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Add core::ptr::assume_moved #3700
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Is there a reason we can not just say changing the upper bits has no impact on a pointer if an appropriate tagging scheme is available, without need for additional methods? |
Yes, the reason is that even if the hardware understands a particular tagging scheme, the memory model in Rust and LLVM does not. Setting a tag on a pointer, even though it has no impact on the hardware side, makes the memory model think the pointer has now been offset outside of its original allocation and thus any access to it is Undefined Behaviour. To be able to do this we need a helper method that simulates a "realloc" from the untagged address to the tagged address to make the memory model happy.
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Cc @rust-lang/opsem |
these should probably be associated functions, not methods. also, this seems to ignore another type of pointer tagging, often used by interpreters, where the bottom bits (otherwise always zero because of alignment) are used to tag the type of the object. |
This question should indeed be answered in the RFC text, not just in the discussion thread. (This RFC could have benefited from a pre-RFC phase, posting it on the forum to get some feedback to ensure that it has all the expected details.) |
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Thanks for posting the RFC! However, I think it wasn't quite ready yet. Here's some first feedback. This RFC is still lacking most of the relevant details, so I didn't proceed beyond the reference-level section.
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# Summary | ||
[summary]: #summary | ||
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Add helper methods on primitive pointer types to facilitate getting and setting the tag of a pointer. |
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The term "tag of a pointer" means a lot of different things, and often it means something different from how you are using the term here. So the RFC should clarify the terminology it uses. This is, AFAIK, not meant to be a general pointer tagging mechanism. No RFC is even needed for that. It is specific for working with hardware that ignores certain bits of a pointer.
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The thinking is that hardware that ignores certain bits of a pointer is only relevant if you're setting those bits to some values, i.e. in the context of pointer tagging. Thus, the way to add support for such hardware is through a mechanism for pointer tagging. Does that reasoning make sense?
This is still a bit of an open question, there are several directions we could go with this. The tricky part is that different architectures have support for something similar, but the details (which exact bits are ignored) may vary.
On that account I'm not sure whether it'd be better to try and make a general high-bit pointer tagging mechanism that could be used across architectures, or whether it'd be better to just upstream e.g. aarch64-specific functions into core_arch and maybe then think about a generic one that just calls into those depending on the platform.
I don't really have very strong opinions here.
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I'm just pointing out the RFC is written in a confusing way. I would suggest to make it very targeted specifically for "supporting hardware that ignores some bits in a pointer", rather than general pointer tagging. This should become clear already in the summary and the first paragraph of the motivation.
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the lower 48 bits, leaving higher bits unused. The remaining bits are for the most part used to | ||
distinguish userspace pointers (0x00) from kernelspace pointers (0xff). | ||
Certain architectures provide extensions, such as TBI on AArch64, that allow programs to make use of | ||
those unused bits to insert custom metadata into the pointer. |
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To be clear, userspace can already do that without hardware support, trivially. By masking out those bots for each load. The hardware feature just makes this slightly more efficient.
I find the introduction to be a bit confusing due to this.
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Currently, Rust does not acknowledge TBI and related architecture extensions that enable the use of | ||
tagged pointers. This could potentially cause issues in cases such as working with TBI-enabled C/C++ | ||
components over FFI, or when writing a tagging memory allocator. |
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Currently, Rust does not acknowledge TBI and related architecture extensions that enable the use of | |
tagged pointers. This could potentially cause issues in cases such as working with TBI-enabled C/C++ | |
components over FFI, or when writing a tagging memory allocator. | |
Currently, Rust does not support directly using TBI and related architecture extensions that simplify the use of | |
tagged pointers. This could potentially cause issues in cases such as working with TBI-enabled C/C++ | |
components over FFI, or when writing a tagging memory allocator. |
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Certain architectures provide extensions, such as TBI on AArch64, that allow programs to make use of | ||
those unused bits to insert custom metadata into the pointer. | ||
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Currently, Rust does not acknowledge TBI and related architecture extensions that enable the use of |
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The RFC should explain what the problem with the use of these extensions in Rust is.
Also, it is very odd to see interop with C/C++ as the motivation here, given that those languages do not support TBI either (which is another relevant fact the RFC should mention).
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Well they don't support it "fully" on a language level, as in you can't just tag arbitrary pointers with no issues, but they sort of do in that there are currently C/C++ components running in production that operate on TBI-enabled pointers.
I want to make it possible to, say, write a custom allocator which internally just calls malloc, puts some tag in the pointer it got from malloc and then returns that pointer to the user.
