9 ms·

Arenas in Rust

134 points by welovebunnies 1y ago

pizlonator 1y ago
Arenas let you use OOBAs to do data attacks, and those can give you the moral equivalent of remote code execution. Like if your arena contains both a buffer that OOB's and a buffer passed to a syscall, then having memory safety around the arena doesn't help you
- Animats 1y ago
  Yes. This is a big headache in graphics, where there are big tables indexed by texture index down at the GPU level. Managing those as the content changes, with the GPU using the data while the CPU alters it, leads to huge complexity in Vulkan graphics. The two times I've had to deal with a really tough debugging problem in Rust, it's been related to some crate which did that.
- achierius 1y ago
  And as a bonus, they also conveniently opt you out of any other hardening features your system malloc might have implemented (like MTE). I certainly appreciate the performance benefits of custom allocators but it's disheartening to see people naively adopting them just as the world's allocators are starting to get their acts together with regards to security.
Animats 1y ago
This is the back-link problem in Rust. You can do back links with weak pointers, but it's rather verbose and involves extra run-time checks. I've been trying to figure out a compile-time approach to that problem. Here's an early version of a tech note on that.[1] It looks possible, but there are many issues. But it's important to look at now. The C++ people are trying to figure out safe backlinks. [1] https://github.com/John-Nagle/technotes/blob/main/docs/rust/rustbacklinks.md https://github.com/John-Nagle/technotes/blob/main/docs/rust/...
- discardable_dan 1y ago
  It's pretty easy to do at runtime without weak pointers, people who write rust are just allergic to reference counting. Which is absurd, because most programmer use RC values thousands of times every day without even thinking about it. It's really this easy: https://pastebin.com/SAeKG7Sw https://pastebin.com/SAeKG7Sw
  - Animats 1y ago
    Option<Rc<RefCell<Node<T>>>> works, but you have to manage your own teardown of the list. If you use Weak, releasing the strong reference to the head of the list tears down the whole list.
    - afdbcreid 1y ago
      You want to handle teardown yourself anyway, as otherwise long lists could overflow the stack on drop. I'm not convinced not using `Weak` is better, though.
- athrowaway3z 1y ago
  My gut says the following, so take with a grain of salt: If your goal is to not do refcounting, You can tie compile time guaranteed access on a 'parent getter' (but only ever as a non-mut reference) by saying: - parent element MUST be present at time of construction - When doing a 'parent get' call, you MUST still prove you're in the parent's lifetime. This can be done at compile time at 0 runtime cost. The problem is, that these constraints drastically reduce the usability of those back-links. There is no compiler solution in Rust. Rust lifetimes form a DAG of (overlapping) ranges. (in terms that some leaf nodes are inductive proofs) To relax those 2 constraints require a bi-directional graph, and I'm not sure if that can ever be computed in a reasonable time.
  - Animats 1y ago
    There are two main cases: - Single ownership with back references that never outlive the owning reference This requires a construct that manages the forward and back references together, preserving the invariant that A->B->A. It's also necessary to insure that no non-weak back reference from B to A exists when A is dropped. - True multiple ownership with weak back references This is the hard case. It might be solveable for complicated situations. See the example in my tech note that links to a Rust Playground file.[1] That's not a contrived example; it's code in actual use. It builds a set where all elements of the set can find the set itself. If two sets are merged (union), all elements move to one set struct, and the other now-empty set struct is discarded. All the back references are maintained during this operation. Most back references seem to fall into one of those two cases. In fact, the second includes the first. The first is so common that it's worth handling separately. The core problem is finding an approach which is 1) powerful enough to be useful on real code, and 2) not too hard to check at compile time. The second case requires some restrictions on coding style. The preferred coding style is short borrows with very narrow scope, typically within one statement. That's normally considered bad, because it means extra time checking for double borrows. But if this kind of borrow can be made compile-time, it's free. The compiler is going to have to check for overlapping borrow scopes, and the more local the borrow scope, the less compile time work is involved. Storing a borrowed link in a structure or a static requires too much compile-time analysis, but if the lifetime of the borrow is very local, the analysis isn't hard. Borrow scopes which include a function call imply the need for cross-function call tree analysis. It's been pointed out to me that traits and generics make this especially hard, because you don't know what the instantiation can do before generics are instantiated. That might require some kind of annotation that says to the compiler "Implementations of function foo for trait Foobar are not allowed to do an upgrade of an Rc<Bar>". Then, at the call, the compiler can tell it's OK for a borrow scope to include that call, and in an implementation of the called function, upgrade of Rc<Bar> is prohibited. In fact, if this is done for all functions that receive an Rc<Bar> parameter, it might be possible to do all this without call tree analysis. It's becomes a type constraint. Need to talk to the type theorists, of which I am not one. All those .borrow() calls are annoying. It might be possible to elide them most of the time, in the same way that you can refer to the item within an Rc without explicitly de-referencing the Rc. That would be nice from an ergonomic perspective. But it runs the risk of generating confusing compiler messages when a problem is detected. It would be like those confusing error messages that arise when type inference can't figure something out or the borrow checker needs explicit lifetime info to resolve an ambiguity. [1] https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=3071faba427b440643d26ac5fe182caa https://play.rust-lang.org/?version=stable&mode=debug&editio...
IshKebab 1y ago
Insightful article. As they say doubly linked lists are approximately useless and never used. But trees were nodes can refer to their parents are extremely common. And I agree, a custom arena is usually the best option here. Even though it seems like "giving up" and going back to C, it's still way better. There's this great comparison of Arenas in Rust: https://donsz.nl/blog/arenas/ https://donsz.nl/blog/arenas/ Some of them have keys that can't be reused too, so you can avoid "use after free" style bugs (at runtime) which is way better than C pointer wrangling. It's not perfect though. I kind of wish Rust did have some kind of support for this stuff natively. Pretty low on my list of Rust desires though - way behind better compile time and fewer async footguns.
- sgarland 1y ago
  > doubly-linked lists are approximately useless Leaf pages in InnoDB’s B+tree implementation is the big one I can think of. Turns out being able to quickly walk pages for a range scan is helpful. I agree that there isn’t wide variety of applications where they jump out as being the best choice. So, yeah, “approximately useless” sounds about right.
echelon 1y ago
