Fr0zka
eaa44bf0c0
Second defect from the full 28-op EffectOverBox audit. Three ops roll a radius as Lerp(MinRadius, MaxRadius, hash01), which lands in [min(A,B), max(A,B)] -- Lerp does not require A <= B. Their EffectOverBox sweeps lattice cells padded by the largest radius a cell could hold, so a pad below the true maximum means the cells are never examined and the op reports Identity for a box its own Eval will fill or carve: no geometry, no collision. FGridColumnMod ~1086 Max(MaxRadius, 0) exposed FShaftFieldSource ~1436 Max(ShaftMaxRadius, ConnectorRadius) exposed FIslandBlobSource ~1803 Max(IslandMinRadius, IslandMaxRadius) already correct The third one is the argument: the concern was met and guarded once in this same file, and the other two shipped without it. Fixed with FMath::Max3, a spelling already used here (~2481) and in VoxelCaveMorphology.cpp. Shipped defaults are ordered correctly (2/5, 2/7, 5/11), so this is a NO-OP at defaults -- that is its acceptance signal, and any moved number means the diff did more than intended. It needs a mis-ordered asset value, which nothing prevents: ClampMin is a per-property floor and UE cannot express "<= that other property". ColumnMinRadius is also settable per-room via UVoxelTerrainOpDefinition. Deliberately NOT fixed by normalising the params: swapping the Lerp endpoints maps the same hash to a different radius, changing geometry and breaking the eight equivalence tests. Only the bound becomes conservative; Eval is byte- identical -- verified, no Eval/Roll/GetCells line in the diff. Also audited and found sound (recorded so they are not re-chased): FPassageCarveOp and the passage bounding sphere, FOriginSpineOp, FBoundarySealOp, FShaftLedgeMod, FCaveArchMod, FDomeMod. Not built -- Jahni builds. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
102 lines
5.2 KiB
Markdown
102 lines
5.2 KiB
Markdown
# Codex task 004 — two box verdicts assume `MinRadius ≤ MaxRadius`; a third one already doesn't
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**Owner:** Codex (Model Luna, xHigh) · **Orchestrator:** Claude · **Branch:** `experimental`
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**Status:** specified, not started
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**Kind:** ⚠️ **correctness of a box verdict.** Same class as task 003. Small fix, closes a class.
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---
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## Why this exists
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Three operators roll a primitive radius from a hash between two designer-set params:
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```cpp
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Out.R = FMath::Lerp(MinRadius, MaxRadius, hash01); // FGridColumnMod ~1129
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Out.R = FMath::Lerp(P.ShaftMinRadius, P.ShaftMaxRadius, hash01); // FShaftFieldSource ~1560
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Out.Rxy = FMath::Lerp(P.IslandMinRadius, P.IslandMaxRadius, hash01); // FIslandBlobSource ~1850
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```
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`FMath::Lerp(A, B, t)` with `t ∈ [0,1]` lands anywhere in `[min(A,B), max(A,B)]` — it does **not**
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require `A ≤ B`.
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Each op's `EffectOverBox` then sweeps a **range of lattice cells** around the query box, padded by
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the largest radius a cell could hold, and tests each rolled primitive exactly. The pad decides which
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cells are *looked at at all*, so a pad smaller than the true maximum radius means **cells are never
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examined**, their primitives are never tested, and the op reports `Identity` for a box that its own
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`Eval` will carve or fill.
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| op | pad used for the cell sweep | correct? |
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|---|---|---|
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| `FGridColumnMod` ~1086 | `FMath::Max(MaxRadius, 0.0f) + ColBlend` | ⛔ **exposed** |
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| `FShaftFieldSource` ~1436 | `FMath::Max(P.ShaftMaxRadius, P.ConnectorRadius) + ExtraReach` | ⛔ **exposed** |
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| `FIslandBlobSource` ~1803 | `const float MaxR = FMath::Max(P.IslandMinRadius, P.IslandMaxRadius);` | ✅ **already correct** |
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**The third one is the point.** Someone hit this exact concern while writing the island source and
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guarded it. The other two shipped without the guard. This task makes the three consistent.
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## How exploitable, stated honestly
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The shipped defaults are correctly ordered (`2/5`, `2/7`, `5/11`), so **nothing is broken out of the
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box.** It needs a mis-ordered asset value — `ColumnMinRadius = 8, ColumnMaxRadius = 4`.
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Nothing prevents that. The `UPROPERTY` metas carry `ClampMin = "1.0"`, which is a per-property
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floor; Unreal has no declarative way to say "must be ≤ that other property". And `ColumnMinRadius`
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is *also* settable per-room through `UVoxelTerrainOpDefinition`, so it is not only the strate asset.
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What makes it worth the five lines: when it does happen, the failure is **invisible and maddening**.
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`Eval` still draws the fat column perfectly, so every tile that gets meshed looks correct; only the
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tiles the classifier *skipped* are missing — no geometry, no collision, in a world that otherwise
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looks right.
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## The fix — five lines, and one thing you must NOT do
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Make each pad use the true envelope:
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```cpp
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// FGridColumnMod ~1086
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const float Reach = FMath::Max3(MinRadius, MaxRadius, 0.0f) + ColBlend;
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// FShaftFieldSource ~1436
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const float Pad = FMath::Max3(P.ShaftMinRadius, P.ShaftMaxRadius, P.ConnectorRadius) + ExtraReach;
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```
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(Use whatever spelling is idiomatic here — check that `FMath::Max3` is already used in this codebase
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before reaching for it; nested `FMath::Max` is fine and matches `FIslandBlobSource`'s existing line.)
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### ⛔ DO NOT "fix it properly" by normalising the params
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The tempting larger fix — swap `Min`/`Max` at resolution time so `Min ≤ Max` always — is **wrong and
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will break the build's tests.** `Eval` computes `Lerp(Min, Max, t)`; swapping the endpoints maps the
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same hash `t` to a *different* radius for the same cell. That changes generated geometry and breaks
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the eight bit-for-bit equivalence tests against the `switch` path.
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**Only the BOUND may become conservative. `Eval` stays byte-identical.** This is the same rule as
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task 003 and the same reason.
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## ⚠️ Invariants
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1. **No `Eval`, `RollColumn`, `RollShaft`, `GetCells`, or `GetCellsAt` body may change.** If your
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diff touches one, you have the wrong site — stop and say so.
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2. **Do not touch `FIslandBlobSource`.** It is already correct and is the reference for this fix.
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3. The change direction is strictly conservative: a wider sweep examines *more* cells, so a verdict
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can only move from `Identity` toward `CarveOnly`/`FillOnly`, never the reverse. Do not add any
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compensating tightening.
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4. Comments are French + English; match the surrounding file. Say **why** the envelope is
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`max(Min, Max)` and not `Max` — the next reader must not "simplify" it back.
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## Acceptance
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- `git diff --stat` shows exactly one file: `Source/VoxelForge/Private/VoxelDensityOpStack.cpp`,
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and a handful of lines.
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- The three ops now agree on the pattern.
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- After the build: **every box-verdict line and every equivalence test is unchanged**, because the
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shipped defaults are correctly ordered and the envelope only differs when they are not. **A change
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in any of those numbers means the diff did something it should not have.** That is this task's
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whole acceptance signal — a *no-op at defaults* is the expected, correct result.
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## Notes for the reviewer (Claude)
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- Confirm both pads changed and `FIslandBlobSource` did not.
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- Confirm no `Lerp` argument order was touched anywhere — that is the failure mode that would look
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like a tidy-up and silently change the world.
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