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Fix negative UFFDIO_COPY result handling - #91
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The kernel can return EAGAIN with uffdio_copy.copy set to -EAGAIN when a non-cooperative memory-map change races with UFFDIO_COPY. Casting that value to usize incorrectly reports a huge partial copy. Only construct PartiallyCopied when copy is positive. Return CopyFailed for zero or negative results. Kernel source: https://github.com/torvalds/linux/blob/95567729173e62e0e60a1f8ad9eb2e1320a8ccac/fs/userfaultfd.c#L1587-L1591 Signed-off-by: Dylan Reid <dgreid@fb.com>
hikalium
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Oct 17, 2026
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LGTM. Thanks for catching this!
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The kernel can return EAGAIN with uffdio_copy.copy set to -EAGAIN when a non-cooperative memory-map change races with UFFDIO_COPY. Casting that value to usize incorrectly reports a huge partial copy.
Only construct PartiallyCopied when copy is positive. Return CopyFailed for zero or negative results.
Kernel source:
https://github.com/torvalds/linux/blob/95567729173e62e0e60a1f8ad9eb2e1320a8ccac/fs/userfaultfd.c#L1587-L1591