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I don't doubt there's C/C++ code in production that causes UB and works anyway, but since we are going for a UB-free approach here as we always do, it is not a fair comparison to claim that C/C++ already supports this in a way Rust wouldn't.
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``` | ||
assert!(ptr.tag() == 0); | ||
let tagged_ptr = unsafe { ptr.with_tag(63) }; | ||
assert!(tagged_ptr.tag() == 63); |
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The guide-level explanation should give a guide for how to think about these functions and what they do.
This is where you have to explain that tag
is basically realloc
. Currently, realloc
suddenly appears in "drawbacks", which nobody reading the RFC will understand.
Remember that an RFC is supposed to be a self-contained document such that everyone who is reasonably well-versed in Rust but knows nothing about the particular problem domain can understand what problem the RFC is trying to solve, and how it is trying to solve it. Your RFC is missing a lot of background and explanation to make it satisfy this requirement.
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[reference-level-explanation]: #reference-level-explanation | ||
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Within Rust's memory model, modifying the high bits offsets the pointer outside of the bounds of | ||
its original allocation, making any use of it Undefined Behaviour. |
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its original allocation, making any use of it Undefined Behaviour. | |
its original allocation, making any load/store with that pointer Undefined Behaviour. |
"any use" is not correct, e.g. wrapping_offset
or ==
on such a pointer are completely fine.
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caller. | ||
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# Reference-level explanation | ||
[reference-level-explanation]: #reference-level-explanation |
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This section should contain the full signature of the newly proposed methods, and their doc comments (see the existing pointer methods for how our doc comments look like), so that we have an exact description of what the proposal even is. You can't just propose two function names and leave the details for later, when the entire reason that an RFC is even needed is that those details are far from simple.
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Yes for sure, the reason they're not there yet is that I wanted to ask for outside opinions before settling in on what those signatures should be and what the exact methods should actually look like. My bad that it didn't come across as intended.
The concrete part of the proposal is that we should have a function/method to set the high-bit tag of a pointer & one to retrieve it, the other details such as the name, where it should live or what it should entail are still in flux.
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[drawbacks]: #drawbacks | ||
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Because the memory model we currently have is not fully compatible with memory tagging and | ||
tagged pointers, setting the high bits of a pointer must be done with great care in order to |
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That's not correct. Pointer tagging works perfectly fine and is a commonly used technique in Rust. The memory model is just not compatible with loading from a tagged pointer without first removing the tag.
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That's why I wrote "not fully compatible" - you can make it work, just not as smoothly as one would like given what the hardware can support.
Besides, if you remove the tag, would that not make the result a different pointer? They won't compare equal after all.
Either way, I can clarify for sure if it's not self evident.
Why can't this be done by the backend? ie. I write my code as |
Is simulating a Consider the following code: void *original = /*...*/;
void *copy = original;
void *tagged = realloc(original, ...); Now, according to the semantics of However, if my understanding of TBI is correct, this is not what happens here. Specifically, Am I misunderstanding TBI or |
you have UB if you try to do any accesses through |
@matthieu-m this model definitely makes some code UB that would be correct when using TBI in an assembly program. However, we have to impose some restrictions to make TBI compatible with higher-level language models such as Rust (and the same goes for C and C++). |
The discrepancy caused by LLVM (and Rust) not understanding the concept of TBI is fairly unfortunate. I think it should be noted in Future Possibilities that the choice of using a That is, while overly restrictive today, the drawback of the selected model is not painting us into a corner as far as I can see. |
Why is this better than explicitly masking off the bits and then having that mask be optimized away? |
Well, sometimes they get ignored, and sometimes all bits matter. This seems highly non-trivial, but I am not an expert on the relevant LLVM passes.
That also sounds like an option, if LLVM supports it. |
It seems like LLVM knows about it, but doesn't currently have a pass that optimizes for it: Seems like it would make more sense to add this functionality to LLVM rather than Rust though. |
Indeed, I should have at least marked it as draft from the get-go, or started with the forum as you suggest. This was intended as a conversation starter, it's by no means a ready proposal. I'm fully expecting to re-write this with more information, just want to get some outside opinions and fresh eyes on the direction first. |
That does sound like something that could be a useful LLVM pass, especially for compatibility with different platforms.
