Comment by ttfkam | Hacker News Reader

Comment by ttfkam | Hacker News Reader

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Every time I hear the term "lowest common denominator" applied to programmers, I reflect on the number of network-accessible remote exploits that provide root access via the Linux kernel over the past three decades.

And yet the folks maintaining the network stack in the Linux kernel are not anywhere near what I or almost anyone else would consider the lowest common denominator programmer.

Sometimes you just have to accept that ALL humans are inherently bad at this, only some are worse than others at various points in their lives. The security and reliability of the product of our output should not be so largely dependent upon our ability to get a full night's sleep, reasonable working hours, and lack of personal conflict. To be sure, we should all strive for those things, but perhaps the choice of tools is correct in prioritizing mitigation of difficulties and vulnerabilities from meatspace.

Sometimes that's a borrow checker. Other times it's a garbage collector. And every once in a while, we need to break out an unsafe block (or the entire C language) to get a job done.

Considering how much more expensive people are than software or hardware, the trade off for easier/simpler access is increasingly straightforward.

While I strongly agree this doesn't seem to match the sentiment of the paper.

Which is prioritizing a lower barrier to entry for new programmers at the cost of safety checks that help prevent humans from making mistakes at runtime.

It seems to me if our conclusion is humans, no matter how skilled, are flawed, we should be willing to do absolutely anything to lower the barrier to entry except removing checks on correctness.

What makes you say that? What safety or correctness features do you think a garbage collector in Rust eschews? On the contrary, gc should make the language safer since it obviates the need for the "unsafe" escape hatch.

See https://users.rust-lang.org/t/bronze-gc-and-aliasing-problem.... The GC allows use after free bugs that are compile time exceptions in Rust.

But the gc is there to test the usability cost of Rust's memory management scheme. A properly written gc would of course not allow for use-after-free bugs.

The problem is that this study is relatively worthless, because the API used by this GC inherently introduces UB. If you have a correct API, the usability tradeoff might be very different.

Can you please point to some results in the study that would have been different if the tracing gc hadn't suffered from use-after-free bugs?

To phrase what my sibling commentor said but in a different way:

This paper is an experiment to see if a particular API could make using Rust easier. This specific API is not sound. This means that the paper's results are kind of irrelevant in a strict sense; this API cannot work in Rust, so it being easier is kind of a moot point. Maybe someone can take their API, make it sound, and then try again, and that's certainly good. But it is a serious flaw in the methodology.

Specifically, this API call is unsound:

  GcRefCell<T>::as_mut(&self) -> &mut T

It's not about the internals of the GC. It's about the API that it exposes to users.

The author has some ideas: https://github.com/mcoblenz/Bronze/issues/2#issuecomment-939... These ones are straightforward, and would probably work, but they would also introduce some friction, and removing friction is the entire point of the exercise. Is it too much? Maybe! It also maybe isn't. You'd need another study to figure that out.

> This paper is an experiment to see if a particular API could make using Rust easier.

According to the author: "The Bronze project is exploring the usability costs of the restrictions that Rust imposes."

That does not require a sound gc to explore. Maybe it would have been different had the goal been to see whether a gc can be bolted onto Rust. Yes, that might be difficult given Rust's semantics and how to deals with pointers. But that doesn't seem to be the goal of the author's research.

Again, it's not about the GC being sound, it's about the API that's being used. There's (imho) not a lot of value in figuring out if an API that can't be used is usable.

There are other GC crates that offer sound APIs. Research using those sounds very promising and good!

Then explain to me why it matters. If none of the toy examples in the research actually required multiple mutable references to the same objects, then why would it matter that the compiler allowed it? I think you missed the point of the research: to "explor[e] the usability costs of the restrictions that Rust imposes."

If they didn’t require it, then that would significantly strengthen the paper! They could have not exposed that API and it would be much more useful. I suspect that they probably did need it though. An immutable GC would only be useful by adding an interior mutability wrapper, which is exactly the kind of friction the paper is trying to avoid.

It matters because the rest of the Rust universe follows this pattern. If they coded their own standard library (and any other allowed libraries) and modified the compiler to not miscompile this usage, then that exploration would be valid. But you’re also going to get friction from miscompiles and fighting with other APIs that do have this property. So it would be more useful to either go all-out ok this idea that Rust’s uniqueness properties are a bad thing, or fix the soundness issues. The halfway step simply confounds too many issues to be truly useful, in my opinion.

I agree that exploring the space is a good idea and useful. My issue is with the methodology, not the concept.

The article does include a thorough description of the tasks. Afaict, none of them requires multiple mutable references to the same objects. It would be strange if any task did as it then wouldn't be implementable in plain Rust.

I think you are missing the point of this research. The author has, given his large sample size, persuasively demonstrated that Rust's borrow checker is confusing to newbies. Data from 428 students is a lot and an order of a magnitude more than many other programming language usability studies. This is in my opinion interesting research, even though I understand why Rust fans doesn't like his results!

