[release/10.0] [mono][sgen] Fix card scanning in LOS non-array objects#125213
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[release/10.0] [mono][sgen] Fix card scanning in LOS non-array objects#125213github-actions[bot] wants to merge 1 commit intorelease/10.0from
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`sgen_card_table_region_begin_scanning` needs to determine whether any cards are marked in the object starting at `start` address. The current card size in 512 bytes. Consider an object with start 500 and size 20, so this object lives within 2 cards. We will check the card associated with the address 500. Once this iteration is done, we will check whether `start + card_size < end` => `500 + 512 < 520`. This is false, so we will no longer scan the second card. The algorithm was expecting the start address to be aligned to the card start, which is what the fix does. This bug is severe, but, in practice, this code path is not as frequent. This bug would only affect objects greater than 8k, that are not arrays.
BrzVlad
approved these changes
Mar 5, 2026
This was referenced Mar 5, 2026
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Backport of #125116 to release/10.0
/cc @BrzVlad
Customer Impact
In some cases, cards marked for non-array LOS objects (objects with size greater than 8K) were being ignored. This means that the GC could fail to observe that some object refs were stored inside these large objects. This can result in GC crashes or other undefined behavior. This can impact all mono workloads. This issue was confirmed to be hit by a customer who also investigated it via a GC debug flag. There are other undiagnosed GC crashes that might hopefully be fixed by this.
Regression
Testing
Was able to create a console app reproducing this bug. Validated the fix.
Risk
Low. The fix just does an address alignment, in order to scan correctly an additional card.