I'd like to see something powerful for safety and speed in Crates.io and Cargo: The ability for crates to specify whether they use certain features of Rust: - unsafe - arena allocation or other future "useful" stuff - proc_macros, eg. serde (a huge compile speed hit) - deep transitive dependency (total #, or depth) - some proxy for the "number of compiler steps" (compiler workload complexity) We should be able to set a crate-wide or workspace-wide policy that can ban other crates that violate these rules. We could make this hand-written early on, but eventually enforced by the compiler. You could then easily guarantee that no downstream dependency uses unsafe code, has a crazy number of dependencies, or has crazy compile times. You could opt-in or opt-out to these things without having to think too much about them or constantly (manually) be on the lookout. This would be hugely beneficial to the Rust ecosystem, safety, code quality, and developer productivity / happiness.
- deleted 1y ago
  [deleted]
- burnt-resistor 1y ago
  Many of these currently exist as lints that can live in a Cargo.toml: [workspace.lints.rust] unsafe_code = "forbid" https://doc.rust-lang.org/rustc/lints/index.html https://doc.rust-lang.org/rustc/lints/index.html Perhaps what would be useful is crates.io determining through analysis which in the set of all lint(s) a crate is compatible with and exposing that as automatic metadata?
mwkaufma 1y ago
> Handles are deterministic. If a bug made your program crash on the last run, it will crash the same way on this run. > Given that the arena will still be subject to array bounds checking, handle bugs won't allow an attacker to overwrite arbitrary memory the way pointer bugs do. Just because you're in-bounds of the arena, doesn't mean you're not stomping on in-bounds neighboring data -- which can introduce "nondeterminism" as the author defines it. These are hand-waving arguments.
- mwkaufma 1y ago
  Anticipating pushback: yes, you can disallow "pointer arithmetic" on handles, and store fingerprints in the "slots" to ensure they still contain the handle's identity to detect user-after-free, but congrats, you've implemented sparse sets, which there are dozen's of C++ implementations of with the same safety guarantees, so it's unclear what rust is bringing in that case (e.g. https://github.com/skypjack/entt/blob/master/src/entt/entity/sparse_set.hpp https://github.com/skypjack/entt/blob/master/src/entt/entity...)
  - at_compile_time 1y ago
    That C++ already has many implementations of sparse sets seems to be a point in favor of sparse sets rather than a point against Rust, especially given that C++ doesn't need them the same way Rust does.
    - mwkaufma 1y ago
      Well, there's several implementations because it's a bit of a leaky abstraction, and implementation details matter/vary across use-cases, which is consistent w/ my experience of rust having heavy fragmentation in applying the array-index pattern for "weak pointers."
  - airstrike 1y ago
    If only devs had any other reason to use Rust.....
sfpotter 1y ago
Here's an implementation of a doubly-linked list which is perfectly fine on modern hardware: an array of structs where each struct contains a pointer to its next and previous elements in addition to whatever other data is being stored. Here's a situation where a traditional pointer-based double-linked list is fine: when the payload is very large (e.g., an entire document in some app).
- cindyllm 1y ago
  [dead]
wasmperson 1y ago
> There are several ways to solve this problem. One way is to avoid using direct references to the particular class of objects at all. Instead, allocate a big array of objects, and refer to them with integer indexes into that array. There are several names that have been used for such arrays and indexes; let's call them arenas and handles. I thought the term "Arena" referred to linear allocators, but maybe it's not so narrowly defined. > At one level, this question is not very fair, because an answer to it as stated would be that one simply does not use doubly linked lists. They have been popular in introductory computer science lectures because they are a neat way to explain pointers in a data structure that's easy to draw on a whiteboard, but they are not a good match to modern hardware. The last time I used one was in the nineties. I know the Linux kernel uses them, but that design was also laid down in the nineties; if you were designing a kernel from scratch today, you would probably not do it that way. The arena data structure you described is inefficient unless it uses a linked list to track empty slots. All general-purpose heap allocators use linked lists in their implementations. Linked lists show up wherever you want pointer stability, low fragmentation, or a way to decouple the storage of objects from their participation in data structures. I struggle to imagine how it would be possible to implement something like Linux without at least one linked list in it. > Essentially you are bypassing the notion of pointers provided directly by the hardware Pointers aren't a hardware concept, they're a language concept. Array indices and pointers both rely on indirect addressing, which is the underlying hardware concept. The "handles" strategy in Rust feels like a kludgy approach not because it bypasses the hardware but because Rust's borrow checker isn't actually involved in ensuring safety in that case, just its bounds checking and mandatory initialization (and if all you need is those two things then...).
crq-yml 1y ago
I'm a bit agnostic about the specific solution these days. In general, early binding(so, static memory and types, formalized arenas and handles, in-line linear logic with few or no loops or branches) debugs more readily than late(dynamic memory allocs and types, raw pointers, virtual functions, runtime configuration). The appeal of late binding is in deferring the final computation to later so that your options stay open, while the converse is true with early binding - if you can write a program that always returns a precomputed answer, that's easier to grasp and verify. When we compare one method of early binding with another, it's probably going to be a comparison of the granularity. Arenas are "stupid simple" - they partition out memory into chunks that limit the blast radius of error, but you still make the error. Ownership logic is a "picky generalization" - it lets you be more intricate and gain some assurances if you put up with the procedures necessary, but it starts to inhibit certain uses because its view into usage is too narrow with too many corner cases. If we take Go's philosophy as an example of "what do you do if you want idioms for a really scalable codebase" - though you can certainly argue that it didn't work - it's that you usually want to lean on the stupid simple stuff and customize what you need for the rest. You don't opt into a picky Swiss Army Knife unless there's a specific problem to solve with it. Larger codebases have proportionately smaller sections that demand intricacy because more and more of the code is going to itself be a method of defining late binding, of configuring and deferring parts of processing. That said, Rust lets you fall back to "stupid simple", it just doesn't pave the way to make that the default.
jmull 1y ago
> Doesn't that mean you are throwing out all the memory safety properties you were hoping to achieve by using Rust in the first place? ...That argument sounds logical, but it's not actually correct. Actually, it is correct. The more generally you use "arenas" the more they are subject to the same kinds of issues as other manual memory management systems. "arenas" can only avoid/reduce the "nondeterminism" and "security" issues of general manual memory management on top of a buffer of bytes by not becoming general manual memory management on top of a buffer of bytes.
- tyre 1y ago
  How could using arenas lead to remote code execution?
  - jmull 1y ago
    You just need to put something executable, in whatever sense, in the arena... values that represent functions to call or dynamic libraries to load, or privileges to do those things, etc.
