Fail closed on any 0xEF lead byte in the metamorphic scan

[thedavidmeister]

Closes #54

Implements the ruling on #54 exactly (#54 (comment)): the metamorphic scan fails closed on ANY bytecode whose first byte is the EIP-3541 reserved 0xEF — not on a designator shape, not on an EOF version match. EIP-3541 means ordinary deployment can never produce such code, so it exists only via protocol features (EOF containers, EIP-7702 delegation designators, whatever the prefix is assigned next) that a legacy disassembler cannot reason about, and the scan must refuse to vouch for any of it. The narrow exact-prefix rule is what caused this bug when 0xEF01 arrived; a format registry would re-arm it for 0xEF02.

What changed

• LibExtrospectMetamorphic.scanMetamorphicRisk(bytes) gains a first-byte gate: when bytecode[0] == 0xEF (new file-level constant EIP3541_RESERVED_LEAD_BYTE), it returns a bitmap of exactly 1 << 0xEF before any opcode scan — the 0xEF byte itself reported as the risky element, per the ruling's mechanism. All other input delegates to the reachable-opcode scan masked by METAMORPHIC_OPS as before. The function is now genuinely total over bytes per the checkNotMetamorphic reports an account with no code as NOT metamorphic #55 ruling: the reserved prefix is the one first byte that never reaches scanEVMOpcodesReachableInBytecode, so EOFBytecodeNotSupported is no longer reachable from the metamorphic pair — EOF input (0xEF00…) now gets the 1 << 0xEF verdict instead of the EOFBytecodeNotSupported revert, which is the verdict change the ruling specifies ("checkNotMetamorphic then reverts as for any nonzero scan"). ABI unchanged; the named return was dropped per org convention while rewriting the function.
• checkNotMetamorphic(bytes) is untouched in code and now reverts Metamorphic(1 << 0xEF) on any reserved-prefix input through the scan it delegates to. The address-taking pair from Account-taking absence checks reject codeless accounts (ruling E+D on #55) #132 inherits both behaviours through delegation, and the codeless guards keep passing unchanged.
• isEOFBytecode is untouched, keeping its isEOFBytecode's two-byte magic test misclassifies EOF in both directions: version-less 0xEF00 blobs revert, 0xEF01 delegation designators pass #53-pinned factual meaning (0xEF00 magic only); the gate lives in the metamorphic scan, not in redefining the predicate. testScanMetamorphicRiskEIP7702DelegationDesignator asserts isEOFBytecode(designator) == false alongside the new verdict to pin that separation.
• Scope is the metamorphic pair only: the raw opcode scans (scanEVMOpcodesReachableInBytecode, scanEVMOpcodesPresentInBytecode) keep their EOFBytecodeNotSupported gate for direct callers, and the sibling designator findings on the beacon predicates and isERC1167Proxy keep their own issues.
• NatSpec/docs: LibExtrospectMetamorphic (header and all four entry points), METAMORPHIC_OPS in EVMOpcodes.sol (notes that bit 0xEF is reported without being a member), IExtrospectV1.scanMetamorphicRisk / checkNotMetamorphic, and the README metamorphic section all state the fail-closed rule and drop the now-false EOFBytecodeNotSupported claims.
• Test oracle: LibExtrospectionSlow.scanMetamorphicRiskSlow carries the same rule from its own SLOW_EIP3541_LEAD_BYTE constant, so the differential fuzz (testScanMetamorphicRiskReference) now runs with no EOF carve-out — total over all bytes. The vm.assume(!isEOFBytecode(...)) / EOF-skip carve-outs in the reference fuzz, the reverts-iff-nonzero fuzz, and both address/bytes equivalence fuzzes are deleted rather than widened: the properties now hold over all (etchable) bytes.

TDD

Failing-tests commit d11887e adds the discriminating tests, re-pins the four metamorphic-pair EOF tests from EOFBytecodeNotSupported to the new verdict, and gives the slow oracle the rule. nix develop -c forge test --no-match-path '*fork*': 369 passed, 17 failed — all 17 in the two metamorphic test files, failing on the old behaviour (zero scans for designators, EOFBytecodeNotSupported reverts, and the delegate-dependent 1 << 0xF4 verdict). The full suite adds the 3 ARBITRUM_RPC_URL fork failures (environmental, identical on main: 377 passed, 3 failed there).

Implementation commit 4200dbd adds the gate and docs. nix develop -c forge test --no-match-path '*fork*': 386 passed, 0 failed.

