docs(spec): one >=64-bit ratio-product rule, guarded in both specs (LAB-2594) - #58
docs(spec): one >=64-bit ratio-product rule, guarded in both specs (LAB-2594)#5827Bslash6 wants to merge 3 commits into
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…AB-2594) The decompression-bomb ratio bound was specified twice with divergent normative text: interop v2 bound the integer width (LAB-1135), wire format said nothing, so an implementer reading wire format alone could legally compute the product in 32-bit pointer-width arithmetic. Both documents now carry the same rule, and CI fails if the copies drift. The rule names the operation rather than a property of the arithmetic -- widen the operand to >=64-bit unsigned BEFORE multiplying -- because that is what defeats the actual defect: 1000 * payload.len() on usize is 64-bit on the author's host and in CI, and wraps only on the shipped wasm32 target. "Compute in >=64 bits" never fires in that author's self-assessment. The "if max_allowed overflows: REJECT" pseudocode is removed rather than reworded. It is not an observable event as written in any target language, and once the operand is widened it is unreachable -- the two 512 MiB caps bound the product below 2^39. Rejecting on overflow is explicitly not a substitute for widening: at 32-bit width it refuses 99.2% of the legal compressed-size range. Narrowing "no floating point" to "no floating-point ratio" (needed to permit JavaScript Number, exact below 2^53) would have sanctioned a truncating integer division accepting up to 1000*cs + (cs-1), so the bound is now required to be computed by multiplication in any arithmetic. interop v2's "corrupts the bound in both directions" was wrong: wrapping can only tighten it, so the failure mode is spurious rejection, never a bypass -- but a total one, collapsing to 704 B at 4.29 MB and to 0 at the 512 MiB cap. Spec text only. cachekit-core already uses checked_mul on u64 and is unchanged; no limit values and no fixture bytes were touched.
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WalkthroughThe specifications now require widened multiplication for decompression-ratio limits. A new checker compares duplicated normative blocks. Mutation tests validate checker failures, and CI runs both safeguards. ChangesDecompression limit consistency
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: ⚪ Minimal · up to The PR is merge-ready after normal checks, with one bounded documentation follow-up: describe the one-eighth impact as approximate for each wrap interval in the affected specification and changelog passages. No actionable merge-blocking risk remains. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
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Inline comments:
In `@spec/interop-v2.md`:
- Around line 286-292: Correct the 32-bit wrapping rationale in
spec/interop-v2.md lines 286-292, spec/wire-format.md lines 441-447, and
CHANGELOG.md lines 45-50: state that wrapping causes rejection only when the
wrapped bound is below original_size, and remove the claims of universal
unreadability and 99.2% rejection while preserving the spurious-rejection
description.
In `@spec/wire-format.md`:
- Line 394: Update the fenced code block in the wire-format specification near
the reported location by declaring the fence language as text, resolving
markdownlint MD040 without changing the block’s contents.
In `@tools/check-spec-duplication.py`:
- Line 51: Update the validation raises in the spec-duplication script,
including the checks around the BEGIN/END sentinels, so they no longer trigger
Ruff TRY003: move each inline message into a purpose-specific exception or add a
narrow justified suppression while preserving the existing failure details.
Apply the same fix in `@tools/test_check_spec_duplication.py` at line 36: Covered
as the same configured inline-exception-message lint violation.
In `@tools/test_check_spec_duplication.py`:
- Line 44: Rename the unused parameter in the “unmodified tree” lambda within
CASES from root to _ while preserving its Callable[[Path], None] signature and
behavior.
