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retoorandClaude Opus 4.8 79ae6f27da cpp-safety: safe-C++ playbook, fix a second UB, and a sanitizer net

Response to incident WA-2026-10-27 (the std::vector max_size() crash). Deep
research archived under research/safe-cpp-practices (8 Tier 1, 37 Tier 2, 2
conclusions) on production-safe C++ that keeps development speed: OpenSSF
compiler hardening, the C++ Core Guidelines bounds profile, integer-overflow
practice, libFuzzer/sanitizer CI, the sanitizer overhead ladder, clang-tidy
ratchets.

The research prescribed an ASan+UBSan+_GLIBCXX_ASSERTIONS build of the parsers.
Standing it up immediately caught a second, independent undefined-behavior bug
that two manual reviews had missed: a protobuf varint loop that shifted by up
to 70 bits (shift >= 64 is UB) on malformed input. Fixed and guarded by a
deterministic test and the sanitizer net.

- src/whatsapp/wa_proto.cpp: guard the wire-0 varint shift (the UB).
- tests/whatsapp_test.cpp: add an overlong-varint hostile case.
- tools/san-whatsapp.sh: the standing sanitizer net for detz::wa parsers.
- WaClient frame capture: a frame that fails to decode/dispatch has its exact
  bytes recorded to <home>/whatsapp/last-error.log (bounded), so the still
  unreproduced live <iq> failure becomes a one-paste deterministic fix. Wired
  from the wizard and the whatsapp_send tool.
- docs/cpp-safety.md: the project-wide playbook and design decisions (four
  layers: overflow-safe checked-reader parsing, cheap release hardening, the
  sanitizer test net, capture). docs/whatsapp.md section 13.6 records the
  follow-up and the honest open item. AGENTS.md makes it a standing rule;
  cap.md and mcp.md inherit it for their wire parsers.

Verified: build, whatsapp_test (incl. hostile+fuzz), make lint, and
tools/san-whatsapp.sh all green; research audit PASS.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

2026-10-27 19:03:32 +01:00

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std::span / subspan: no default bounds checking (cppreference, Embedonix, ModernesCpp)

Metadata

Field	Value
Institute	Molodetz Institute — Research & Retrieval Division
Subject	safe-cpp-practices
Document type	Source Dossier
Tier	1
Source title	std::span / subspan: no default bounds checking (cppreference, Embedonix, ModernesCpp)
Source URL	https://en.cppreference.com/cpp/container/span
Source publisher / author	cppreference.com; Embedonix; ModernesCpp
Source type	vendor docs
Source date (published)	Unknown — not stated
Date researched	2026-10-27
Researched by	tai (agent), model: Claude Opus 4.8, on behalf of retoor
Research method	web search synthesis
Confidence	Medium
Status	Active

Summary

std::span is the right type for a non-owning view of bytes, but its operator[] and subspan do not range-check by default; the parser must validate before it slices.

Key findings

subspan with an offset past the end is undefined behavior; it does not throw.
span::operator[] has no runtime check in C++20; at() only arrives in C++26.
For packet parsing, confirm the frame holds header plus the claimed payload length before reading the payload.
GSL and tcb::span check preconditions only when NDEBUG is not defined, so bounds checking is an implementation choice, not a standard guarantee.

Pros / strengths

Clear, correct caveat that prevents a false sense of safety from using span.

Cons / limitations / caveats

Synthesized from several secondary pages; the ModernesCpp claim that span checks bounds conflicts with the others and is the weaker reading.

Relevance to tai

detz's decoder is a checked reader over a byte view: it validates length against remaining bytes before every slice, never relying on span/subspan to catch a bad offset.

Changelog

Date	Change
2026-10-27	Initial dossier created (web search synthesis).

2.0 KiB Raw Blame History