Crate sptr

source · [−]

Expand description

This library provides a stable polyfill for Rust’s Strict Provenance experiment.

Mapping to STD APIs:

This crate “overlays” a bunch of unstable std apis, here are the mappings:

core::ptr (sptr)

pub fn invalid<T>(addr: usize) -> *const T;
pub fn invalid_mut<T>(addr: usize) -> *mut T;
pub fn from_exposed_addr<T>(addr: usize) -> *const T;
pub fn from_exposed_addr_mut<T>(addr: usize) -> *mut T;

core::pointer (sptr::Strict)

pub fn addr(self) -> usize;
pub fn expose_addr(self) -> usize;
pub fn with_addr(self, addr: usize) -> Self;
pub fn map_addr(self, f: impl FnOnce(usize) -> usize) -> Self;

NON-STANDARD EXTENSIONS (disabled by default, use at your own risk)

sptr::uptr (feature = uptr)
sptr::iptr (feature = uptr)
sptr::OpaqueFnPtr (feature = opaque_fn)

Swapping between sptr and core::ptr should be as simple as switching between sptr:: and ptr:: for static functions. For methods, you must import sptr::Strict into your module for the extension trait’s methods to overlay std. The compiler will (understandably) complain that you are overlaying std, so you will need to also silence that as seen in the following example:

#![allow(unstable_name_collisions)]
use sptr::Strict;

let ptr = sptr::invalid_mut::<u8>(1);
println!("{}", ptr.addr());

By default, this crate will also mark methods on pointers as “deprecated” if they are incompatible with strict_provenance. If you don’t want this, set default-features = false in your Cargo.toml.

Rust is the canonical source of definitions for these APIs and semantics, but the docs here will vaguely try to mirror the docs checked into Rust.

The following explanation of the model should also appear at the top of core::ptr:

Strict Provenance

The following text is non-normative, insufficiently formal, and is an extremely strict interpretation of provenance. It’s ok if your code doesn’t strictly conform to it.

Strict Provenance is an experimental set of APIs that help tools that try to validate the memory-safety of your program’s execution. Notably this includes Miri and CHERI, which can detect when you access out of bounds memory or otherwise violate Rust’s memory model.

Provenance must exist in some form for any programming language compiled for modern computer architectures, but specifying a model for provenance in a way that is useful to both compilers and programmers is an ongoing challenge. The Strict Provenance experiment seeks to explore the question: what if we just said you couldn’t do all the nasty operations that make provenance so messy?

What APIs would have to be removed? What APIs would have to be added? How much would code have to change, and is it worse or better now? Would any patterns become truly inexpressible? Could we carve out special exceptions for those patterns? Should we?

A secondary goal of this project is to see if we can disamiguate the many functions of pointer<->integer casts enough for the definition of usize to be loosened so that it isn’t pointer-sized but address-space/offset/allocation-sized (we’ll probably continue to conflate these notions). This would potentially make it possible to more efficiently target platforms where pointers are larger than offsets, such as CHERI and maybe some segmented architecures.

Provenance

This section is non-normative and is part of the Strict Provenance experiment.

Pointers are not simply an “integer” or “address”. For instance, it’s uncontroversial to say that a Use After Free is clearly Undefined Behaviour, even if you “get lucky” and the freed memory gets reallocated before your read/write (in fact this is the worst-case scenario, UAFs would be much less concerning if this didn’t happen!). To rationalize this claim, pointers need to somehow be more than just their addresses: they must have provenance.

When an allocation is created, that allocation has a unique Original Pointer. For alloc APIs this is literally the pointer the call returns, and for local variables and statics, this is the name of the variable/static. This is mildly overloading the term “pointer” for the sake of brevity/exposition.

The Original Pointer for an allocation is guaranteed to have unique access to the entire allocation and only that allocation. In this sense, an allocation can be thought of as a “sandbox” that cannot be broken into or out of. Provenance is the permission to access an allocation’s sandbox and has both a spatial and temporal component:

Spatial: A range of bytes that the pointer is allowed to access.
Temporal: The lifetime (of the allocation) that access to these bytes is tied to.

Spatial provenance makes sure you don’t go beyond your sandbox, while temporal provenance makes sure that you can’t “get lucky” after your permission to access some memory has been revoked (either through deallocations or borrows expiring).

Provenance is implicitly shared with all pointers transitively derived from The Original Pointer through operations like offset, borrowing, and pointer casts. Some operations may shrink the derived provenance, limiting how much memory it can access or how long it’s valid for (i.e. borrowing a subfield and subslicing).

Shrinking provenance cannot be undone: even if you “know” there is a larger allocation, you can’t derive a pointer with a larger provenance. Similarly, you cannot “recombine” two contiguous provenances back into one (i.e. with a fn merge(&[T], &[T]) -> &[T]).

A reference to a value always has provenance over exactly the memory that field occupies. A reference to a slice always has provenance over exactly the range that slice describes.

If an allocation is deallocated, all pointers with provenance to that allocation become invalidated, and effectively lose their provenance.

