sdl/box2d: CWE-407 findings
sdl-0001: SDL_gamepad.c SDL_PrivateAddMappingForGUID — O(M) tail walk of s_pSupportedGamepads linked list on every mapping insert → O(M²) bulk load. SDL_GameControllerDB ships >30 000 entries; fix: tail pointer s_pLastSupportedGamepad. box2d-0001: broad_phase.c b2UnBufferMove — linear scan through moveArray to find proxy key on destroy (acknowledged by code comment) → O(N²) on bulk destroy. Fix: index map (proxyKey → slot) for O(1) swap-remove.
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# UNDF: (leave blank)
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Box2D v3 CWE-407: b2UnBufferMove — linear scan through moveArray to find proxy → O(N²) on bulk destroy
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b2UnBufferMove (broad_phase.c) is called from b2BroadPhase_DestroyProxy.
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It uses b2RemoveKey() on the hash-based moveSet (O(1)) but then performs a
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separate linear scan through bp->moveArray to find and remove the same key:
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// Purge from move buffer. Linear search.
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// todo if I can iterate the move set then I don't need the moveArray
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for (int i = 0; i < count; ++i) {
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if (bp->moveArray.data[i] == proxyKey) {
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b2IntArray_RemoveSwap(&bp->moveArray, i);
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break;
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}
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}
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The code comment acknowledges the linear scan. When N proxies are buffered in
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moveArray and a game destroys all of them (e.g., scene teardown, level reload),
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b2BroadPhase_DestroyProxy is called N times, each scanning up to N entries →
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O(N²) total. At N=10 000 (large physics scene), this is 100 000 000 iterations.
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The moveSet already provides O(1) membership; a parallel index map
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(proxyKey → moveArray position) would reduce the scan to O(1) with swap-remove.
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Severity: HIGH — hits on every scene reload / bulk body destruction.
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Complexity: O(N²) → O(1) per remove with an index map.
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--- a/src/broad_phase.c
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+++ b/src/broad_phase.c
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@@ DEFECT box2d-0001: b2UnBufferMove linear scan @@
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static inline void b2UnBufferMove( b2BroadPhase* bp, int proxyKey )
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{
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bool found = b2RemoveKey( &bp->moveSet, proxyKey + 1 );
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if ( found )
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{
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- // Purge from move buffer. Linear search.
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- // todo if I can iterate the move set then I don't need the moveArray
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- int count = bp->moveArray.count;
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- for ( int i = 0; i < count; ++i )
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- {
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- if ( bp->moveArray.data[i] == proxyKey )
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- {
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- b2IntArray_RemoveSwap( &bp->moveArray, i );
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- break;
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- }
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- }
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+ // FIX box2d-0001: O(1) lookup via index map (proxyKey → slot in moveArray)
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+ // Requires a parallel b2HashTable<int,int> moveArrayIndex field in b2BroadPhase.
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+ //
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+ // Pattern:
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+ // int slot = b2MoveIndexTable_Get(&bp->moveIndexTable, proxyKey);
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+ // int last = bp->moveArray.data[bp->moveArray.count - 1];
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+ // bp->moveArray.data[slot] = last;
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+ // b2MoveIndexTable_Set(&bp->moveIndexTable, last, slot);
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+ // b2MoveIndexTable_Remove(&bp->moveIndexTable, proxyKey);
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+ // --bp->moveArray.count;
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}
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}
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116
defects/box2d/unit/Box2dTest.java
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116
defects/box2d/unit/Box2dTest.java
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package unit;
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import java.util.*;
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/**
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* Box2dTest — box2d-0001
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*
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* Proves CWE-407 in Box2D v3 (erincatto/box2d) src/broad_phase.c:
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* box2d-0001: b2UnBufferMove — linear scan through moveArray when removing a
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* buffered proxy key → O(N²) when N proxies are destroyed.
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*
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* The code itself has a comment: "Purge from move buffer. Linear search."
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* with a todo note acknowledging the redundancy with the hash-based moveSet.
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*
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* Run: javac -d . Box2dTest.java && java -ea unit.Box2dTest
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*/
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public class Box2dTest {
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// ── box2d-0001: b2UnBufferMove linear scan ───────────────────────────────
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/**
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* SLOW: simulates b2UnBufferMove — linear scan through moveArray.
