67 lines
2.4 KiB
Markdown
67 lines
2.4 KiB
Markdown
# UNDF: UNDF-2026-000000103
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# hbase-0002: BaseLoadBalancer.randomAssignment — usedSNs ArrayList.contains() O(n²) in assignment loop
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## Severity
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HIGH — called for every region assignment and on every random assignment fallback during region
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open; O(S²) where S = number of servers; materializes on large clusters during rolling restart
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or mass region reassignment.
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## File
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`hbase-balancer/src/main/java/org/apache/hadoop/hbase/master/balancer/BaseLoadBalancer.java`
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## Lines
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465 (`usedSNs` declaration), 470 and 478 (`usedSNs.contains()` inside do-while and for loops)
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## Pattern
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CWE-407: O(n) ArrayList.contains() inside two nested loops.
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```java
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// DEFECTIVE (line 465)
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List<ServerName> usedSNs = new ArrayList<>(servers.size());
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// DEFECTIVE (line 467-473) — do-while loop, up to numServers * 4 iterations
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do {
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int i = rand.nextInt(numServers);
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sn = servers.get(i);
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if (!usedSNs.contains(sn)) { // O(usedSNs.size()) per iteration
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usedSNs.add(sn);
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}
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} while (cluster.wouldLowerAvailability(regionInfo, sn) && iterations++ < maxIterations);
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// DEFECTIVE (line 477-486) — fallback for loop over all servers
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if (iterations >= maxIterations) {
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for (ServerName unusedServer : servers) {
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if (!usedSNs.contains(unusedServer)) { // O(usedSNs.size()) per server
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```
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`usedSNs` is an `ArrayList`. In the do-while loop, `contains` is called up to `numServers * 4`
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times, each O(usedSNs.size()). In the fallback for-loop, `contains` is called for each server —
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O(S) iterations, each O(S) = O(S²) total.
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For a cluster with S=500 servers (typical large HBase), the fallback path costs O(250,000)
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operations rather than O(500).
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## Fix
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Replace `ArrayList` with `LinkedHashSet`:
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```java
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// FIXED
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Set<ServerName> usedSNs = new LinkedHashSet<>(servers.size());
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```
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`contains()` and `add()` both become O(1). No behavior change — the `usedSNs` collection is only
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tested for membership, never indexed.
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## Complexity
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- Before: O(S²) worst-case per assignment (fallback path) + O(S) per do-while amortized
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- After: O(S) worst-case per assignment
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## Impact
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`randomAssignment()` is called:
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1. During initial bulk assignment on cluster startup
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2. For each region that needs reassignment when no preferred server is available
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3. During rolling restart — every region gets reassigned
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On a 500-server cluster with 200k regions, the fallback path triggered by `wouldLowerAvailability`
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can make the balancer loop take minutes instead of seconds.
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