Re:Always Bump Downwards (2019)
by ridiculous_fish on Tuesday September 19, 2023 @01:42PM
(#37573435)
I think this doesn't work because `aligned + size` may wrap all the way around into the valid region again. For example if aligned == ptr + 1, and size is usize::MAX, we will end up with new_ptr == ptr and the allocation will wrongly succeed.
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The straightforward answer is: don't tolerate ridiculous alignments.
You'd need to check for that, though.
No, just restrict the domain of the alignment input to like, u8. Maybe u16. Either way, it is easy to ensure your bump allocation space is far enough away from SIZE_MAX.
Interesting point. I modified my example to test what you described. I had to play with the compilation flags to get the allocs to not be optimized out and to not panic when the integer overflow happens, but otherwise I didn't change the logic. I'm pretty sure my implementation is correctly handling the case you mention, evidenced by it returning a null pointer.
Link: https://godbolt.org/z/f1jGW6Pa3
Update: NVM, definitely not being handled correctly. https://godbolt.org/z/cMTe1o979