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The non-normative rationale claimed 32-bit wrapping makes "every entry whose compressed payload is >= 4.29 MB permanently unreadable — 99.2 % of the legal range". That conflates two different mechanisms and overstates the wrapping case by about 8x. 99.2 % is the share of the legal compressed-size range sitting above the first wrap point (4,294,968 B). It is the correct figure for *reject-on-overflow*, which refuses every payload past that threshold — and the section elsewhere uses it correctly for exactly that purpose, to argue a checked multiply is not a substitute for widening. It is not the figure for silent wrapping. The wrapped bound sweeps [0, 2^32) in steps of 1000, so it lands below the 512 MiB uncompressed cap — the only region where it can refuse a legal entry at all — for exactly 1/8 of each 2^32/1000 ~= 4.29 MB cycle, since the cap is 2^29 and the modulus 2^32. Across the other 7/8 the wrapped bound still exceeds every permitted original_size and the entry is accepted. Inside that eighth, rejection needs original_size to exceed the wrapped bound. Verified numerically: CodeRabbit's counterexample of 5,242,880 B wraps to 947,912,704 B, comfortably above the cap, so nothing is rejected there. The worst positions do stand as written (704 B at the threshold, 0 at the cap). Rewrote the claim in both copies of the shared block, added a sentence naming the two figures as distinct so the 99.2 % above does not read as contradicting the 1/8 below, and corrected the matching CHANGELOG entry. The normative rule is untouched: the >= 64-bit widening MUST, the multiplication requirement, and the no-ratio prohibition all stand exactly as the panel left them. Only non-normative rationale changed. Also in this pass, both cheap and both real: - MD040 on the bomb-detection fence (wire-format.md:394) — tagged `text`, matching the convention already used at lines 85, 139 and 578. - ARG005 on the "unmodified tree" case — renamed the unused lambda parameter to `_`. Not applied: TRY003 on the four ValueError messages in check-spec-duplication.py and one in its test. The finding attributes these to "the repository's TRY003 rule", and no such rule exists — this repo has no pyproject.toml, no ruff.toml, no pre-commit config, and verify.yml runs no Python linter. TRY003 is flake8-tryceratops and is not in ruff's default select (E4, E7, E9, F), so it can only come from CodeRabbit's own expanded ruleset. Satisfying it means custom exception classes for four raise sites in a 134-line stdlib-only guard whose documented contract is to name the exact defect. Rebutted on the PR. check-spec-duplication and its 9-case mutation suite both pass, so the two copies remain identical modulo operand name and the guard still fails closed. Full stdlib verify suite green. CodeRabbit-Resolved: interop-v2.md:292:Correct the 32-bit wrapping impac CodeRabbit-Resolved: wire-format.md:394:Specify a language for this fenc CodeRabbit-Resolved: test_check_spec_duplication.py:44:Rename the unused
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Inline comments:
In `@spec/interop-v2.md`:
- Around line 290-291: Update the impact wording in spec/interop-v2.md lines
290-291, spec/wire-format.md lines 445-446, and CHANGELOG.md lines 48-49 to
consistently describe the per-wrap-interval fraction as approximately ⅛, or
explicitly scope the exact ⅛ figure to the full 2²⁹-byte payload period.
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Panel re-run at high stakes on cd840aa, because that commit changed crypto/protocol spec text after the previous panel had reviewed it — the gate keys off current HEAD, not "a panel ran on this ticket once". All four agents confirmed the arithmetic correction is right; all four found defects in how I wrote it. Every figure below re-verified independently before acting. Restored, and this is the one that mattered. The rewrite silently dropped "permanently unreadable ... as a hard error rather than a cache miss" — the only clause in the note that told an implementer what happens to their application. Two agents flagged it independently. The note had been left stating frequency with no symptom and no persistence, which reads as a rare recoverable edge case; the truth is the wrapped bound is a pure function of the size, so an affected entry fails identically on every read, forever. Correcting an overstatement into an understatement is not a fix. Fixed a backwards argument, also found independently by two agents. The closing sentence cited 99.2 % for reject-on-overflow against 1/8 for wrapping and concluded "which is precisely why a checked multiply is not a substitute for widening" — on those figures it ranks the silent defect as 8x milder than the loud one, steering a reader toward the unchecked multiply. The real reason a checked multiply is no substitute is that it is unnecessary: after both size caps the product is < 2^39 and cannot overflow 64 bits. Both are non-conforming; the