- simonask 1y ago
  It all comes down to whether pointers into the arena do something different than normal pointers when they are dangling. Normal pointers cause UB. That’s astronomically bad. If your arena pointers crash your program in a well-defined way, that’s already a much better situation.
  - 1718627440 1y ago
    The reason pointers are UB is that they could be anything including another object, code or memory mapped hardware interfaces. The analog here would be if you would just use the same index into a different arena, that trouble also wouldn't be bounded to just one arena.
lenkite 1y ago
I wish Rust would finalize its custom memory allocators api (allocator_api) and put it in the stdlib as soon as possible. That feature has been unstable for a decade now.
- whytevuhuni 1y ago
  I'd really like them not to hurry (or at least not "ASAP"). Among features that Rust should get right, this is definitely one of them. And from its tracking issue [1], it seems there's alternatives with really important trade-offs (especially the very interesting storage API alternative), and it'd be really sad if we got stuck with something that ends up being unusable in many cases people actually want to use them on. [1] https://github.com/rust-lang/rust/issues/32838 https://github.com/rust-lang/rust/issues/32838
  - vacuity 1y ago
    Fully agree with you. The feature needs to be done right, and an overhaul like the storage proposal is probably necessary. I think the allocator feature is one of those features that Rust historically had leeway on, because it's still relatively new, but now a bright path for the years (decades) ahead is crucial. Unstable-but-implemented-and-maybe-stabilizing is still better than stable-but-sucks-forever.
oflebbe 1y ago
I think I will love FORTRAN again
asplake 1y ago
> The last time I used [a doubly linked list] was in the nineties. I know the Linux kernel uses them, but that design was also laid down in the nineties; if you were designing a kernel from scratch today, you would probably not do it that way. Curious to know in the Linux context whether moves away from doubly linked lists have ever been tested?
- gavinsyancey 1y ago
  Linux uses them because it has a lot of lists of objects that very rarely or never need to be iterated over, but where it needs to be fast to - Insert a new item - Delete a specific item (that you have a pointer to - Move a specific item from list A to list B - Get the next item to work on And where pointers to an item need to be stable. Doubly-linked lists are very fast for all of that; their main downside is they are slow to iterate over due to cache incoherency but that doesn't matter if you very rarely or never iterate through the entire list. And since you need to have stable pointers to items in the list that don't change when you insert/remove items, most other data structures would need an extra level of indirection so you lose the cache coherency benefit anyways.
  - monocasa 1y ago
    And they also value for the size of the set to not be bounded, and for those operations to occur without allocation.
procaryote 1y ago
I kinda think rust is a dead-end. The learning curve is really challenging for most people, and you gain all these new constraints. It competes against languages that give you memory safety by having garbage collection, or I guess even by using arenas in a few cases. GC comes with a performance hit, but a bit of a performance hit is usually much easier to deal with than having a smaller pool of effective developers Worse is better
- pimeys 1y ago
  Nah. It's like learning vim, you get over the first shock and then you have a lifetime friend. And another thing is with current coding agents, like the gpt-5-codex, Rust really shines here. The agent can easily guide you through the first borrow checker issues, and the compiler helps the agent to write good code. Source: been writing Rust for a decade now. It is easily the best language I've ever used, still.
  - t43562 1y ago
    I'm experiencing that shock now. Can't create a global instance of my self-made heap datastructure. I'm longing for C memory management at the moment - it seems a tiny price to pay. Now I learn that linked lists aren't possible and one has to use some, frankly, bullshit scheme to pretend to have them. IMO the documentation isn't really preparing people for the new reality of what can be done and how to do things.
    - binarymax 1y ago
      The way I think about rust is this: you can experience the pain up front while you’re getting your program to compile, or you can experience pain later during runtime in random unpredictable ways. Rust is the former. It takes time to learn how to reduce the up front pain, but I’m so much more confident in the end result. I say this having years of coding background in languages like C, C#, Java, JavaScript, Python, etc.
      - t43562 1y ago
Someone 1y ago
Given Rust’s memory safety goal, IMO the proper way to solve the circular reference problem in the context of Rust would be in the language. It should have a way to express what cycles may exist, maybe also how the code will break them when needed. Have people been thinking about that?
- SkiFire13 1y ago
  There haven't been any serious proposals yet, but some ideas have been floating around from time to time. For example this blog post has some ideas that might allow them https://smallcultfollowing.com/babysteps/blog/2024/03/04/borrow-checking-without-lifetimes/ https://smallcultfollowing.com/babysteps/blog/2024/03/04/bor... The hardest problems I think is that this requires a fundamental change to the language that something might be already borrowed just by existing. It also likely needs to introduce a notion of "stable" places that will not move when you move a prefix of them, e.g. to allow moving a `Box` holding self-referential data. In other words, there's a lot of bikeshedding to do.
- afdbcreid 1y ago
  First, there are external crates offering safe self-referential structs. A builtin language feature will for sure be more convenient (and also probably more performant, as most of those crates require an allocation) but it's just really hard to design. Even in very old self-referential crates soundness issues are still being discovered to this day. So it will need to a lot of time and care to design.
prngl 1y ago
> The last time I used [a doubly linked list] was in the nineties. I know the Linux kernel uses them, but that design was also laid down in the nineties; if you were designing a kernel from scratch today, you would probably not do it that way. This and sibling comments about the supposed uselessness or outdatedness of linked lists taught me something about the disconnect between many systems engineers resistant to Rust and many members of the (evangelical) Rust community. To address the technical topic first: a doubly linked list is what you reach for when you’re looking for an intrusive data structure with fast append and fast delete. You need to iterate over it, but don’t need random access. Queues where you may need to remove elements. Think a list of completions that can be cancelled. Why an intrusive data structure? Because you don’t want to, or can’t allocate. Because you want only a non-owning reference to the object, and you don’t want to have to manage any other lifetimes or dynamic (potentially unbounded, or arbitrarily bounded) allocations. Intrusive data structures are a great tool to minimize allocations, and to keep allocations (and ownership) logically separated across modules. And now onto the cultural topic. That a member of the Rust community might not know this (which is of course totally fine) is what taught me something, which was maybe obvious to many: Rust brings safety to a C++ style language and audience. For the same reasons many dislike C++, many dislike Rust. For the same reasons many would opt for C over C++ when they have the choice (either across all their projects or for certain projects), many continue to opt for C over Rust. Rust will not be taking over systems programming for the same reasons C++ did not. Well, by the numbers, it probably did (and Rust probably will too), so maybe better to say that it will not stamp out C for systems programming for the same reasons C++ did not. And it’s not just because of the stubbornness of C programmers. Systems programming has 2 cultures and practice groups: the C camp, and the C++ camp. The C++ camp is a big tent. I’d argue it includes Rust and Swift (and maybe D). Zig belongs to the C camp. Safety is important, and maybe Rust has the right model for it, but it is not a solution for the C camp. It likely did not intend to be, and again, by the numbers, replacing C++ is probably the more important task.
