Ensure from_box works if boxed.addr() == 1#52
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SmallBox::from_box will reuse the allocation from its argument as ptr in smallbox. However, said allocation might be equal to 1, which is the INLINE_SENTINAL value. This would cause the resulting smallbox to read from its uninitialized inline storage space when dereferenced. Note that this problem likely cannot be triggered on a desktop OS since the first page is typically set up to segfault when read. Thus if the type is not a ZST, then the allocator won't have given out a pointer in the first page, and if the type is a ZST, then reading from the uninitialized storge space is perfectly fine behavior. However, bare metal programs could concivibly run into this problem, so this commit causes from_box to copy from boxed into a newly created smallbox if boxed.addr() == INLINE_SENTINEL.
Owner
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I rather like to revert #45 considering this case. |
Author
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That seems reasonable. I'll close this request. |
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SmallBox::from_box will reuse the allocation from its argument as ptr in smallbox. However, said allocation might have an address of 1, which is the INLINE_SENTINAL value.
This would cause the resulting smallbox to read from its uninitialized inline storage space when de-referenced.
Note that this problem likely cannot be triggered on a desktop OS since the first page is typically set up to segfault when read. Thus if the type is not a ZST, then the allocator won't have given out a pointer in the first page, and if the type is a ZST, then reading from the uninitialized storage space is perfectly fine behavior.
However, bare metal programs could conceivably run into this problem, so this commit causes from_box to copy from boxed into a newly created smallbox if boxed.addr() == INLINE_SENTINEL.