numbers compare blast radius, not acceptability. Dropped a false superlative I introduced. "The worst positions are severe — 704 B" is wrong: the wrapped bound lands only on multiples of gcd(1000, 2^32) = 8, and its nonzero floor is 8 B at compressed_size = 115,964,117, which is 88x below 704. A third party calibrating a conformance test to "the worst case" would have built it 88x too weak. Now states the floor and names the gcd that produces it. Fixed a false equation: `2^32/1000 ~= 4.29` MB put the unit outside the code span, so the span asserted 2^32/1000 ~= 4.29 — off by 10^6. Dropped "exactly" from the 1/8 claim: measured density is 0.1249998, not 1/8 exactly, and "exactly" is the wrong word in a section whose subject is that approximate claims about integer arithmetic caused this bug. Dropped the "exceeds every permitted original_size" clause, false at compressed_size = 335,544,320 where the wrapped bound equals 2^29. Cut per the pragmatism review: the sweep sentence, the 7/8 complement, and the restatement of the normative pseudocode inside a non-normative note. Restating a normative rule in the rationale is how the two drift. The CHANGELOG carried a third copy of the derivation; trimmed to the outcome. The one-off vulgar-fraction glyph is gone. Scoped the note's headline. It claimed "the failure direction under pointer-width arithmetic is fail-closed, never a bypass", which is true of the ratio product and false in general: nothing in either document binds the width of original_size, and truncating it IS a bypass — a declared 4,294,968,296 becomes 1000, clears the 512 MiB cap, clears the ratio bound, and is accepted where 64-bit rejects. Verified. The headline now says "the ratio product's failure direction" and flags original_size as a separate unbound obligation, so the spec no longer claims a property it does not have. Closing the gap is out of scope for a docs-only PR on a different bound and is filed as LAB-2734, together with the conformance vector that cannot detect it: reject_declared_size_bomb declares 2^40, whose low 32 bits are zero, so a truncating reader sees 0 and rejects on the length mismatch instead — passing the vector for the wrong reason. One correction to my own previous commit message. It rebutted TRY003 on the grounds that no such rule is configured here, then applied ARG005, which is equally unconfigured — arguing both sides in one commit. The honest line is cost, not provenance: ARG005 and MD040 are one token each and match conventions the repo already follows, while TRY003 wants custom exception classes for four raise sites in a 134-line stdlib guard whose documented contract is to name the exact defect. The rebuttal stands on that basis. Not applied: a second erratum footnote for this correction. The existing one exists because the old text pointed the wrong *direction*, which mis-prioritises a fix. Overstating severity errs safe and changes nothing an implementer does. check-spec-duplication passes, its 9-case mutation suite still fails closed, and all stdlib verify legs are green.
Expert panel — high stakes, re-run at
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| # | Finding | Found by |
|---|---|---|
| 1 | The rewrite silently deleted "permanently unreadable … as a hard error rather than a cache miss" — the only clause telling an implementer what happens to their application. Left frequency with no symptom or persistence, reading as a rare recoverable edge case. Restored, with the reason it is permanent (the bound is a pure function of the size). | craftsman + pragmatism, independently |
| 2 | The closing contrast argued backwards: citing 99.2 % (reject-on-overflow) against 1/8 (wrapping) and concluding that is "why a checked multiply is not a substitute" ranks the silent defect as 8× milder than the loud one. Real reason: it is unnecessary — after both caps the product is < 2³⁹ and cannot overflow 64 bits. | bug-hunter + security, independently |
| 3 | False superlative I introduced. 704 B is not a worst position: the bound lands only on multiples of gcd(1000, 2³²) = 8, floor 8 B at compressed_size = 115,964,117 — 88× below 704. A conformance test calibrated to "the worst case" would be 88× too weak. |
bug-hunter |
| 4 | `2³²/1000 ≈ 4.29` MB is a false equation as rendered — unit outside the code span, so the span asserts 2³²/1000 ≈ 4.29, off by 10⁶. |
craftsman |
| 5 | "exactly ⅛" — measured density is 0.1249998, not exactly 1/8. "Exactly" is the wrong word in a section about approximate integer-arithmetic claims causing this bug. | bug-hunter |
| 6 | "exceeds every permitted original_size" is false at compressed_size = 335,544,320, where the wrapped bound equals 2²⁹. Clause cut. |
bug-hunter + craftsman |
Also cut per the pragmatism review: the sweep sentence, the 7/8 complement, and a restatement of the normative pseudocode inside a non-normative note (that is how the two drift). CHANGELOG carried a third copy of the derivation — trimmed to the outcome.