QA

• Discriminating tests:

  • testScanMetamorphicRiskEIP7702DelegationDesignator — the issue's verified repro inverted: 0xef0100 || 0x1234…7890 (length 23, isEOFBytecode false) scans to exactly 1 << 0xEF instead of 0.
  • testCheckNotMetamorphicRevertsOnEIP7702DelegationDesignator — the same designator reverts Metamorphic(1 << 0xEF) instead of passing (the repro's checkNotMetamorphic line inverted).
  • testScanMetamorphicRiskEIP7702DelegationDesignatorDelegateIndependent — delegate 0x5BF4…0000 (the issue thread's coin-flip case, previously 1 << DELEGATECALL) now reports the same 1 << 0xEF as every other designator: the verdict no longer depends on the address's hex digits.
  • testScanMetamorphicRiskBareReservedByte (hex"EF", length 1), testScanMetamorphicRiskFutureReservedPrefix (0xEF02…), testScanMetamorphicRiskEOFReservedPrefix / testCheckNotMetamorphicRevertsOnEOFReservedPrefix (0xEF00…, re-pinned from EOFBytecodeNotSupported) — ANY first byte 0xEF, not a shape or version whitelist.
  • testScanMetamorphicRiskReservedPrefixFuzz / testCheckNotMetamorphicReservedPrefixFuzz — 0xEF || tail for arbitrary fuzzed tails: always exactly 1 << 0xEF / always Metamorphic(1 << 0xEF).
  • Address entry points: testScanMetamorphicRiskAddressEIP7702DelegationDesignator, testCheckNotMetamorphicAddressRevertsOnEIP7702DelegationDesignator (a real etched 23-byte designator read back through account.code), plus the re-pinned test*AddressEOFReservedPrefix tests.
  • Totality: testScanMetamorphicRiskReference (differential vs the slow oracle, EOF carve-out deleted), testCheckNotMetamorphicFuzz (reverts-iff-nonzero over ALL bytes, EOF skip deleted), both *AddressEquivalenceFuzz tests (non-EOF assume deleted; designators and EOF code now included via assumeEtch).
  • Unchanged behaviour pinned by the untouched existing tests: empty bytes scan zero and pass, codeless accounts revert CodelessAccount at the address boundary, clean/risky legacy bytecode verdicts identical, ExtrospectConstants and all non-metamorphic suites untouched.

• Mutations applied (each applied to the working tree at 4200dbd, nix develop -c forge test --no-match-path '*fork*' run in full, then reverted; baseline for that command is 386 passed, 0 failed):

  #	Mutant (applied to src/lib/LibExtrospectMetamorphic.sol, then reverted)	Outcome
  M1	0xEF gate deleted entirely (revert to pre-fix behaviour)	KILLED: 369 passed, 17 failed — every new/changed test
  M2	first-byte comparison inverted == → !=	KILLED: 348 passed, 38 failed — reserved-prefix tests plus every clean/exact-bitmap legacy test
  M3	fail-open: gate returns 0 instead of 1 << 0xEF (the EIP-7702 delegated accounts are reported as NOT metamorphic #54 bug class itself)	KILLED: 372 passed, 14 failed — every reserved-prefix test on both the scan and the check, bytes and address entry points
  M4	length guard boundary > 0 → > 1 (bare 0xEF escapes the gate)	KILLED: 382 passed, 4 failed — testScanMetamorphicRiskBareReservedByte plus the reference and reserved-prefix fuzzes with 1-byte counterexamples
  M5	wrong reported bit: uint256(2) << 0xEF (= 1 << 0xF0 CREATE, still a METAMORPHIC_OPS member so the check still reverts)	KILLED: 372 passed, 14 failed — the exact-bitmap assertions and exact-revert-data expectations pin bit 0xEF itself
  M6	checkNotMetamorphic(bytes) masks the scan with & METAMORPHIC_OPS, silently ignoring bit 0xEF	KILLED: 382 passed, 4 failed — the check-level designator/EOF tests and the reverts-iff-nonzero fuzz
  M7	gate reads the LAST byte instead of the first	KILLED: 370 passed, 16 failed — designator and prefix tests (0x…90 last byte falls through to the legacy scan)

  All 7 mutants killed, 0 survivors; working tree verified clean after the campaign.

• Oracle: the RULING comment on EIP-7702 delegated accounts are reported as NOT metamorphic #54 (EIP-7702 delegated accounts are reported as NOT metamorphic #54 (comment)), read independently of the implementation: fail closed on ANY 0xEF first byte; mechanism is scanMetamorphicRisk(bytes) reporting bit 0xEF as the risky element while staying total per the checkNotMetamorphic reports an account with no code as NOT metamorphic #55 ruling; checkNotMetamorphic reverts as for any nonzero scan; isEOFBytecode keeps its isEOFBytecode's two-byte magic test misclassifies EOF in both directions: version-less 0xEF00 blobs revert, 0xEF01 delegation designators pass #53 meaning; scope is the metamorphic pair only. Expected values in the tests are constructed from that text as uint256(1) << 0xEF literals and abi.encodeWithSelector(Metamorphic.selector, …), not imported from the implementation (which uses its own constant), and the slow oracle re-derives the rule from its own EIP-3541 constant.

