% The Unsafe Rust Programming Language

This document is about advanced functionality and low-level development practices in the Rust Programming Language. Most of the things discussed won't matter to the average Rust programmer. However if you wish to correctly write unsafe code in Rust, this text contains invaluable information.

This document seeks to complement The Rust Programming Language Book (TRPL). Where TRPL introduces the language and teaches the basics, TURPL dives deep into the specification of the language, and all the nasty bits necessary to write Unsafe Rust. TURPL does not assume you have read TRPL, but does assume you know the basics of the language and systems programming. We will not explain the stack or heap, we will not explain the syntax.

A Tale Of Two Languages

Rust can be thought of as two different languages: Safe Rust, and Unsafe Rust. Any time someone opines the guarantees of Rust, they are almost surely talking about Safe Rust. However Safe Rust is not sufficient to write every program. For that, we need the Unsafe Rust superset.

Most fundamentally, writing bindings to other languages (such as the C exposed by your operating system) is never going to be safe. Rust can't control what other languages do to program execution! However Unsafe Rust is also necessary to construct fundamental abstractions where the type system is not sufficient to automatically prove what you're doing is sound.

Indeed, the Rust standard library is implemented in Rust, and it makes substantial use of Unsafe Rust for implementing IO, memory allocation, collections, synchronization, and other low-level computational primitives.

Upon hearing this, many wonder why they would not simply just use C or C++ in place of Rust (or just use a "real" safe language). If we're going to do unsafe things, why not lean on these much more established languages?

The most important difference between C++ and Rust is a matter of defaults: Rust is 100% safe by default. Even when you opt out of safety in Rust, it is a modular action. In deciding to work with unchecked uninitialized memory, this does not suddenly make dangling or null pointers a problem. When using unchecked indexing on x, one does not have to suddenly worry about indexing out of bounds on y. C and C++, by contrast, have pervasive unsafety baked into the language. Even the modern best practices like unique_ptr have various safety pitfalls.

It should also be noted that writing Unsafe Rust should be regarded as an exceptional action. Unsafe Rust is often the domain of fundamental libraries. Anything that needs to make FFI bindings or define core abstractions. These fundamental libraries then expose a safe interface for intermediate libraries and applications to build upon. And these safe interfaces make an important promise: if your application segfaults, it's not your fault. They have a bug.

And really, how is that different from any safe language? Python, Ruby, and Java libraries can internally do all sorts of nasty things. The languages themselves are no different. Safe languages regularly have bugs that cause critical vulnerabilities. The fact that Rust is written with a healthy spoonful of Unsafe Rust is no different. However it does mean that Rust doesn't need to fall back to the pervasive unsafety of C to do the nasty things that need to get done.

What does unsafe mean?

Rust tries to model memory safety through the unsafe keyword. Interestingly, the meaning of unsafe largely revolves around what its absence means. If the unsafe keyword is absent from a program, it should not be possible to violate memory safety under any conditions. The presence of unsafe means that there are conditions under which this code could violate memory safety.

To be more concrete, Rust cares about preventing the following things:

• Dereferencing null/dangling pointers
• Reading uninitialized memory
• Breaking the pointer aliasing rules (TBD) (llvm rules + noalias on &mut and & w/o UnsafeCell)
• Invoking Undefined Behaviour (in e.g. compiler intrinsics)
• Producing invalid primitive values:
  • dangling/null references
  • a bool that isn't 0 or 1
  • an undefined enum discriminant
  • a char larger than char::MAX
  • A non-utf8 str
• Unwinding into an FFI function
• Causing a data race

That's it. That's all the Undefined Behaviour in Rust. Libraries are free to declare arbitrary requirements if they could transitively cause memory safety issues, but it all boils down to the above actions. Rust is otherwise quite permisive with respect to other dubious operations. Rust considers it "safe" to:

• Deadlock
• Leak memory
• Fail to call destructors
• Access private fields
• Overflow integers
• Delete the production database

However any program that does such a thing is probably incorrect. Rust just isn't interested in modeling these problems, as they are much harder to prevent in general, and it's literally impossible to prevent incorrect programs from getting written.

There are several places unsafe can appear in Rust today, which can largely be grouped into two categories:

• There are unchecked contracts here. To declare you understand this, I require you to write unsafe elsewhere:

  • On functions, unsafe is declaring the function to be unsafe to call. Users of the function must check the documentation to determine what this means, and then have to write unsafe somewhere to identify that they're aware of the danger.
  • On trait declarations, unsafe is declaring that implementing the trait is an unsafe operation, as it has contracts that other unsafe code is free to trust blindly.