Scoped, not fixed here — filed as LAB-2734 (security, Size M)
The note's headline claimed "the failure direction under pointer-width arithmetic is fail-closed, never a bypass". True of the ratio product, false in general: nothing in either document binds the width of original_size, and truncating it is a real bypass — a declared 4,294,968,296 becomes 1000, clears the 512 MiB cap and the ratio bound, and is accepted where 64-bit rejects. Verified.
The headline is now scoped to "the ratio product's failure direction" with original_size flagged as a separate unbound obligation, so the spec no longer claims a property it lacks. Adding the normative clause is new normative text on a different bound — out of scope for this PR, and it carries its own panel gate.
LAB-2734 also captures the second half: reject_declared_size_bomb declares 2⁴⁰, whose low 32 bits are zero, so a truncating reader sees original_size = 0 and rejects on the length mismatch instead — passing the vector for the wrong reason. The one published vector aimed at this bound is blind to the defect. Distinct axis from LAB-2718 (fail-closed wrap undemonstrable); this one is a fail-open truncation that is both unbound and undetectable.
Not applied
- A second erratum footnote. The existing one exists because the old text pointed the wrong direction, which mis-prioritises a fix. Overstating severity errs safe and changes nothing an implementer does.
- TRY003 (CodeRabbit,
tools/check-spec-duplication.py:51,53,58,61and its test at:36). Rebutted below.
CodeRabbit disposition
| Finding | Disposition |
|---|---|
interop-v2.md:292 — correct the 32-bit wrapping impact statement |
Applied. Independently verified: your counterexample at 5,242,880 B wraps to 947,912,704 B, above the cap, so nothing is rejected there. 99.2 % was the share of the range above the first wrap point — right for reject-on-overflow, wrong for wrapping. Good catch; it survived a prior panel. |
wire-format.md:394 — MD040 |
Applied — ```text, matching lines 85/139/578 and interop-v2's already-tagged copy of the same pseudocode. |
test_check_spec_duplication.py:44 — ARG005 |
Applied — lambda _: None. |
check-spec-duplication.py:51 — TRY003 |
Rebutted, see below. |
On TRY003. The finding attributes these to "the repository's TRY003 rule". There is no such rule: this repo has no pyproject.toml, no ruff.toml, no .pre-commit-config.yaml, and verify.yml runs no Python linter. TRY003 is flake8-tryceratops and is not in ruff's default select (E4, E7, E9, F), so it can only come from your own expanded ruleset.
I want to be straight about the basis, because "the rule isn't configured here" would have rebutted ARG005 and MD040 too, and I applied both. The real line is cost against benefit. ARG005 and MD040 are one token each and move the code toward conventions the repo already follows. TRY003 wants custom exception classes for four raise sites in a 134-line stdlib-only guard whose documented contract is precisely to "raise ValueError naming the exact defect" — the diagnostic strings are the design, and four exception classes to relocate them is a net loss in a script this size. If the repo later adopts a ruff config, that is the moment to revisit, and it is a repo-wide decision rather than one this PR should make.
Panel: bug-hunter-supreme, security-specialist, code-craftsman, catchphrase-agent — parallel, high stakes. check-spec-duplication and its 9-case mutation suite pass; all stdlib verify legs green.
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Closes LAB-2594.
