Undefined Behavior With Box And Once_map In Rust

Hey guys! Today, we're diving into a tricky situation involving Rust's once_map crate, Box values, and the Miri interpreter. It turns out that combining these elements can lead to undefined behavior, which is definitely something we want to avoid in our code. Let's break down the issue and explore potential solutions.

The Problem: Miri Flags Undefined Behavior

When you're working with Rust, you want to ensure your code is not only functional but also safe. Miri, a Rust interpreter and a part of the Clippy tool, helps catch undefined behavior at compile time. Consider the following code snippet:

use once_map::OnceMap;

fn main() {
 let map = OnceMap::new();
 map.insert(0, |_| Box::new(0));
}

This simple program uses the once_map crate to insert a Box<i32> into the map. However, when you run this code with Miri, you might encounter an error indicating undefined behavior. The error message points to a retagging issue within the once_map crate's synchronization module (src/sync.rs).

The core of the problem lies in how once_map handles the Box value internally. Miri's error message suggests that a SharedReadOnly retag is being attempted on a memory location, but the required tag is missing from the borrow stack. This violation of Stacked Borrows rules (an experimental feature in Rust) signals a potential bug. Specifically, Miri indicates that the retagging at alloc26540[0x0..0x4] is failing, which involves a SharedReadOnly retag happening during the insert operation on the OnceMap.

Deep Dive into the Error

To better understand the error, let's dissect the key components:

• OnceMap: This is a concurrent map that ensures each key is initialized only once. It's designed for scenarios where you need to lazily initialize values in a thread-safe manner.
• Box<i32>: Box is a smart pointer that allocates memory on the heap. It provides ownership and ensures that the allocated memory is deallocated when the Box goes out of scope.
• Miri: As mentioned earlier, Miri is a Rust interpreter that detects undefined behavior by checking memory access patterns and borrow rules at runtime. It is an invaluable tool for writing safe and correct Rust code.
• Stacked Borrows: This is an experimental memory model used by Miri to track borrows and detect potential data races and memory safety violations. Stacked Borrows maintains a stack of active borrows for each memory location, and operations like retagging must adhere to specific rules.

The error arises because of the interaction between once_map's internal synchronization mechanisms and the Box's memory management. The insert operation involves retagging memory, and if the borrow stack isn't in the expected state, Miri flags it as undefined behavior.

Possible Causes and Connections

The error message in the original report suggests a potential link to an issue in the stable_deref_trait crate. The stable_deref_trait crate provides a trait called StableDeref, which indicates that a type's Deref implementation (used for dereferencing) maintains a stable memory address. However, there are known issues when StableDeref is implemented for types like Box because the underlying memory might be reallocated or moved, invalidating the stability guarantee. It is important to use stable deref in your application.

The stable_deref_trait Connection

The maintainers of stable_deref_trait are aware of these issues, particularly concerning types like Box. When a Box is reallocated (e.g., when resizing a Vec<Box<T>>), the memory address of the contained T might change. This invalidates the StableDeref guarantee and can lead to undefined behavior if other parts of the code rely on the address remaining constant.

The once_map crate might be relying on StableDeref assumptions internally, which could explain why using Box triggers the Miri error. If once_map assumes that the memory address of the inserted value remains stable, it could lead to incorrect memory access and violations of Rust's borrow rules.

Potential Solutions and Mitigation Strategies

Given the potential undefined behavior, it's crucial to address this issue if you're using once_map with Box values. Here are a few strategies to consider:

1. Avoid Storing Box Directly: Instead of storing Box<T> directly in the once_map, consider storing T and wrapping it in a Box only when needed. This can reduce the potential for memory-related issues within once_map.

2. Custom StableDeref: As suggested in the original report, the once_map crate could define its own version of StableDeref that excludes Box. This would prevent once_map from making incorrect assumptions about the stability of Box's memory address.

3. Use Alternative Data Structures: If possible, explore alternative data structures that don't rely on StableDeref assumptions or have better support for Box. For example, you could use a regular HashMap protected by a Mutex, although this might have performance implications.

4. Careful Memory Management: Review the code that interacts with once_map and Box to ensure that memory is being managed correctly. Pay close attention to any operations that might cause memory reallocation or movement.

Code Example: Avoiding Direct Box Storage

Here's an example of how you can avoid storing Box directly in the once_map:

use once_map::OnceMap;

fn main() {
 let map: OnceMap<i32, i32> = OnceMap::new();
 map.insert(0, |_| 0); // Store i32 directly

 // Wrap in Box when needed
 let boxed_value = map.get(&0).map(|value| Box::new(*value));

 match boxed_value {
 Some(boxed) => println!("Value: {}", boxed),
 None => println!("Value not found"),
 }
}

In this example, we store i32 directly in the OnceMap instead of Box<i32>. When we need a Box, we create it on demand using the value from the map. This approach avoids potential issues with memory management inside the OnceMap.

Conclusion: Navigating Undefined Behavior

Encountering undefined behavior can be a daunting experience, but with tools like Miri and a deeper understanding of Rust's memory model, you can identify and address these issues effectively. In the case of once_map and Box, it's essential to be aware of the potential pitfalls and consider alternative approaches to ensure your code remains safe and reliable.

By understanding the interplay between once_map, Box, and stable_deref_trait, you can write more robust and maintainable Rust code. Keep experimenting, keep learning, and don't be afraid to dive deep into the intricacies of Rust's memory management. Happy coding, folks!

For more information on Rust's memory model, check out the official Rust documentation on ownership and borrowing.