- alfiedotwtf 1y ago
  > To address the technical topic first: a doubly linked list is what you reach for when you’re looking for an intrusive data structure with fast append and fast delete. You need to iterate over it, but don’t need random access. Queues where you may need to remove elements. Think a list of completions that can be cancelled. > > Why an intrusive data structure? Because you don’t want to, or can’t allocate. Because you want only a non-owning reference to the object, and you don’t want to have to manage any other lifetimes or dynamic (potentially unbounded, or arbitrarily bounded) allocations. Intrusive data structures are a great tool to minimize allocations, and to keep allocations (and ownership) logically separated across modules. If you really wanted this in Rust, you could probably get away with just initialising a Vec::with_capacity(), then have each element’s next and prev be indexes into the vec.
andoando 1y ago
I am really confused here. 1. Implementing a linked list is a one and done deal and should be included in a (standard) library. 2. Would a classic pointer based implementation of a linked list in C really be a cause of security vulnerability??
- gaanbal 1y ago
  yea it's already in the standard library and it's very straightforward
  - vacuity 1y ago
    The one in the standard library is non-intrusive and allocates space on the heap per node. In situations where a linked list implementation, in Rust code, no less, is deemed necessary, alternatives should probably be used.
Wowfunhappy 1y ago
The only Rust programs I've written are simple toys, and I don't know enough computer science to fully understand the problem space here. However, the first thing that comes to mind is, what if you just used unsafe for your doubly linked list? I understand this defeats the point of using Rust. But if the argument is "I'm using C because I actually need doubly linked lists and Rust makes that hard/slow", well C is also unsafe, so using Rust with an unsafe block for your doubly linked list seems like an improvement. I feel like at least part of the reason not to do this is "because the Rust community will yell at you." Well, maybe they shouldn't do that? If you were using C no one would care, so switching to unsafe Rust shouldn't imply the sky is falling.
- vacuity 1y ago
  Unsafe is quite plainly the right tool for the job. Yes, people can mess up unsafe code, but that really is just how things are (until we get magic formally verifying compilers?!). I appreciate your levelheaded take, rather than advocating for only C or exaggerated concern for safe Rust.
- School-Cotton 1y ago
  Why would the rust community “yell at you” for using unsafe somewhere where it’s clearly needed? By the way, unsafe doesn’t defeat the purpose of rust. The entire point of rust is that it lets you build safe abstractions on top of unsafe code, not that it eliminates unsafe code entirely.
- rstuart4133 1y ago
  The answer to "why not use unsafe" is it's hard to do correctly. What you have to do is pretty well documented https://doc.rust-lang.org/nomicon/working-with-unsafe.html https://doc.rust-lang.org/nomicon/working-with-unsafe.html but it's fiddly, tedious, easy to get wrong (you have to understand what the compiler is doing internally), and there are no automated checks to verify you have got it right. A newbie has almost no hope of getting right in the first 5 tries (5 is entirely made up.) But, Rust already provides a linked list in it's standard library: https://doc.rust-lang.org/std/collections/struct.LinkedList.html https://doc.rust-lang.org/std/collections/struct.LinkedList.... So a Rust programmer might say there is no need use unsafe if you want a linked list. There is a "source" link on that page, so it isn't hard to see how it's done. Yes, it does use "unsafe". Where it gets a little complex is a C programmer will look at that implementation and scoff at the linked list the Rust standard library provides. Amazingly the C and Rust programmers agree that this implementation is mostly useless. We know that because the Rust documentation says: "NOTE: It is almost always better to use Vec or VecDeque because array-based containers are generally faster, more memory efficient, and make better use of CPU cache [than this Rust linked list]." When a C programmer uses a linked list, he almost invariably uses what someone above termed above an "intrusive" implementation. An intrusive implementation is typically faster, more memory efficient, and makes just as good use of the CPU cache as the Rust Vec or VecDeque. An intrusive implementation reserves a bit of memory inside of the object you want to put in a linked list for the linked list code. That memory is typically "owned" by the linked list code, as in it manages it's lifetime, whereas the lifetime of the object the memory is in is managed by some other mechanism. For example, if you delete some unrelated item it list, it might have to reach in and modify the linked list pointer in this item. But Rust has a simple memory model: if you modify an object you have to prove you own it. The linked list code reaching into another object can not do that. That's one example, but intrusive linked lists break Rust's ownership and borrowing rules in so many ways, they are like oil and water. As far as I can tell there is no way to provide an intrusive linked list library in Rust that is safe to use, which is to say all code that uses it would have to be flagged as unsafe too. This probably raises as more questions that it answered, and you are probably wondering how could two implementations of the same thing (a linked list) be so different. I'm not going to go down that rabbit hole, other than to remark it's clear a lot of Rust programmers commenting here don't understand the flexibility the intrusive method gives you. But their instincts are right about one thing: they are dangerous. It's the sort of danger a C programmer is entirely comfortable with, but that same danger is what has lead to C programmers being responsible for a of CVE's. They are right about another thing too: if you put a bit of thought into it, you can usually come up with something that is for all practical purposes just as fast, and the Rust compiler can prove is safe. Sometimes though, "a bit of thought" is an understatement, and sometimes it feels like you are in a straight jacket. The article code be read is someone straining against that straight jacket, doing a lot of thinking and coming up with what a C programmer would call a fairly ugly (and slower) solution. The old C programmer in me says that's a fair summary. But I also have to acknowledge it is a very safe solution.
alazoral 1y ago
What I find is everything that I find interesting is cyclic, and so Rust has poor ergonomics for writing things I’m interested in. This is sad for me because it has the best compiler errors in the business. If I could get Swift with Rust’s compiler errors (Rust’s compiler in general is far better behaved) I’d be a happy chap.