The strict provenance experiment is mostly only interested in exploring stricter spatial provenance. In this sense it can be thought of as a subset of the more ambitious and formal Stacked Borrows research project, which is what tools like Miri are based on. In particular, Stacked Borrows is necessary to properly describe what borrows are allowed to do and when they become invalidated. This necessarily involves much more complex temporal reasoning than simply identifying allocations. Adjusting APIs and code for the strict provenance experiment will also greatly help Stacked Borrows.

Pointer Vs Addresses

This section is non-normative and is part of the Strict Provenance experiment.

One of the largest historical issues with trying to define provenance is that programmers freely convert between pointers and integers. Once you allow for this, it generally becomes impossible to accurately track and preserve provenance information, and you need to appeal to very complex and unreliable heuristics. But of course, converting between pointers and integers is very useful, so what can we do?

Also did you know WASM is actually a “Harvard Architecture”? As in function pointers are handled completely differently from data pointers? And we kind of just shipped Rust on WASM without really addressing the fact that we let you freely convert between function pointers and data pointers, because it mostly Just Works? Let’s just put that on the “pointer casts are dubious” pile.

Strict Provenance attempts to square these circles by decoupling Rust’s traditional conflation of pointers and usize (and isize), and defining a pointer to semantically contain the following information:

The address-space it is part of.
The address it points to, which can be represented by a usize.
The provenance it has, defining the memory it has permission to access.

Under Strict Provenance, a usize cannot accurately represent a pointer, and converting from a pointer to a usize is generally an operation which only extracts the address. It is therefore impossible to construct a valid pointer from a usize because there is no way to restore the address-space and provenance. In other words, pointer-integer-pointer roundtrips are not possible (in the sense that the resulting pointer is not dereferencable).

The key insight to making this model at all viable is the with_addr method:

    /// Creates a new pointer with the given address.
    ///
    /// This performs the same operation as an `addr as ptr` cast, but copies
    /// the *address-space* and *provenance* of `self` to the new pointer.
    /// This allows us to dynamically preserve and propagate this important
    /// information in a way that is otherwise impossible with a unary cast.
    ///
    /// This is equivalent to using `wrapping_offset` to offset `self` to the
    /// given address, and therefore has all the same capabilities and restrictions.
    pub fn with_addr(self, addr: usize) -> Self;

So you’re still able to drop down to the address representation and do whatever clever bit tricks you want as long as you’re able to keep around a pointer into the allocation you care about that can “reconstitute” the other parts of the pointer. Usually this is very easy, because you only are taking a pointer, messing with the address, and then immediately converting back to a pointer. To make this use case more ergonomic, we provide the map_addr method.

To help make it clear that code is “following” Strict Provenance semantics, we also provide an addr method which promises that the returned address is not part of a pointer-usize-pointer roundtrip. In the future we may provide a lint for pointer<->integer casts to help you audit if your code conforms to strict provenance.

Using Strict Provenance

Most code needs no changes to conform to strict provenance, as the only really concerning operation that wasn’t obviously already Undefined Behaviour is casts from usize to a pointer. For code which does cast a usize to a pointer, the scope of the change depends on exactly what you’re doing.

In general you just need to make sure that if you want to convert a usize address to a pointer and then use that pointer to read/write memory, you need to keep around a pointer that has sufficient provenance to perform that read/write itself. In this way all of your casts from an address to a pointer are essentially just applying offsets/indexing.

This is generally trivial to do for simple cases like tagged pointers as long as you represent the tagged pointer as an actual pointer and not a usize. For instance:

// #![feature(strict_provenance)]
#![allow(unstable_name_collisions)]
use sptr::Strict;

unsafe {
    // A flag we want to pack into our pointer
    static HAS_DATA: usize = 0x1;
    static FLAG_MASK: usize = !HAS_DATA;

    // Our value, which must have enough alignment to have spare least-significant-bits.
    let my_precious_data: u32 = 17;
    assert!(core::mem::align_of::<u32>() > 1);

    // Create a tagged pointer
    let ptr = &my_precious_data as *const u32;
    let tagged = ptr.map_addr(|addr| addr | HAS_DATA);

    // Check the flag:
    if tagged.addr() & HAS_DATA != 0 {
        // Untag and read the pointer
        let data = *tagged.map_addr(|addr| addr & FLAG_MASK);
        assert_eq!(data, 17);
    } else {
        unreachable!()
    }
}

(Yes, if you’ve been using AtomicUsize for pointers in concurrent datastructures, you should be using AtomicPtr instead. If that messes up the way you atomically manipulate pointers, we would like to know why, and what needs to be done to fix it.)

Something more complicated and just generally evil like a XOR-List requires more significant changes like allocating all nodes in a pre-allocated Vec or Arena and using a pointer to the whole allocation to reconstitute the XORed addresses.

