RFR: 8351081: Off-by-one error in ShenandoahCardCluster

[duke]

On Tue, 4 Mar 2025 04:06:00 GMT, Cesar Soares Lucas <cslucas at openjdk.org> wrote:

> Given certain values for the variables in [this expression](https://github.com/openjdk/jdk/blob/a87dd1a75f78cf872df49bea83ba48af8acfa2fd/src/hotspot/share/gc/shenandoah/shenandoahScanRemembered.cpp#L173) the result of the computation can be equal to `_ rs->total_cards()` which will lead to segmentation fault, for instance in [starts_object(card_at_end)](https://github.com/openjdk/jdk/blob/a87dd1a75f78cf872df49bea83ba48af8acfa2fd/src/hotspot/share/gc/shenandoah/shenandoahScanRemembered.hpp#L393). The problem happens, though, because the `_object_starts` array doesn't have a [guarding entry](https://github.com/openjdk/jdk/blob/a87dd1a75f78cf872df49bea83ba48af8acfa2fd/src/hotspot/share/gc/shenandoah/shenandoahCardTable.cpp#L37) at the end. This pull request adjusts the allocation of `_object_starts` to include an additional entry at the end to account for this situation.
> 
> Tested with JTREG tier 1-4, x86_64 & AArch64 on Linux.

@JohnTortugo 
Your change (at version 9a4ac53343aaa62b055241f90bd6d610a483ed66) is now ready to be sponsored by a Committer.

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PR Comment: https://git.openjdk.org/jdk/pull/23882#issuecomment-2698667853