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[flang] Fix heap overflow in Real formatting.

Authored by Meinersbur on Oct 4 2020, 4:15 AM.



The reinterpret_cast is problematic because 1. it violates the strict aliasing assumption and 2. the types can be of different size. In particular, BinaryFloatingPointNumber is always a power-of-2 whereas the source type can be smaller (like for 80-bit real). Therefore, when dereferencing the BinaryFloatingPointNumber, the accesses exceeds the source type's allocation size.

Fix by using memcpy to a zero-initialized buffer of the larger size.

Found using AddressSanitizer.

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Meinersbur created this revision.Oct 4 2020, 4:15 AM
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Meinersbur requested review of this revision.Oct 4 2020, 4:15 AM

This works, but maybe the memcpy should be in a constructor for BinaryFloatingPointNumber -- it already has a memcpy in its default constructor, after an assertion that the size is not smaller (which would be removed).

Copy integer part-wise instead of memcpy. memcpy/reinterpret_cast assumed platform endianess.

I think a better fix would be to generalize ToUInt64 as a template elsewhere. I'll prepare a patch and add you as the reviewer.

Thanks, looking forward to you patch.

Meinersbur abandoned this revision.Oct 19 2020, 8:40 AM

Superseded by D89435