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moonlock/DECISIONS.md
nevaforget 65ea523b36
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Details
fix: audit fixes — CString zeroize, FP account check, PAM timeout, blur downscale (v0.6.5) 
Address findings from second triple audit (quality, performance, security):

- Wrap PAM CString password in Zeroizing<CString> to wipe on drop (S-H1)
- Add check_account() for pam_acct_mgmt after fingerprint unlock,
  with resume_async() to restart FP on transient failure (S-M1)
- 30s PAM timeout with generation counter to prevent stale result
  interference from parallel auth attempts (S-M3)
- Downscale wallpaper to max 1920px before GPU blur, reducing work
  by ~4x on 4K wallpapers (P-M1)
- exit(1) instead of return on no-monitor after lock.lock() (Q-2.1)
2026-03-30 00:24:43 +02:00

4.8 KiB
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Decisions

Architectural and design decisions for Moonlock, in reverse chronological order.

2026-03-30 – Second audit: zeroize CString, FP account check, PAM timeout, blur downscale

  • Who: Nyx, Dom
  • Why: Second triple audit (quality, performance, security) found: CString password copy not zeroized (HIGH), fingerprint unlock bypassing pam_acct_mgmt (MEDIUM), no PAM timeout leaving user locked out on hanging modules (MEDIUM), GPU blur on full wallpaper resolution (MEDIUM), no-monitor edge case doing return instead of exit(1) (MEDIUM).
  • Tradeoffs: PAM timeout (30s) uses a generation counter to avoid stale result interference — adds complexity but prevents parallel PAM sessions. FP restart after failed account check re-claims the device, adding a D-Bus round-trip, but prevents permanent FP death on transient failures. Blur downscale to 1920px cap trades negligible quality for ~4x less GPU work on 4K wallpapers.
  • How: (1) Zeroizing<CString> wraps password in auth.rs, zeroize/std feature enabled. (2) check_account() calls pam_acct_mgmt after FP match; resume_async() restarts FP on transient failure. (3) auth_generation counter invalidates stale PAM results; 30s timeout re-enables UI. (4) MAX_BLUR_DIMENSION caps blur input at 1920px, sigma scaled proportionally. (5) exit(1) on no-monitor after lock.lock().

2026-03-28 – Remove embedded wallpaper from binary

  • Who: Nyx, Dom
  • Why: Wallpaper is installed by moonarch to /usr/share/moonarch/wallpaper.jpg. Embedding a 374K JPEG in the binary is redundant. GTK background color (Catppuccin Mocha base) is a clean fallback.
  • Tradeoffs: Without moonarch installed AND without config, lockscreen shows plain dark background instead of wallpaper. Acceptable — that's the expected minimal state.
  • How: Remove wallpaper.jpg from GResources, return None from resolve_background_path when no file found, skip background picture creation when no texture available.

2026-03-28 – Audit-driven security and lifecycle fixes (v0.6.0)

  • Who: Nyx, Dom
  • Why: Triple audit (quality, performance, security) revealed a critical D-Bus signal spoofing vector, fingerprint lifecycle bugs, and multi-monitor performance issues.
  • Tradeoffs: cleanup_dbus() extraction adds a method but clarifies the stop/match ownership; running_flag: Rc<Cell<bool>> adds a field but prevents race between async restart and stop; sender validation adds a check per signal but closes the only known auth bypass.
  • How: (1) Validate D-Bus VerifyStatus sender against fprintd's unique bus name. (2) Extract cleanup_dbus() from stop(), call it on verify-match. (3) Rc<Cell<bool>> running flag checked after await in restart_verify_async. (4) Consistent 3s D-Bus timeouts. (5) Panic hook before logging. (6) Blur and avatar caches shared across monitors. (7) Peek icon disabled. (8) Symlink rejection for background_path. (9) TOML parse errors logged.

2026-03-28 – GPU blur via GskBlurNode replaces CPU blur

  • Who: Nyx, Dom
  • Why: CPU-side Gaussian blur (image crate) blocked the GTK main thread for 500ms–2s on 4K wallpapers at cold cache. Disk cache mitigated repeat starts but added ~100 lines of complexity.
  • Tradeoffs: GPU blur quality is slightly different (box-blur approximation vs true Gaussian), acceptable for wallpaper. Removes image and dirs dependencies entirely. No disk cache needed.
  • How: Snapshot::push_blur() + GskRenderer::render_texture() on connect_realize. Blur happens once on the GPU when the widget gets its renderer, producing a concrete gdk::Texture. Zero startup latency.

2026-03-28 – Optional background blur via image crate (superseded)

  • Who: Nyx, Dom
  • Why: Consistent with moonset/moongreet — blurred wallpaper as lockscreen background is a common UX pattern
  • Tradeoffs: Adds image crate dependency (~15 transitive crates); CPU-side Gaussian blur at load time adds startup latency proportional to image size and sigma. Acceptable because blur runs once and the texture is shared across monitors.
  • How: load_background_texture(bg_path, blur_radius) loads texture, optionally applies imageops::blur(), returns gdk::Texture. Config option background_blur: Option<f32> in TOML.

2026-03-28 – Shared wallpaper texture pattern (aligned with moonset/moongreet)

  • Who: Nyx, Dom
  • Why: Previously loaded wallpaper per-window via Picture::for_filename(). Multi-monitor setups decoded the JPEG redundantly. Blur feature requires texture pixel access anyway.
  • Tradeoffs: Slightly more code in main.rs (texture loaded before window creation), but avoids redundant decoding and enables the blur feature.
  • How: load_background_texture() in lockscreen.rs decodes once, create_background_picture() wraps shared gdk::Texture in gtk::Picture. Same pattern as moonset/moongreet.