They can be compatible. Memory tagging does use the TBI bits. Pointer authentication uses an arbitrary number of bits, and the kernel configures the width and whether the TBI bits are preserved. So you can use both, it just costs you 8 bits of signature.
Moreover, this can be configured independently for code and data pointers. iOS turns off TBI on code pointers to get 8 more bits of signature. That's not a problem for memory tagging because memory tagging isn't particularly useful for code pointers anyway.
> Does iOS currently use different signature sizes?
Code and data live in the same address space, and the address-space needs of the system are the main input to the basic signature width, so the basic signatures widths are currently the same, and the only difference is TBI.
You could imagine a system where code was always loaded into a restricted subset of the address space and so code pointers could use wider signatures.
> Can I write an application that uses the top bits of data pointers?
Apple's ABIs actually consider the top 8 bits of data pointers to be outside the addressable range on all its 64-bit targets, including x86_64. ARM64 TBI just means that you don't need to explicitly mask off those bits before doing loads and stores. But there are caveats:
- ARMv8.5 memory tagging uses bits 56-59, so you should probably stick to just the top four bits in case Apple ever uses memory tagging.
- IIRC the first ARM64 iOS release didn't enable TBI, so if your deployment target goes really far back, you do still need to mask.
- The ABI for pointers expects those bits to be clear on normal ABI boundaries. This means you need to mask before handing pointers off to other code; on the upside, however, you don't need to worry about those bits being set when you receive a pointer.
Moreover, this can be configured independently for code and data pointers. iOS turns off TBI on code pointers to get 8 more bits of signature. That's not a problem for memory tagging because memory tagging isn't particularly useful for code pointers anyway.