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slice_count overflow for very large allocations #876

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slice_count overflow for very large allocations#876
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@colesbury

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@colesbury
colesbury
opened on Apr 12, 2024
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This was seen in python/cpython#117755 on s390x (Linux on IBMz) for an allocation of 0x7fffffffffffffff bytes:

mimalloc/src/segment.c

Lines 617 to 618 in f199b88

slice->slice_count = (uint32_t)slice_count;
mi_assert_internal(slice->slice_count == slice_count); // no overflow?

The mi_segment_os_alloc is successful (possibly due to overcommit), but then the slice_count is truncated, which leads to either an assertion error (in debug builds) or floating point error due to a divide by zero.

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