[release/11.0] JIT: guard overflow in loop cloning offset limits - #134350
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Positive array length offsets can wrap negative and let the fast clone run without bounds checks. Guard offsets that can overflow and add regression coverage for decreasing loops. Fixes #133823 > [!NOTE] > This pull request description was generated with GitHub Copilot. --------- Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 336fb408-8580-4f7b-b3e6-087280a6294a
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Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch |
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@dotnet/jit-contrib ptal |
JulieLeeMSFT
approved these changes
Sep 21, 2026
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Backport of #133834 to release/11.0
/cc @AndyAyersMS
Customer Impact
Loop cloning can incorrectly remove array bounds checks when a positive
arr.Length + offsetloop limit overflows to a negative value. Safe C# can then perform an out-of-bounds read and terminate with a native access violation instead of throwingIndexOutOfRangeException. This reproduces on .NET 11 RC1 and RC2; .NET 10 is correct. See #133823.Regression
Introduced during the .NET 11 cycle by #129309, which extended loop cloning to handle span/stride and constant-offset limits.
Testing
Added regression coverage for decreasing loops with overflowing positive offsets. The test fails before the fix and verifies that the required bounds exception is preserved after the fix. The original PR also passed JIT CI; Mono exclusions were added because this is CoreCLR-specific loop-cloning coverage.
Risk
Low. The change is localized to loop-cloning condition derivation. It rejects offsets that cannot be represented safely instead of selecting the unchecked fast clone, so the fallback retains the existing bounds checks.