p < q is only defined if they point into the same thing, I think?
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It's only required to work within objects. I also don't think typical uses of pointer authentication will impact anything other than pointers not directly available to C without compiler intrinsics (return addresses) and function pointers. It won't be used for most regular data.
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I believe people are working on making malloc produce authenticated pointers.
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Verifying that pointers passed to free and realloc were created by malloc doesn't seem particularly useful. Maybe I'm misunderstanding how it would be used. I also don't find this to be a particularly compelling feature. ARMv8.5 MTE (memory tagging) is a great feature though.
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I'd have preferred if they'd implemented hardware-based shadow stacks rather than pointer authentication. It would be much better for the primary use case. A very weak probabilistic mitigation competing for the same bits as memory tagging and ASLR just isn't all that compelling.