The snippet above will currently compile & work "fine" on a TBI system, except that Miri will rightly complain that the code has UB. The end goal of this proposal is to create an interface for top-byte tagging that does not break the memory model. This is separate from making those always safe to dereference, for which the LLVM pass would be helpful. |
It's not different. Methods already exist to do what you are trying to do: fn mask_addr(addr: usize) -> usize {
addr & 0xFFFFFFFFFFFFFF
}
let tag = 60;
let tagged_ptr = (&value as *const _).map_addr(|addr| addr | (tag << 56));
let val = unsafe { *tagged_ptr.map_addr(mask_addr) }; This is completely sound under MIRI and doesn't require any extensions to the Rust abstract machine. The only bit that's missing is a compiler optimization that erases the |
If I'm understanding what you're suggesting correctly, under this model the users would need to write out |
@RalfJung Should I then re-write this based on the already received comments and then post on Rust Internals? |
Fair enough - the proposed API seems a bit too high level for this allocator use-case though? Wouldn't the primitive operation be something like "realloc" but where you specify the target address? And there doesn't need to be an explicit |
It certainly could be if that's the community consensus, I don't have very strong views on what the exact API should look like - my intention when posting this was to get opinions on that exact question. Next time around I'll go through Internals first, I suppose I took the request for comments term a bit too literally for how it's used here :)
True as well, the intent there is just for convenience. Because different architectures can use different bits for the tagging it'd make sense to have a corresponding |
I also think a lower-level API that focuses on the |
The realloc approach would also support cases where virtual memory mappings are used for a similar purpose on platforms without hardware support for pointer tagging (ie. where you map the same physical memory to two or more virtual address ranges). |
I don't think Rust will have standard APIs for manipulating page tables? ;) I was going to say, I don't think this is ready yet for a portable API. It makes little sense to try and sketch a portable API that has exactly one target implementation. The RFC should focus in providing APIs for platform-specific capabilities, e.g. in |
that doesn't matter if you can still use This is kinda similar to how Rust doesn't have a |
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i meant that you'd |
The realloc-like interface could be neat in that it would let us avoid all the otherwise present difficulties with making the interface portable. With a signature like say (name TBC)
And similar for other platforms as needed. How does that sound? |
I've now posted a re-written version of this here: https://internals.rust-lang.org/t/pre-rfc-core-simulate-realloc/21745 |
Yeah, that could be done with an intrinsic like the one backing pointer tagging.
Yes that is roughly what I had in mind when suggesting a realloc-like interface. :) |
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Updated the RFC based on the discussion on internals: |
#[unstable(feature = "ptr_assume_moved", issue = "none")] | ||
#[cfg_attr(not(bootstrap), rustc_simulate_allocator)] | ||
#[allow(fuzzy_provenance_casts)] | ||
pub unsafe fn assume_moved<T>(original: *mut T, new_address: usize) -> *mut T { |
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Bikeshed: I find "assume" to be not a great choice here, since usually "assume(X)" means "either X is true or we have UB". But here it's actually fine for the allocation to not have moved at all.
It's more like, we are telling the AM that a move happened. I found "simulate" a better term than "assume". We could also use some other term of art like "axiomatic_move" or "abstract_move".
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I think this is mostly subjective, arguments can be made for several different names. As I said in the internals thread, if someone from the libs/opsem team blesses a particular name I'm happy to just change it to that, whatever it may be. I don't have a particularly strong preference of my own.
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It's also easy to rename later so the name does not have to be final in the RFC. (That should be mentioned under "open questions".)
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Added that section back as requested :)
is straightforward to write a 'correct' pointer tagging implementation by simply doing a `inttoptr` | ||
cast inside the memory allocator implementation, be it a custom C `malloc` or using a custom C++ | ||
`Allocator`. The goal of this effort is to create a Rust API for implementing this type of | ||
functionality that is guaranteed to be free of Undefined Behaviour. |
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FWIW doing this in the global_allocator in Rust is "probably also fine", same as in C/C++. Those are not inlined and so the compiler won't see the problematic pointer manipulations. The allocator is already special and it could be possible to fold this magic into that existing magic. (It'd still be nicer to do something proper. I just don't want there to be the impression that Rust somehow supports this less than C/C++ do.)
The main case where new language support is required is when doing this inside regular code.
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Add a helper for primitive pointer types to facilitate modifying the address of a pointer. This
mechanism is intended to enable the use of architecture features such as AArch64 Top-Byte Ignore
(TBI) to facilitate use-cases such as high-bit pointer tagging. An example application of this
mechanism would be writing a tagging memory allocator.
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