> Afaict, none of them requires multiple mutable references to the same objects

They ask to store objects ("turtles") into a single Vec. Two turtles from that Vec can breed to create a child turtle stored in that same vec. Parents must retain a reference (a real ref, not an index or other workaround) to their children, meaning that children have multiple refs. Children can become parents themselves, so all the turtles are mutable.

There you have it: multiple mutable references to the same object. With proper Rust you'll need some kind of RefCell to implement this (convoluted) design. The runtime check will ensure a runtime panic if you try to make the same object mutable via different RefCells (trying to breed a turle with itself). With BronzeGc the compiler will believe that they are different objects, and UB-optimize accordingly.

I don’t think you understand why, given that as myself and several people have stated, the issue isn’t with the results. It’s with the methods. I have no opinion either way about the results, because as far as I can tell, they result from an incoherent premise, which means the conclusion is also incoherent.

> An immutable GC would only be useful by adding an interior mutability wrapper, which is exactly the kind of friction the paper is trying to avoid.

This would be the most idiomatic approach, yes. They could use a code transformation to automate the 'wrapping' and present an intuitive, frictionless interface to the developer.

Totally, and that’s one of the ideas floated in the issue. I think depending on how it could be done it may not even lost out on ergonomics in most cases, but I haven’t worked through it myself.

The problem isn't that the GC suffered from use after free. The problem is that the GC allowed the user to have 2 references to a mutable object which is UB in Rust (and a compile time error without unsafe code). The problem with the API is that without some pretty fundamental changes to the language, the only times a GC doesn't error, and Rust does is when the user tries to do undefined behavior. Removing the UB is also pretty much impossible because if you allow these references, you remove one of the main tools the Rust compiler uses to optimize code.

Having simultaneous mutable references to an object is UB in Rust because the memory manager cannot detect use-after-free errors. What you are talking about is not a feature but a limitation of Rust's borrow checker. But when using a gc multiple multiple references it not a problem at all so there is no reason to prevent it. This has almost nothing to do with performance. Restricting the number of mutable references to one does not mean that the compiler can emit significantly faster code.

> Having simultaneous mutable references to an object is UB in Rust

This is correct.

> because the memory manager cannot detect use-after-free errors.

... this is not. It is UB because the language declares it UB. It is absolutely to the core of the design of the language itself. All Rust code relies on this property to work. Something that breaks it is in fact broken, regardless of any other aspect of the program.

Quoting the article:

> those who [used the GC] required only about a third as much time (4 hours vs. 12 hours).

Surely you can devote a chunk of your newfounded time to find use-after-free bugs.

If that worked, people wouldn't still be finding use after free bugs in commonly used C libraries. The whole reason to use Rust is that it turns most of C's UB into compile time errors so that your code isn't horribly broken in the first place.

I don't know how to find use-after-free bugs in arbitrary code, no matter how much time I devote to the task. I just can't keep enough state in my head at once.

What does "use after free" mean in a garbage collected environment? Isn't the purpose of GC to automatically free after last use.

It means the GC design is fundamentally broken.

> A key tradeoff is that Bronze does not guarantee thread safety

Data races are currently prevented by the borrow checker. Any GC in order to provide equivalent correctness would need to do so as well.

If you read the paper, they needed to throw out the thread safety from Rust's borrow checker in order to make the GC work. That is a massive surface area for bugs they are opening back up to make the language easier for beginners.

I just added a clarification to the README about this. The main issue is that the current implementation keeps track of roots with a shadow stack technique (https://llvm.org/docs/GarbageCollection.html#using-llvm-gcwr...), which is not thread-safe. It wasn't worth the engineering work for this particular study since the tasks didn't require more than one thread. A practical implementation, of course, would need to be thread-safe.

I'm sure the GC implementation itself could be made thread safe. But the paper mentions that Bronze lets you have multiple mutable references to an object. Doesn't this open the door to the user's code having data races, whereas it would be safe by default in vanilla Rust?

Because their gc is a proof-of-concept created specifically for this research. It doesn't even deallocate memory. Most likely, they didn't have their students write any threaded code so thread-safety wasn't a concern. For a production ready tracing gc, of course they would add thread safety, it's not a big problem.

No, this is the fundamental tradeoff they made to make the Bronze'd Rust easy to use:

> Rust permits only one mutable reference to a value at a time... With Bronze, mutation is permitted through all references to each garbage-collected object, with no extra effort. A key tradeoff is that Bronze does not guarantee thread safety; as in other garbage collected languages, it is the programmer’s responsibility to ensure safety.

Allowing mutability anywhere is what fundamentally makes Bronze easier to learn, and more error-prone.

The paper's author has already explained to you how thread safety can be achieved in a production-ready gc. This is not something that is particularly difficult to engineer and is quite orthogonal to whether one chooses to use tracing gc, ref counting or Rust-like borrow checking.

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