• I am declaring that I have, to the best of my knowledge, adhered to the unchecked contracts:

  • On trait implementations, unsafe is declaring that the contract of the unsafe trait has been upheld.
  • On blocks, unsafe is declaring any unsafety from an unsafe operation within to be handled, and therefore the parent function is safe.

There is also #[unsafe_no_drop_flag], which is a special case that exists for historical reasons and is in the process of being phased out. See the section on destructors for details.

Some examples of unsafe functions:

• slice::get_unchecked will perform unchecked indexing, allowing memory safety to be freely violated.
• ptr::offset in an intrinsic that invokes Undefined Behaviour if it is not "in bounds" as defined by LLVM (see the lifetimes section for details).
• mem::transmute reinterprets some value as having the given type, bypassing type safety in arbitrary ways. (see the conversions section for details)
• All FFI functions are unsafe because they can do arbitrary things. C being an obvious culprit, but generally any language can do something that Rust isn't happy about. (see the FFI section for details)

As of Rust 1.0 there are exactly two unsafe traits:

• Send is a marker trait (it has no actual API) that promises implementors are safe to send to another thread.
• Sync is a marker trait that promises that threads can safely share implementors through a shared reference.

All other traits that declare any kind of contract really can't be trusted to adhere to their contract when memory-safety is at stake. For instance Rust has PartialOrd and Ord to differentiate between types which can "just" be compared and those that implement a total ordering. However you can't actually trust an implementor of Ord to actually provide a total ordering if failing to do so causes you to e.g. index out of bounds. But if it just makes your program do a stupid thing, then it's "fine" to rely on Ord.

The reason this is the case is that Ord is safe to implement, and it should be impossible for bad safe code to violate memory safety. Rust has traditionally avoided making traits unsafe because it makes unsafe pervasive in the language, which is not desirable. The only reason Send and Sync are unsafe is because thread safety is a sort of fundamental thing that a program can't really guard against locally (even by-value message passing still requires a notion Send).

Working with unsafe

Rust generally only gives us the tools to talk about safety in a scoped and binary manner. Unfortunately reality is significantly more complicated than that. For instance, consider the following toy function:

fn do_idx(idx: usize, arr: &[u8]) -> Option<u8> {
    if idx < arr.len() {
        unsafe {
            Some(*arr.get_unchecked(idx))
        }
    } else {
        None
    }
}

Clearly, this function is safe. We check that the index is in bounds, and if it is, index into the array in an unchecked manner. But even in such a trivial function, the scope of the unsafe block is questionable. Consider changing the < to a <=:

fn do_idx(idx: usize, arr: &[u8]) -> Option<u8> {
    if idx <= arr.len() {
        unsafe {
            Some(*arr.get_unchecked(idx))
        }
    } else {
        None
    }
}

This program is now unsound, an yet we only modified safe code. This is the fundamental problem of safety: it's non-local. The soundness of our unsafe operations necessarily depends on the state established by "safe" operations. Although safety is modular (we still don't need to worry about about unrelated safety issues like uninitialized memory), it quickly contaminates the surrounding code.

Trickier than that is when we get into actual statefulness. Consider a simple implementation of Vec:

// Note this defintion is insufficient. See the section on lifetimes.
struct Vec<T> {
    ptr: *mut T,
    len: usize,
    cap: usize,
}

// Note this implementation does not correctly handle zero-sized types.
// We currently live in a nice imaginary world of only positive fixed-size
// types.
impl<T> Vec<T> {
    fn push(&mut self, elem: T) {
        if self.len == self.cap {
            // not important for this example
            self.reallocate();
        }
        unsafe {
            ptr::write(self.ptr.offset(len as isize), elem);
            self.len += 1;
        }
    }
}

This code is simple enough to reasonably audit and verify. Now consider adding the following method:

    fn make_room(&mut self) {
        // grow the capacity
        self.cap += 1;
    }

This code is safe, but it is also completely unsound. Changing the capacity violates the invariants of Vec (that cap reflects the allocated space in the Vec). This is not something the rest of Vec can guard against. It has to trust the capacity field because there's no way to verify it.

unsafe does more than pollute a whole function: it pollutes a whole module. Generally, the only bullet-proof way to limit the scope of unsafe code is at the module boundary with privacy.