Confirm OUTPUT_FLIP (1.0→999.0), no integrity warning
`requirements.txt`	Python dependencies
`expected_output.txt`	Expected terminal output
`SHA256SUMS_T1.txt`	File integrity hashes
`logs/`	Execution logs for all steps

Environment

jax 0.6.2
flax 0.10.7
orbax-checkpoint 0.11.37
Python 3.10

Note on Warnings

The "Sharding info not provided" warning emitted during restore() is a topology advisory that appears identically for both baseline and tampered checkpoints. It is not related to data integrity and provides no signal that tensor data has changed.

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