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STATUS_INVALID_LDT_SIZE
LDT size changes are constrained by descriptor-table geometry
STATUS_INVALID_LDT_SIZE is more specific than a general bad-buffer failure. The NTSTATUS definition identifies two invalid conditions: an attempt to grow an LDT through this size-setting operation, or a size that is not an even number of selectors. The size is therefore interpreted in terms of descriptor entries, not as an arbitrary byte allocation.
An x86 selector references a descriptor, and Microsoft documents the descriptor representation through LDT_ENTRY. Selector-inspection APIs return that descriptor data for a thread. In compatibility code, keeping byte counts, selector counts, and descriptor sizes distinct is essential: a value copied from a byte-oriented API into a selector-count calculation can be aligned as an integer yet still be invalid for the LDT contract.
Capture the old size, requested size, selector count, and caller operation before any rounding. Verify the unit used by every intermediate field and whether the code path is trying to grow rather than shrink or otherwise modify the existing table. Do not “fix” the request by rounding the size down, because that can silently discard descriptor entries and invalidate selectors still referenced by 16-bit code. Correct the source calculation and preserve the compatibility subsystem state that owns the LDT.
What to inspect
- Log both byte size and derived selector count; never assume they use the same unit.
- Compare requested and existing LDT sizes to detect an unsupported growth attempt.
- Trace selector users before reducing a table size; arbitrary truncation can invalidate live segmented references.
References
- Microsoft: LDT_ENTRY
- Microsoft: GetThreadSelectorEntry
- Microsoft: NTVDM and 16-bit app support
- Microsoft: Wow64GetThreadSelectorEntry
- Microsoft Open Specifications: NTSTATUS values
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