134 points by welovebunnies 1y ago

Arenas let you use OOBAs to do data attacks, and those can give you the moral equivalent of remote code execution. Like if your arena contains both a buffer that OOB's and a buffer passed to a syscall, then having memory safety around the arena doesn't help you

Animats 1y ago
Yes. This is a big headache in graphics, where there are big tables indexed by texture index down at the GPU level. Managing those as the content changes, with the GPU using the data while the CPU alters it, leads to huge complexity in Vulkan graphics. The two times I've had to deal with a really tough debugging problem in Rust, it's been related to some crate which did that.
achierius 1y ago
And as a bonus, they also conveniently opt you out of any other hardening features your system malloc might have implemented (like MTE). I certainly appreciate the performance benefits of custom allocators but it's disheartening to see people naively adopting them just as the world's allocators are starting to get their acts together with regards to security.

Animats 1y ago

This is the back-link problem in Rust. You can do back links with weak pointers, but it's rather verbose and involves extra run-time checks. I've been trying to figure out a compile-time approach to that problem. Here's an early version of a tech note on that.[1] It looks possible, but there are many issues. But it's important to look at now. The C++ people are trying to figure out safe backlinks. [1] https://github.com/John-Nagle/technotes/blob/main/docs/rust/rustbacklinks.md https://github.com/John-Nagle/technotes/blob/main/docs/rust/...

discardable_dan 1y ago
It's pretty easy to do at runtime without weak pointers, people who write rust are just allergic to reference counting. Which is absurd, because most programmer use RC values thousands of times every day without even thinking about it. It's really this easy: https://pastebin.com/SAeKG7Sw https://pastebin.com/SAeKG7Sw
- Animats 1y ago
  Option<Rc<RefCell<Node<T>>>> works, but you have to manage your own teardown of the list. If you use Weak, releasing the strong reference to the head of the list tears down the whole list.
  - afdbcreid 1y ago
    You want to handle teardown yourself anyway, as otherwise long lists could overflow the stack on drop. I'm not convinced not using `Weak` is better, though.
athrowaway3z 1y ago
My gut says the following, so take with a grain of salt: If your goal is to not do refcounting, You can tie compile time guaranteed access on a 'parent getter' (but only ever as a non-mut reference) by saying: - parent element MUST be present at time of construction - When doing a 'parent get' call, you MUST still prove you're in the parent's lifetime. This can be done at compile time at 0 runtime cost. The problem is, that these constraints drastically reduce the usability of those back-links. There is no compiler solution in Rust. Rust lifetimes form a DAG of (overlapping) ranges. (in terms that some leaf nodes are inductive proofs) To relax those 2 constraints require a bi-directional graph, and I'm not sure if that can ever be computed in a reasonable time.
- Animats 1y ago
  There are two main cases: - Single ownership with back references that never outlive the owning reference This requires a construct that manages the forward and back references together, preserving the invariant that A->B->A. It's also necessary to insure that no non-weak back reference from B to A exists when A is dropped. - True multiple ownership with weak back references This is the hard case. It might be solveable for complicated situations. See the example in my tech note that links to a Rust Playground file.[1] That's not a contrived example; it's code in actual use. It builds a set where all elements of the set can find the set itself. If two sets are merged (union), all elements move to one set struct, and the other now-empty set struct is discarded. All the back references are maintained during this operation. Most back references seem to fall into one of those two cases. In fact, the second includes the first. The first is so common that it's worth handling separately. The core problem is finding an approach which is 1) powerful enough to be useful on real code, and 2) not too hard to check at compile time. The second case requires some restrictions on coding style. The preferred coding style is short borrows with very narrow scope, typically within one statement. That's normally considered bad, because it means extra time checking for double borrows. But if this kind of borrow can be made compile-time, it's free. The compiler is going to have to check for overlapping borrow scopes, and the more local the borrow scope, the less compile time work is involved. Storing a borrowed link in a structure or a static requires too much compile-time analysis, but if the lifetime of the borrow is very local, the analysis isn't hard. Borrow scopes which include a function call imply the need for cross-function call tree analysis. It's been pointed out to me that traits and generics make this especially hard, because you don't know what the instantiation can do before generics are instantiated. That might require some kind of annotation that says to the compiler "Implementations of function foo for trait Foobar are not allowed to do an upgrade of an Rc<Bar>". Then, at the call, the compiler can tell it's OK for a borrow scope to include that call, and in an implementation of the called function, upgrade of Rc<Bar> is prohibited. In fact, if this is done for all functions that receive an Rc<Bar> parameter, it might be possible to do all this without call tree analysis. It's becomes a type constraint. Need to talk to the type theorists, of which I am not one. All those .borrow() calls are annoying. It might be possible to elide them most of the time, in the same way that you can refer to the item within an Rc without explicitly de-referencing the Rc. That would be nice from an ergonomic perspective. But it runs the risk of generating confusing compiler messages when a problem is detected. It would be like those confusing error messages that arise when type inference can't figure something out or the borrow checker needs explicit lifetime info to resolve an ambiguity. [1] https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=3071faba427b440643d26ac5fe182caa https://play.rust-lang.org/?version=stable&mode=debug&editio...

IshKebab 1y ago

Insightful article. As they say doubly linked lists are approximately useless and never used. But trees were nodes can refer to their parents are extremely common. And I agree, a custom arena is usually the best option here. Even though it seems like "giving up" and going back to C, it's still way better. There's this great comparison of Arenas in Rust: https://donsz.nl/blog/arenas/ https://donsz.nl/blog/arenas/ Some of them have keys that can't be reused too, so you can avoid "use after free" style bugs (at runtime) which is way better than C pointer wrangling. It's not perfect though. I kind of wish Rust did have some kind of support for this stuff natively. Pretty low on my list of Rust desires though - way behind better compile time and fewer async footguns.

sgarland 1y ago
> doubly-linked lists are approximately useless Leaf pages in InnoDB’s B+tree implementation is the big one I can think of. Turns out being able to quickly walk pages for a range scan is helpful. I agree that there isn’t wide variety of applications where they jump out as being the best choice. So, yeah, “approximately useless” sounds about right.

