Fix MSan false positive in fiber stack memory#100138
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alexey-milovidov merged 5 commits intomasterfrom Mar 22, 2026
Merged
Fix MSan false positive in fiber stack memory#100138alexey-milovidov merged 5 commits intomasterfrom
alexey-milovidov merged 5 commits intomasterfrom
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Fiber stacks are heap-allocated via `aligned_alloc`, so MSan considers them entirely uninitialized. Unlike the program's main stack (which the OS zero-initializes), fiber stack bytes that are never explicitly written to remain "uninitialized" in MSan's shadow. This causes false positives when stack slots are reused across function calls within the fiber — MSan traces the origin back to an earlier allocation in a completely unrelated function. The fix calls `__msan_unpoison` on the fiber stack after allocation. This is safe because MSan's per-variable lifetime tracking (via `__lifetime.start` / `__lifetime.end` compiler intrinsics) still properly resets shadow state for each local variable, so real uninitialized value bugs are still detected. https://s3.amazonaws.com/clickhouse-test-reports/json.html?PR=98677&sha=8536dc82c7ed23a6a5ebf073f36235f5be6ea9d7&name_0=PR&name_1=Stress%20test%20%28arm_msan%29 Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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Workflow [PR], commit [296cd06] Summary: ✅ AI ReviewSummaryThis PR fixes an MSan false positive by unpoisoning memory allocated for ClickHouse Rules
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This was referenced Mar 21, 2026
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LLVM Coverage Report
PR changed lines: PR changed-lines coverage: 100.00% (3/3, 0 noise lines excluded) |
azat
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Apr 2, 2026
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| /// Mark the fiber stack memory as initialized for MSan. | ||
| /// Unlike the program's main stack (which the OS zero-initializes), fiber stacks are | ||
| /// heap-allocated via aligned_alloc, so MSan considers them uninitialized. |
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@alexey-milovidov can you explain why? MSan should handle aligned_alloc
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Fiber stacks are heap-allocated via
aligned_alloc, so MSan considers them entirely uninitialized. Unlike the program's main stack (which the OS zero-initializes), fiber stack bytes that are never explicitly written to remain "uninitialized" in MSan's shadow. This causes false positives when stack slots are reused across function calls within the fiber — MSan traces the origin back to an earlier allocation in a completely unrelated function.The fix calls
__msan_unpoisonon the fiber stack after allocation. This is safe because MSan's per-variable lifetime tracking (via__lifetime.start/__lifetime.endcompiler intrinsics) still properly resets shadow state for each local variable, so real uninitialized value bugs are still detected.https://s3.amazonaws.com/clickhouse-test-reports/json.html?PR=98677&sha=8536dc82c7ed23a6a5ebf073f36235f5be6ea9d7&name_0=PR&name_1=Stress%20test%20%28arm_msan%29
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