spec/wire-format.md§ Security Limits specified the decompression-bomb ratio bound with no integer width and an overflow test no target language can perform.spec/interop-v2.md:256bound the same product normatively (LAB-1135, #53) — two documents, one bound, divergent normative text, and the weaker one is what a third-party implementer reading wire format alone would follow. Spec trust bug, not a live vulnerability:cachekit-corehas always been correct.What changed
One rule, stated in full in both documents, guarded against drift. The
>= 64-bit unsignedobligation now appears in both § Security Limits sections with identical rationale (identical text modulo each document's operand name). Stating it twice is deliberate — an implementer reads one document standalone — sotools/check-spec-duplication.pycompares the two copies in CI, with a 9-case mutation suite that re-arms the exact LAB-2594 divergence and asserts the guard fails.The rule names the operation, not a property of the arithmetic: widen the operand to
>= 64-bitunsigned before multiplying. That is what defeats the real defect —1000 * payload.len()onusizeis 64-bit on the author's host and in 64-bit CI, and wraps only on the shipped wasm32 target, so a rule phrased as "compute in >= 64 bits" never fires in that author's self-assessment. Rustu64, Pythonintand JavaScriptNumber(exact below 2⁵³; product < 2³⁹, noBigInt) all satisfy it, so no fallback is offered.if max_allowed overflows: REJECTis removed, not reworded. It is not an observable event as written (Rust wraps silently in release, JSNumberloses precision rather than overflowing, Python integers are arbitrary-precision — this repo's owntools/interop-v2-reference.pycan never take that branch), and once the operand is widened it is unreachable: the two 512 MiB caps bound the product below 2³⁹. Rejecting on overflow is explicitly not an accepted substitute for widening — at 32-bit width it refuses 99.2 % of the legal compressed-size range. Wire format's pseudocode now enforces both caps inline rather than leaving the precondition to a collapsed flow section.The bound MUST be computed by multiplication. Narrowing the old blanket "no floating point" rule to "no floating-point ratio" was necessary to permit JavaScript
Number, but on its own it sanctionedoriginal_size / compressed_size > 1000, whose truncation accepts up to1000·cs + (cs − 1)— looser than this spec permits. Division is now forbidden in any arithmetic.interop v2's "corrupts the bound in both directions" corrected. It cannot. Wrapping begins at
⌈2³²/1000⌉ = 4,294,968B (~4.29 MB) and can only tighten the bound (wrapped(p) = p mod 2³² < 2³² ≤ p, andoriginal_size ≤ 512 MiB < 2³²cannot itself wrap, so the comparison direction is preserved). The failure mode is spurious rejection, never a bypass — but a total one: the wrapped bound collapses to 704 B at that threshold and to 0 at the 512 MiB cap, so every entry with a ≥ 4.29 MB compressed payload is permanently unreadable on such a target. Marked non-normative, with the corrected claim recorded in place per this repo's precedent.Verification
Every arithmetic claim in the new text was verified by execution, not by reading — exhaustively over
cs ∈ [1, 2²⁹]with realuint32_twrapping,uint64_t, and IEEE-754 doubles: zero cases where wrapping loosens the bound, first divergence at exactlycs = 4,294,968, wrapped bound 704 at that point and 0 at2²⁹, zero inexact double products.Full
verify.ymlsuite green locally (13 legs, including the new guard and its mutation suite). Fixture bytes untouched — no SDK re-vendors.Review gates
Crypto/protocol expert panel: run at critical stakes, FIX-FIRST, all surviving findings applied in this diff. The panel caught a real contradiction in the first draft: a "checked multiply and REJECT on overflow" escape hatch that mandated the exact behaviour the next paragraph called a conformance defect. Three of four agents found it independently. It is gone, replaced by the widening requirement. The float-division hole and the CI drift guard also came from the panel.
Two findings were accepted but not fixed here:
original_sizeintest-vectors/interop-v2.jsonis 237 B; divergence begins at a 4,294,968 B compressed payload, so a wrapping decoder passes 100 % of the published suite. Closing it needs a limits-only vector shape plus a fixture version bump (forcing py/ts re-vendors) — out of scope for a spec-text change, filed separately and noted in the CHANGELOG.Documentation gate: this change is the documentation. Both spec documents, the CHANGELOG, and the interop v2 Design Decisions table (which said the rule was "carried over verbatim" — now states both copies are normative and CI-guarded) are updated in this diff. No SDK README, docs.cachekit.io page, or feature-matrix cell references this bound.
Scope
Spec text plus one CI guard. No SDK code —
cachekit-core/src/byte_storage.rsalready useschecked_mulonu64and is deliberately unchanged. No limit values changed. No fixture bytes changed. The msgpack element-count and depth axes (LAB-2503/2504/2505) are orthogonal bounds and untouched.Summary by CodeRabbit
Documentation
Chores