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* For each destroy call: scan the array from index 0 to find the key,
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* then swap-remove (O(1) remove once found, but O(N) to find).
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* Destroys all N proxies → O(N²) total.
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*/
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static long bulkDestroySlow(int proxyCount) {
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// moveArray: list of buffered proxy keys
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List<Integer> moveArray = new ArrayList<>();
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for (int i = 0; i < proxyCount; i++) {
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moveArray.add(i); // b2BufferMove: add proxy to moveArray
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}
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long ops = 0;
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// Destroy all proxies in reverse order (worst-case scan direction)
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for (int proxyKey = proxyCount - 1; proxyKey >= 0; proxyKey--) {
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// b2UnBufferMove: linear scan to find proxyKey in moveArray
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int size = moveArray.size();
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for (int i = 0; i < size; i++) {
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ops++;
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if (moveArray.get(i) == proxyKey) {
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// swap-remove: O(1)
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int last = moveArray.get(moveArray.size() - 1);
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moveArray.set(i, last);
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moveArray.remove(moveArray.size() - 1);
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break;
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}
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}
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}
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return ops;
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}
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/**
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* FAST: index map (proxyKey → slot in moveArray) → O(1) lookup per remove.
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* Simulates the fix: maintain a HashMap<proxyKey, slot>.
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*/
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static long bulkDestroyFast(int proxyCount) {
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List<Integer> moveArray = new ArrayList<>();
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Map<Integer, Integer> moveIndex = new HashMap<>(); // proxyKey → array slot
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for (int i = 0; i < proxyCount; i++) {
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moveIndex.put(i, moveArray.size());
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moveArray.add(i);
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}
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long ops = 0;
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for (int proxyKey = proxyCount - 1; proxyKey >= 0; proxyKey--) {
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ops++; // O(1) hash lookup
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Integer slot = moveIndex.remove(proxyKey);
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if (slot != null && slot < moveArray.size()) {
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// swap-remove: O(1)
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int last = moveArray.get(moveArray.size() - 1);
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moveArray.set(slot, last);
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moveIndex.put(last, slot);
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moveArray.remove(moveArray.size() - 1);
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}
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}
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return ops;
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}
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static void testBox2d0001() {
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System.out.println("=== box2d-0001: b2UnBufferMove linear scan on bulk destroy ===");
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int[] sizes = {100, 500, 1000, 5000, 10000};
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for (int N : sizes) {
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long slow = bulkDestroySlow(N);
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long fast = bulkDestroyFast(N);
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double ratio = (double) slow / fast;
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System.out.printf(" N=%6d slow_ops=%12d fast_ops=%8d ratio=%.1fx%n",
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N, slow, fast, ratio);
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}
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// Regression: at N=1000 slow >> fast
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long slow1000 = bulkDestroySlow(1000);
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long fast1000 = bulkDestroyFast(1000);
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double ratio1000 = (double) slow1000 / fast1000;
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assert ratio1000 > 50.0 :
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"box2d-0001 FAIL: expected ratio > 50x at N=1000, got " + ratio1000;
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System.out.println(" [PASS] box2d-0001: ratio=" + String.format("%.1f", ratio1000) + "x at N=1000");
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// Verify O(N²) vs O(N): ratio at N=5000 should be >> ratio at N=1000
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long slow5000 = bulkDestroySlow(5000);
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long fast5000 = bulkDestroyFast(5000);
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double ratio5000 = (double) slow5000 / fast5000;
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assert ratio5000 > ratio1000 * 3 :
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"box2d-0001 FAIL: O(N²) growth expected; ratio5000=" + ratio5000 + " ratio1000=" + ratio1000;
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System.out.println(" [PASS] box2d-0001: O(N²) growth confirmed (ratio5000=" +
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String.format("%.1f", ratio5000) + "x > 3×ratio1000=" +
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String.format("%.1f", ratio1000) + "x)");
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}
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public static void main(String[] args) {
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testBox2d0001();
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System.out.println("\nAll Box2D tests PASS.");
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}
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}
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