Situations where a valid pointer must be created from just an address, such as baremetal code accessing a memory-mapped interface at a fixed address, are an open question on how to support. These situations will still be allowed, but we might require some kind of “I know what I’m doing” annotation to explain the situation to the compiler. It’s also possible they need no special attention at all, because they’re generally accessing memory outside the scope of “the abstract machine”, or already using “I know what I’m doing” annotations like “volatile”.

Under Strict Provenance is is Undefined Behaviour to:

Access memory through a pointer that does not have provenance over that memory.
offset a pointer to or from an address it doesn’t have provenance over. This means it’s always UB to offset a pointer derived from something deallocated, even if the offset is 0. Note that a pointer “one past the end” of its provenance is not actually outside its provenance, it just has 0 bytes it can load/store.

But it is still sound to:

Create an invalid pointer from just an address (see ptr::invalid). This can be used for sentinel values like null or to represent a tagged pointer that will never be dereferencable. In general, it is always sound for an integer to pretend to be a pointer “for fun” as long as you don’t use operations on it which require it to be valid (offset, read, write, etc).
Forge an allocation of size zero at any sufficiently aligned non-null address. i.e. the usual “ZSTs are fake, do what you want” rules apply but this only applies for actual forgery (integers cast to pointers). If you borrow some struct’s field that happens to be zero-sized, the resulting pointer will have provenance tied to that allocation and it will still get invalidated if the allocation gets deallocated. In the future we may introduce an API to make such a forged allocation explicit.
wrapping_offset a pointer outside its provenance. This includes invalid pointers which have “no” provenance. Unfortunately there may be practical limits on this for a particular platform, and it’s an open question as to how to specify this (if at all). Notably, CHERI relies on a compression scheme that can’t handle a pointer getting offset “too far” out of bounds. If this happens, the address returned by addr will be the value you expect, but the provenance will get invalidated and using it to read/write will fault. The details of this are architecture-specific and based on alignment, but the buffer on either side of the pointer’s range is pretty generous (think kilobytes, not bytes).
Compare arbitrary pointers by address. Addresses are just integers and so there is always a coherent answer, even if the pointers are invalid or from different address-spaces/provenances. Of course, comparing addresses from different address-spaces is generally going to be meaningless, but so is comparing Kilograms to Meters, and Rust doesn’t prevent that either. Similarly, if you get “lucky” and notice that a pointer one-past-the-end is the “same” address as the start of an unrelated allocation, anything you do with that fact is probably going to be gibberish. The scope of that gibberish is kept under control by the fact that the two pointers still aren’t allowed to access the other’s allocation (bytes), because they still have different provenance.
Perform pointer tagging tricks. This falls out of wrapping_offset but is worth mentioning in more detail because of the limitations of CHERI. Low-bit tagging is very robust, and often doesn’t even go out of bounds because types ensure size >= align (and over-aligning actually gives CHERI more flexibility). Anything more complex than this rapidly enters “extremely platform-specific” territory as certain things may or may not be allowed based on specific supported operations. For instance, ARM explicitly supports high-bit tagging, and so CHERI on ARM inherits that and should support it.

Pointer-usize-pointer roundtrips and ‘exposed’ provenance

This section is non-normative and is part of the Strict Provenance experiment.

As discussed above, pointer-usize-pointer roundtrips are not possible under Strict Provenance. However, there exists legacy Rust code that is full of such roundtrips, and legacy platform APIs regularly assume that usize can capture all the information that makes up a pointer. There also might be code that cannot be ported to Strict Provenance (which is something we would like to hear about).

For situations like this, there is a fallback plan, a way to ‘opt out’ of Strict Provenance. However, note that this makes your code a lot harder to specify, and the code will not work (well) with tools like Miri and CHERI.

This fallback plan is provided by the expose_addr and from_exposed_addr methods (which are equivalent to as casts between pointers and integers). expose_addr is a lot like addr, but additionally adds the provenance of the pointer to a global list of ‘exposed’ provenances. (This list is purely conceptual, it exists for the purpose of specifying Rust but is not materialized in actual executions, except in tools like Miri.) from_exposed_addr can be used to construct a pointer with one of these previously ‘exposed’ provenances. from_exposed_addr takes only addr: usize as arguments, so unlike in with_addr there is no indication of what the correct provenance for the returned pointer is – and that is exactly what makes pointer-usize-pointer roundtrips so tricky to rigorously specify! There is no algorithm that decides which provenance will be used. You can think of this as “guessing” the right provenance, and the guess will be “maximally in your favor”, in the sense that if there is any way to avoid undefined behavior, then that is the guess that will be taken. However, if there is no previously ‘exposed’ provenance that justifies the way the returned pointer will be used, the program has undefined behavior.

Using expose_addr or from_exposed_addr (or the equivalent as casts) means that code is not following Strict Provenance rules. The goal of the Strict Provenance experiment is to determine whether it is possible to use Rust without expose_addr and from_exposed_addr. If this is successful, it would be a major win for avoiding specification complexity and to facilitate adoption of tools like CHERI and Miri that can be a big help in increasing the confidence in (unsafe) Rust code.