> Others don't as it breaks things like mmap of prebuilt hashtables.

Can you elaborate? An mmap of prebuilt hash tables doesn’t work well in practice of the mmapped area contains pointers regardless of provenance, and an mmap of a hashtable that uses integer offsets doesn’t involve pointers.

The only real issue I see is if the mmap contains objects but isn’t itself laid out like an object in the language in question, and you need to generate a pointer to one of those objects. (So mmapping an array of structs that don’t contain pointers is fine, but mmapping a mess that contains integer offsets referencing various things in the mmap that don’t nicely line up like an array is harder.)

But I imagine that a pointer provenance system could have an operation that takes as input an mmap, an offset and a type and returns a pointer to the object with the type in question at the offset in question. It would check that the type makes sense (no pointers!) and could, if needed for the degree of safety require, also check for invalid aliasing.