• Category check: the ruling asks for (1) the gate on ANY 0xEF first byte, not designator-shape or EOF-version — covered: first-byte comparison only, pinned for 0xEF00, 0xEF01 designators, 0xEF02, bare 0xEF, and fuzzed tails; (2) the specific mechanism — bit 0xEF in the scan's bitmap, bytes functions total, check reverting on nonzero — covered as specified, including the EOF verdict change that totality entails; (3) isEOFBytecode untouched per isEOFBytecode's two-byte magic test misclassifies EOF in both directions: version-less 0xEF00 blobs revert, 0xEF01 delegation designators pass #53 — covered, no diff on the predicate and the designator test asserts its false answer; (4) scope confined to the metamorphic pair — covered: no change to the raw opcode scans' EOF gate or to the beacon/ERC1167 predicates, whose designator findings keep their own issues. Nothing outside that scope was changed.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes

  • Metamorphic-risk checks now safely handle all bytecode beginning with the reserved 0xEF prefix.
  • EOF containers, delegation designators, and future reserved formats report the reserved-prefix risk instead of producing an unsupported-bytecode error.
  • Raw opcode scanning continues to reject EOF bytecode appropriately.

• Documentation

  • Clarified the fail-closed behavior and risk reporting for reserved-prefix bytecode across the relevant API documentation.

• Tests

  • Expanded coverage for EOF, delegation designators, arbitrary 0xEF-prefixed data, and fuzzed inputs.

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📒 Files selected for processing (3)

• README.md
• src/lib/LibExtrospectMetamorphic.sol
• test/lib/LibExtrospectionSlow.sol

Walkthrough

scanMetamorphicRisk now returns only 1 << 0xEF for any bytecode beginning with 0xEF. checkNotMetamorphic therefore reverts with Metamorphic(1 << 0xEF). Tests cover EOF, EIP-7702 delegation designators, future prefixes, and arbitrary tails.

Changes

Metamorphic Risk Handling

Layer / File(s)	Summary
Reserved-prefix scanner behavior src/lib/LibExtrospectMetamorphic.sol, src/lib/EVMOpcodes.sol, src/interface/IExtrospectV1.sol, README.md	The scanner detects a leading 0xEF and returns 1 << 0xEF before legacy opcode scanning. Documentation describes the updated scan and check behavior.
Reference implementation and scan validation test/lib/LibExtrospectionSlow.sol, test/src/lib/LibExtrospectMetamorphic.scanMetamorphicRisk.t.sol	The slow scanner mirrors the reserved-prefix handling. Tests cover EOF bytecode, EIP-7702 delegation designators, future prefixes, arbitrary tails, and address equivalence.
Check behavior validation test/src/lib/LibExtrospectMetamorphic.checkNotMetamorphic.t.sol	Bytes and address checks now expect Metamorphic(1 << 0xEF). Fuzz tests include bytecode beginning with 0xEF.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: 🔵 Low · up to 4200d

The change correctly rejects any bytecode beginning with 0xEF, but the accompanying documentation should qualify its explanation of how such code can exist across different chain histories and activation rules. The PR is mergeable with explicit owner follow-up to correct that wording.

Sequence Diagram(s)

sequenceDiagram
  participant Caller
  participant LibExtrospectMetamorphic
  participant LegacyOpcodeScanner
  Caller->>LibExtrospectMetamorphic: scanMetamorphicRisk(bytecode)
  LibExtrospectMetamorphic->>LibExtrospectMetamorphic: detect leading 0xEF
  LibExtrospectMetamorphic-->>Caller: return 1 << 0xEF
  Caller->>LibExtrospectMetamorphic: checkNotMetamorphic(bytecode)
  LibExtrospectMetamorphic-->>Caller: revert Metamorphic(1 << 0xEF)
  LibExtrospectMetamorphic->>LegacyOpcodeScanner: scan non-0xEF bytecode

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Suggested reviewers: claude

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: fail-closed handling for any bytecode beginning with 0xEF.
Linked Issues check	✅ Passed	The changes address issue #54 by marking every 0xEF-prefixed input risky and propagating the result through metamorphic checks.
Out of Scope Changes check	✅ Passed	The implementation, documentation, oracle, and tests remain focused on the linked issue and stated metamorphic-scan objectives.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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Actionable comments posted: 1

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Inline comments:
In `@src/lib/LibExtrospectMetamorphic.sol`:
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• README.md
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• src/lib/LibExtrospectMetamorphic.sol
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