echelon 1y ago

I'd like to see something powerful for safety and speed in Crates.io and Cargo: The ability for crates to specify whether they use certain features of Rust: - unsafe - arena allocation or other future "useful" stuff - proc_macros, eg. serde (a huge compile speed hit) - deep transitive dependency (total #, or depth) - some proxy for the "number of compiler steps" (compiler workload complexity) We should be able to set a crate-wide or workspace-wide policy that can ban other crates that violate these rules. We could make this hand-written early on, but eventually enforced by the compiler. You could then easily guarantee that no downstream dependency uses unsafe code, has a crazy number of dependencies, or has crazy compile times. You could opt-in or opt-out to these things without having to think too much about them or constantly (manually) be on the lookout. This would be hugely beneficial to the Rust ecosystem, safety, code quality, and developer productivity / happiness.

deleted 1y ago
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burnt-resistor 1y ago
Many of these currently exist as lints that can live in a Cargo.toml: [workspace.lints.rust] unsafe_code = "forbid" https://doc.rust-lang.org/rustc/lints/index.html https://doc.rust-lang.org/rustc/lints/index.html Perhaps what would be useful is crates.io determining through analysis which in the set of all lint(s) a crate is compatible with and exposing that as automatic metadata?

mwkaufma 1y ago

> Handles are deterministic. If a bug made your program crash on the last run, it will crash the same way on this run. > Given that the arena will still be subject to array bounds checking, handle bugs won't allow an attacker to overwrite arbitrary memory the way pointer bugs do. Just because you're in-bounds of the arena, doesn't mean you're not stomping on in-bounds neighboring data -- which can introduce "nondeterminism" as the author defines it. These are hand-waving arguments.

mwkaufma 1y ago
Anticipating pushback: yes, you can disallow "pointer arithmetic" on handles, and store fingerprints in the "slots" to ensure they still contain the handle's identity to detect user-after-free, but congrats, you've implemented sparse sets, which there are dozen's of C++ implementations of with the same safety guarantees, so it's unclear what rust is bringing in that case (e.g. https://github.com/skypjack/entt/blob/master/src/entt/entity/sparse_set.hpp https://github.com/skypjack/entt/blob/master/src/entt/entity...)
- at_compile_time 1y ago
  That C++ already has many implementations of sparse sets seems to be a point in favor of sparse sets rather than a point against Rust, especially given that C++ doesn't need them the same way Rust does.
  - mwkaufma 1y ago
    Well, there's several implementations because it's a bit of a leaky abstraction, and implementation details matter/vary across use-cases, which is consistent w/ my experience of rust having heavy fragmentation in applying the array-index pattern for "weak pointers."
- airstrike 1y ago
  If only devs had any other reason to use Rust.....

sfpotter 1y ago

Here's an implementation of a doubly-linked list which is perfectly fine on modern hardware: an array of structs where each struct contains a pointer to its next and previous elements in addition to whatever other data is being stored. Here's a situation where a traditional pointer-based double-linked list is fine: when the payload is very large (e.g., an entire document in some app).

cindyllm 1y ago
[dead]

wasmperson 1y ago

> There are several ways to solve this problem. One way is to avoid using direct references to the particular class of objects at all. Instead, allocate a big array of objects, and refer to them with integer indexes into that array. There are several names that have been used for such arrays and indexes; let's call them arenas and handles. I thought the term "Arena" referred to linear allocators, but maybe it's not so narrowly defined. > At one level, this question is not very fair, because an answer to it as stated would be that one simply does not use doubly linked lists. They have been popular in introductory computer science lectures because they are a neat way to explain pointers in a data structure that's easy to draw on a whiteboard, but they are not a good match to modern hardware. The last time I used one was in the nineties. I know the Linux kernel uses them, but that design was also laid down in the nineties; if you were designing a kernel from scratch today, you would probably not do it that way. The arena data structure you described is inefficient unless it uses a linked list to track empty slots. All general-purpose heap allocators use linked lists in their implementations. Linked lists show up wherever you want pointer stability, low fragmentation, or a way to decouple the storage of objects from their participation in data structures. I struggle to imagine how it would be possible to implement something like Linux without at least one linked list in it. > Essentially you are bypassing the notion of pointers provided directly by the hardware Pointers aren't a hardware concept, they're a language concept. Array indices and pointers both rely on indirect addressing, which is the underlying hardware concept. The "handles" strategy in Rust feels like a kludgy approach not because it bypasses the hardware but because Rust's borrow checker isn't actually involved in ensuring safety in that case, just its bounds checking and mandatory initialization (and if all you need is those two things then...).

crq-yml 1y ago

I'm a bit agnostic about the specific solution these days. In general, early binding(so, static memory and types, formalized arenas and handles, in-line linear logic with few or no loops or branches) debugs more readily than late(dynamic memory allocs and types, raw pointers, virtual functions, runtime configuration). The appeal of late binding is in deferring the final computation to later so that your options stay open, while the converse is true with early binding - if you can write a program that always returns a precomputed answer, that's easier to grasp and verify. When we compare one method of early binding with another, it's probably going to be a comparison of the granularity. Arenas are "stupid simple" - they partition out memory into chunks that limit the blast radius of error, but you still make the error. Ownership logic is a "picky generalization" - it lets you be more intricate and gain some assurances if you put up with the procedures necessary, but it starts to inhibit certain uses because its view into usage is too narrow with too many corner cases. If we take Go's philosophy as an example of "what do you do if you want idioms for a really scalable codebase" - though you can certainly argue that it didn't work - it's that you usually want to lean on the stupid simple stuff and customize what you need for the rest. You don't opt into a picky Swiss Army Knife unless there's a specific problem to solve with it. Larger codebases have proportionately smaller sections that demand intricacy because more and more of the code is going to itself be a method of defining late binding, of configuring and deferring parts of processing. That said, Rust lets you fall back to "stupid simple", it just doesn't pave the way to make that the default.

jmull 1y ago

> Doesn't that mean you are throwing out all the memory safety properties you were hoping to achieve by using Rust in the first place? ...That argument sounds logical, but it's not actually correct. Actually, it is correct. The more generally you use "arenas" the more they are subject to the same kinds of issues as other manual memory management systems. "arenas" can only avoid/reduce the "nondeterminism" and "security" issues of general manual memory management on top of a buffer of bytes by not becoming general manual memory management on top of a buffer of bytes.

