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Comment by quotemstr | Hacker News Reader

You can't do that in C++ or any language. You need to do your own relocations and remember enough information to do them. You can't count on any particular virtual address being available on a modern system, not if you want to take advantage of ASLR.

The trouble is that we have to mark relocated pages dirty because the kernel isn't smart enough to understand that it can demand fault and relocate on its own. Well, either that, or do the relocation anew on each access.

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It works with C++ if you use boost::interprocess. Its data structures use offset_ptr internally rather than assuming every pointer is on the heap.

Sure. But that counts as "doing your own relocations". Unsafe Rust could do the same, yes?

What is being relocated?

If you use offsets instead of pointers you're doing relocations "on the fly"

I don't know enough about Rust to say. If it doesn't have the concept of a 'fancy pointer' then I assume no, you'd have to essentially reproduce what boost::interprocess does.

I'm still learning Rust, but iiuc you could do this by creating an OffsetPtr type that implements the Deref trait (https://doc.rust-lang.org/std/ops/trait.Deref.html). This is exactly a "fancy pointer" as you describe.

That introduces different data-types, rather than using the existing ones (instantiated with different pointer-types).

Indeed. I don't know if there's a plan for the standard type to move to offset-ptr, or if there's even a std::offset_ptr, but it would be great if there was.

For us, some of the 'different data type' pain was alleviated with transparent comparators. YMMV.

Edit: It seems C++11 has added some form of support for it... 'fancy pointers'

https://en.cppreference.com/w/cpp/named_req/Allocator#Fancy_...

I don't see what the issue in doing this is in C++.

The only thing that'll break will be the pointers and references to things outside of the mmap'd area.

By that logic you can do it in unsafe Rust as well then. Obviously in safe Rust having potentially dangling "pointers and references to things outside of the mmap'd area" is a big no-no.

And note that even intra-area pointers would have to be offset if the base address changes. Unless you go through the trouble of only storing relative offsets to begin with, but the performance overhead might be significant.

libsigsegv (slow) or userfaultfd (less slow) can be used for this purpose.

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