P0 MMS Reporting: fail closed on zero-payload BIT STRING underflow

[masarray]

Confirmed root cause

src/mms/reporting.cpp:decode_bit_field on current main computed (raw.size()-1)*8 - unused with size_t. Header-only MMS BIT STRING with 1..7 unused bits has zero payload and underflows to a huge iteration bound, indexing beyond the received frame buffer. This is a network-originated malformed-input safety issue.

Fix

• Validate unused-bit count does not exceed actual payload bits before subtraction or byte indexing; throw existing MmsReportingFormatError.
• Preserve normal padded nonempty BIT STRING decoding and existing report state machine.
• Add deterministic malformed OptFlds and inclusion-vector regression for unused counts 1 and 7, plus valid padded report control.

Scope and provenance

Two core files only; no alternate decoder, GUI workaround, thread, packet format or breaking API. Rediscovered while auditing old unmerged draft #133; deliberately applied the focused patch to today's main rather than force-merging the stale draft.

Gates

Require exact-head C++ core/reporting + sanitizers (if triggered), CI and clean review before merge. Physical vendor interoperability remains separately unverified.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-17T01:34:06.394382Z	ddacad7	PR opened

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