tyre 1y ago
How could using arenas lead to remote code execution?
- jmull 1y ago
  You just need to put something executable, in whatever sense, in the arena... values that represent functions to call or dynamic libraries to load, or privileges to do those things, etc.
simonask 1y ago
It all comes down to whether pointers into the arena do something different than normal pointers when they are dangling. Normal pointers cause UB. That’s astronomically bad. If your arena pointers crash your program in a well-defined way, that’s already a much better situation.
- 1718627440 1y ago
  The reason pointers are UB is that they could be anything including another object, code or memory mapped hardware interfaces. The analog here would be if you would just use the same index into a different arena, that trouble also wouldn't be bounded to just one arena.

lenkite 1y ago

I wish Rust would finalize its custom memory allocators api (allocator_api) and put it in the stdlib as soon as possible. That feature has been unstable for a decade now.

whytevuhuni 1y ago
I'd really like them not to hurry (or at least not "ASAP"). Among features that Rust should get right, this is definitely one of them. And from its tracking issue [1], it seems there's alternatives with really important trade-offs (especially the very interesting storage API alternative), and it'd be really sad if we got stuck with something that ends up being unusable in many cases people actually want to use them on. [1] https://github.com/rust-lang/rust/issues/32838 https://github.com/rust-lang/rust/issues/32838
- vacuity 1y ago
  Fully agree with you. The feature needs to be done right, and an overhaul like the storage proposal is probably necessary. I think the allocator feature is one of those features that Rust historically had leeway on, because it's still relatively new, but now a bright path for the years (decades) ahead is crucial. Unstable-but-implemented-and-maybe-stabilizing is still better than stable-but-sucks-forever.

oflebbe 1y ago

I think I will love FORTRAN again

asplake 1y ago

> The last time I used [a doubly linked list] was in the nineties. I know the Linux kernel uses them, but that design was also laid down in the nineties; if you were designing a kernel from scratch today, you would probably not do it that way. Curious to know in the Linux context whether moves away from doubly linked lists have ever been tested?

gavinsyancey 1y ago
Linux uses them because it has a lot of lists of objects that very rarely or never need to be iterated over, but where it needs to be fast to - Insert a new item - Delete a specific item (that you have a pointer to - Move a specific item from list A to list B - Get the next item to work on And where pointers to an item need to be stable. Doubly-linked lists are very fast for all of that; their main downside is they are slow to iterate over due to cache incoherency but that doesn't matter if you very rarely or never iterate through the entire list. And since you need to have stable pointers to items in the list that don't change when you insert/remove items, most other data structures would need an extra level of indirection so you lose the cache coherency benefit anyways.
- monocasa 1y ago
  And they also value for the size of the set to not be bounded, and for those operations to occur without allocation.

procaryote 1y ago

I kinda think rust is a dead-end. The learning curve is really challenging for most people, and you gain all these new constraints. It competes against languages that give you memory safety by having garbage collection, or I guess even by using arenas in a few cases. GC comes with a performance hit, but a bit of a performance hit is usually much easier to deal with than having a smaller pool of effective developers Worse is better

pimeys 1y ago
Nah. It's like learning vim, you get over the first shock and then you have a lifetime friend. And another thing is with current coding agents, like the gpt-5-codex, Rust really shines here. The agent can easily guide you through the first borrow checker issues, and the compiler helps the agent to write good code. Source: been writing Rust for a decade now. It is easily the best language I've ever used, still.
- t43562 1y ago
  I'm experiencing that shock now. Can't create a global instance of my self-made heap datastructure. I'm longing for C memory management at the moment - it seems a tiny price to pay. Now I learn that linked lists aren't possible and one has to use some, frankly, bullshit scheme to pretend to have them. IMO the documentation isn't really preparing people for the new reality of what can be done and how to do things.
  - binarymax 1y ago
    The way I think about rust is this: you can experience the pain up front while you’re getting your program to compile, or you can experience pain later during runtime in random unpredictable ways. Rust is the former. It takes time to learn how to reduce the up front pain, but I’m so much more confident in the end result. I say this having years of coding background in languages like C, C#, Java, JavaScript, Python, etc.
    - t43562 1y ago

Someone 1y ago

Given Rust’s memory safety goal, IMO the proper way to solve the circular reference problem in the context of Rust would be in the language. It should have a way to express what cycles may exist, maybe also how the code will break them when needed. Have people been thinking about that?

SkiFire13 1y ago
There haven't been any serious proposals yet, but some ideas have been floating around from time to time. For example this blog post has some ideas that might allow them https://smallcultfollowing.com/babysteps/blog/2024/03/04/borrow-checking-without-lifetimes/ https://smallcultfollowing.com/babysteps/blog/2024/03/04/bor... The hardest problems I think is that this requires a fundamental change to the language that something might be already borrowed just by existing. It also likely needs to introduce a notion of "stable" places that will not move when you move a prefix of them, e.g. to allow moving a `Box` holding self-referential data. In other words, there's a lot of bikeshedding to do.
afdbcreid 1y ago
First, there are external crates offering safe self-referential structs. A builtin language feature will for sure be more convenient (and also probably more performant, as most of those crates require an allocation) but it's just really hard to design. Even in very old self-referential crates soundness issues are still being discovered to this day. So it will need to a lot of time and care to design.

prngl 1y ago

> The last time I used [a doubly linked list] was in the nineties. I know the Linux kernel uses them, but that design was also laid down in the nineties; if you were designing a kernel from scratch today, you would probably not do it that way. This and sibling comments about the supposed uselessness or outdatedness of linked lists taught me something about the disconnect between many systems engineers resistant to Rust and many members of the (evangelical) Rust community. To address the technical topic first: a doubly linked list is what you reach for when you’re looking for an intrusive data structure with fast append and fast delete. You need to iterate over it, but don’t need random access. Queues where you may need to remove elements. Think a list of completions that can be cancelled. Why an intrusive data structure? Because you don’t want to, or can’t allocate. Because you want only a non-owning reference to the object, and you don’t want to have to manage any other lifetimes or dynamic (potentially unbounded, or arbitrarily bounded) allocations. Intrusive data structures are a great tool to minimize allocations, and to keep allocations (and ownership) logically separated across modules. And now onto the cultural topic. That a member of the Rust community might not know this (which is of course totally fine) is what taught me something, which was maybe obvious to many: Rust brings safety to a C++ style language and audience. For the same reasons many dislike C++, many dislike Rust. For the same reasons many would opt for C over C++ when they have the choice (either across all their projects or for certain projects), many continue to opt for C over Rust. Rust will not be taking over systems programming for the same reasons C++ did not. Well, by the numbers, it probably did (and Rust probably will too), so maybe better to say that it will not stamp out C for systems programming for the same reasons C++ did not. And it’s not just because of the stubbornness of C programmers. Systems programming has 2 cultures and practice groups: the C camp, and the C++ camp. The C++ camp is a big tent. I’d argue it includes Rust and Swift (and maybe D). Zig belongs to the C camp. Safety is important, and maybe Rust has the right model for it, but it is not a solution for the C camp. It likely did not intend to be, and again, by the numbers, replacing C++ is probably the more important task.

