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Fix UB in const_fixed_iterator with negative offsets - #164
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sizeof(value_type) * val converted a negative difference_type to unsigned, so it += -1, it[-1] and std::prev() overflowed the pointer. Cast the size to difference_type before multiplying.
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Sanitizer PR is in #165 |
joto
approved these changes
Oct 15, 2026
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Moving a fixed-size packed iterator backwards by a negative offset calculated the byte offset with unsigned arithmetic. The negative value wrapped around to a huge unsigned number before it was added to the pointer. On all of the platforms we support, we likely still computed the correct address by accident, but strictly speaking, though, it's pointer overflow, which is UB and UBSan flags it. This only affects backwards moves with a negative offset, indexing with a negative index and
std::prev().The offset is now computed in signed arithmetic. I added a unit test for negative offsets, which fails under UBSan without this change. We should probably add a few sanitizer jobs to our CI as well to catch issues like this.