alfiedotwtf 1y ago
> To address the technical topic first: a doubly linked list is what you reach for when you’re looking for an intrusive data structure with fast append and fast delete. You need to iterate over it, but don’t need random access. Queues where you may need to remove elements. Think a list of completions that can be cancelled. > > Why an intrusive data structure? Because you don’t want to, or can’t allocate. Because you want only a non-owning reference to the object, and you don’t want to have to manage any other lifetimes or dynamic (potentially unbounded, or arbitrarily bounded) allocations. Intrusive data structures are a great tool to minimize allocations, and to keep allocations (and ownership) logically separated across modules. If you really wanted this in Rust, you could probably get away with just initialising a Vec::with_capacity(), then have each element’s next and prev be indexes into the vec.

andoando 1y ago

I am really confused here. 1. Implementing a linked list is a one and done deal and should be included in a (standard) library. 2. Would a classic pointer based implementation of a linked list in C really be a cause of security vulnerability??

gaanbal 1y ago
yea it's already in the standard library and it's very straightforward
- vacuity 1y ago
  The one in the standard library is non-intrusive and allocates space on the heap per node. In situations where a linked list implementation, in Rust code, no less, is deemed necessary, alternatives should probably be used.

Wowfunhappy 1y ago

The only Rust programs I've written are simple toys, and I don't know enough computer science to fully understand the problem space here. However, the first thing that comes to mind is, what if you just used unsafe for your doubly linked list? I understand this defeats the point of using Rust. But if the argument is "I'm using C because I actually need doubly linked lists and Rust makes that hard/slow", well C is also unsafe, so using Rust with an unsafe block for your doubly linked list seems like an improvement. I feel like at least part of the reason not to do this is "because the Rust community will yell at you." Well, maybe they shouldn't do that? If you were using C no one would care, so switching to unsafe Rust shouldn't imply the sky is falling.

vacuity 1y ago
Unsafe is quite plainly the right tool for the job. Yes, people can mess up unsafe code, but that really is just how things are (until we get magic formally verifying compilers?!). I appreciate your levelheaded take, rather than advocating for only C or exaggerated concern for safe Rust.
School-Cotton 1y ago
Why would the rust community “yell at you” for using unsafe somewhere where it’s clearly needed? By the way, unsafe doesn’t defeat the purpose of rust. The entire point of rust is that it lets you build safe abstractions on top of unsafe code, not that it eliminates unsafe code entirely.
rstuart4133 1y ago
The answer to "why not use unsafe" is it's hard to do correctly. What you have to do is pretty well documented https://doc.rust-lang.org/nomicon/working-with-unsafe.html https://doc.rust-lang.org/nomicon/working-with-unsafe.html but it's fiddly, tedious, easy to get wrong (you have to understand what the compiler is doing internally), and there are no automated checks to verify you have got it right. A newbie has almost no hope of getting right in the first 5 tries (5 is entirely made up.) But, Rust already provides a linked list in it's standard library: https://doc.rust-lang.org/std/collections/struct.LinkedList.html https://doc.rust-lang.org/std/collections/struct.LinkedList.... So a Rust programmer might say there is no need use unsafe if you want a linked list. There is a "source" link on that page, so it isn't hard to see how it's done. Yes, it does use "unsafe". Where it gets a little complex is a C programmer will look at that implementation and scoff at the linked list the Rust standard library provides. Amazingly the C and Rust programmers agree that this implementation is mostly useless. We know that because the Rust documentation says: "NOTE: It is almost always better to use Vec or VecDeque because array-based containers are generally faster, more memory efficient, and make better use of CPU cache [than this Rust linked list]." When a C programmer uses a linked list, he almost invariably uses what someone above termed above an "intrusive" implementation. An intrusive implementation is typically faster, more memory efficient, and makes just as good use of the CPU cache as the Rust Vec or VecDeque. An intrusive implementation reserves a bit of memory inside of the object you want to put in a linked list for the linked list code. That memory is typically "owned" by the linked list code, as in it manages it's lifetime, whereas the lifetime of the object the memory is in is managed by some other mechanism. For example, if you delete some unrelated item it list, it might have to reach in and modify the linked list pointer in this item. But Rust has a simple memory model: if you modify an object you have to prove you own it. The linked list code reaching into another object can not do that. That's one example, but intrusive linked lists break Rust's ownership and borrowing rules in so many ways, they are like oil and water. As far as I can tell there is no way to provide an intrusive linked list library in Rust that is safe to use, which is to say all code that uses it would have to be flagged as unsafe too. This probably raises as more questions that it answered, and you are probably wondering how could two implementations of the same thing (a linked list) be so different. I'm not going to go down that rabbit hole, other than to remark it's clear a lot of Rust programmers commenting here don't understand the flexibility the intrusive method gives you. But their instincts are right about one thing: they are dangerous. It's the sort of danger a C programmer is entirely comfortable with, but that same danger is what has lead to C programmers being responsible for a of CVE's. They are right about another thing too: if you put a bit of thought into it, you can usually come up with something that is for all practical purposes just as fast, and the Rust compiler can prove is safe. Sometimes though, "a bit of thought" is an understatement, and sometimes it feels like you are in a straight jacket. The article code be read is someone straining against that straight jacket, doing a lot of thinking and coming up with what a C programmer would call a fairly ugly (and slower) solution. The old C programmer in me says that's a fair summary. But I also have to acknowledge it is a very safe solution.

alazoral 1y ago

What I find is everything that I find interesting is cyclic, and so Rust has poor ergonomics for writing things I’m interested in. This is sad for me because it has the best compiler errors in the business. If I could get Swift with Rust’s compiler errors (Rust’s compiler in general is far better behaved) I’d be a happy chap.