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Diffstat (limited to 'rust/kernel/types.rs')
| -rw-r--r-- | rust/kernel/types.rs | 389 |
1 files changed, 389 insertions, 0 deletions
diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs new file mode 100644 index 000000000..fdb778e65 --- /dev/null +++ b/rust/kernel/types.rs @@ -0,0 +1,389 @@ +// SPDX-License-Identifier: GPL-2.0 + +//! Kernel types. + +use crate::init::{self, PinInit}; +use alloc::boxed::Box; +use core::{ + cell::UnsafeCell, + marker::{PhantomData, PhantomPinned}, + mem::MaybeUninit, + ops::{Deref, DerefMut}, + ptr::NonNull, +}; + +/// Used to transfer ownership to and from foreign (non-Rust) languages. +/// +/// Ownership is transferred from Rust to a foreign language by calling [`Self::into_foreign`] and +/// later may be transferred back to Rust by calling [`Self::from_foreign`]. +/// +/// This trait is meant to be used in cases when Rust objects are stored in C objects and +/// eventually "freed" back to Rust. +pub trait ForeignOwnable: Sized { + /// Type of values borrowed between calls to [`ForeignOwnable::into_foreign`] and + /// [`ForeignOwnable::from_foreign`]. + type Borrowed<'a>; + + /// Converts a Rust-owned object to a foreign-owned one. + /// + /// The foreign representation is a pointer to void. + fn into_foreign(self) -> *const core::ffi::c_void; + + /// Borrows a foreign-owned object. + /// + /// # Safety + /// + /// `ptr` must have been returned by a previous call to [`ForeignOwnable::into_foreign`] for + /// which a previous matching [`ForeignOwnable::from_foreign`] hasn't been called yet. + unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> Self::Borrowed<'a>; + + /// Converts a foreign-owned object back to a Rust-owned one. + /// + /// # Safety + /// + /// `ptr` must have been returned by a previous call to [`ForeignOwnable::into_foreign`] for + /// which a previous matching [`ForeignOwnable::from_foreign`] hasn't been called yet. + /// Additionally, all instances (if any) of values returned by [`ForeignOwnable::borrow`] for + /// this object must have been dropped. + unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self; +} + +impl<T: 'static> ForeignOwnable for Box<T> { + type Borrowed<'a> = &'a T; + + fn into_foreign(self) -> *const core::ffi::c_void { + Box::into_raw(self) as _ + } + + unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> &'a T { + // SAFETY: The safety requirements for this function ensure that the object is still alive, + // so it is safe to dereference the raw pointer. + // The safety requirements of `from_foreign` also ensure that the object remains alive for + // the lifetime of the returned value. + unsafe { &*ptr.cast() } + } + + unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self { + // SAFETY: The safety requirements of this function ensure that `ptr` comes from a previous + // call to `Self::into_foreign`. + unsafe { Box::from_raw(ptr as _) } + } +} + +impl ForeignOwnable for () { + type Borrowed<'a> = (); + + fn into_foreign(self) -> *const core::ffi::c_void { + core::ptr::NonNull::dangling().as_ptr() + } + + unsafe fn borrow<'a>(_: *const core::ffi::c_void) -> Self::Borrowed<'a> {} + + unsafe fn from_foreign(_: *const core::ffi::c_void) -> Self {} +} + +/// Runs a cleanup function/closure when dropped. +/// +/// The [`ScopeGuard::dismiss`] function prevents the cleanup function from running. +/// +/// # Examples +/// +/// In the example below, we have multiple exit paths and we want to log regardless of which one is +/// taken: +/// ``` +/// # use kernel::types::ScopeGuard; +/// fn example1(arg: bool) { +/// let _log = ScopeGuard::new(|| pr_info!("example1 completed\n")); +/// +/// if arg { +/// return; +/// } +/// +/// pr_info!("Do something...\n"); +/// } +/// +/// # example1(false); +/// # example1(true); +/// ``` +/// +/// In the example below, we want to log the same message on all early exits but a different one on +/// the main exit path: +/// ``` +/// # use kernel::types::ScopeGuard; +/// fn example2(arg: bool) { +/// let log = ScopeGuard::new(|| pr_info!("example2 returned early\n")); +/// +/// if arg { +/// return; +/// } +/// +/// // (Other early returns...) +/// +/// log.dismiss(); +/// pr_info!("example2 no early return\n"); +/// } +/// +/// # example2(false); +/// # example2(true); +/// ``` +/// +/// In the example below, we need a mutable object (the vector) to be accessible within the log +/// function, so we wrap it in the [`ScopeGuard`]: +/// ``` +/// # use kernel::types::ScopeGuard; +/// fn example3(arg: bool) -> Result { +/// let mut vec = +/// ScopeGuard::new_with_data(Vec::new(), |v| pr_info!("vec had {} elements\n", v.len())); +/// +/// vec.try_push(10u8)?; +/// if arg { +/// return Ok(()); +/// } +/// vec.try_push(20u8)?; +/// Ok(()) +/// } +/// +/// # assert_eq!(example3(false), Ok(())); +/// # assert_eq!(example3(true), Ok(())); +/// ``` +/// +/// # Invariants +/// +/// The value stored in the struct is nearly always `Some(_)`, except between +/// [`ScopeGuard::dismiss`] and [`ScopeGuard::drop`]: in this case, it will be `None` as the value +/// will have been returned to the caller. Since [`ScopeGuard::dismiss`] consumes the guard, +/// callers won't be able to use it anymore. +pub struct ScopeGuard<T, F: FnOnce(T)>(Option<(T, F)>); + +impl<T, F: FnOnce(T)> ScopeGuard<T, F> { + /// Creates a new guarded object wrapping the given data and with the given cleanup function. + pub fn new_with_data(data: T, cleanup_func: F) -> Self { + // INVARIANT: The struct is being initialised with `Some(_)`. + Self(Some((data, cleanup_func))) + } + + /// Prevents the cleanup function from running and returns the guarded data. + pub fn dismiss(mut self) -> T { + // INVARIANT: This is the exception case in the invariant; it is not visible to callers + // because this function consumes `self`. + self.0.take().unwrap().0 + } +} + +impl ScopeGuard<(), fn(())> { + /// Creates a new guarded object with the given cleanup function. + pub fn new(cleanup: impl FnOnce()) -> ScopeGuard<(), impl FnOnce(())> { + ScopeGuard::new_with_data((), move |_| cleanup()) + } +} + +impl<T, F: FnOnce(T)> Deref for ScopeGuard<T, F> { + type Target = T; + + fn deref(&self) -> &T { + // The type invariants guarantee that `unwrap` will succeed. + &self.0.as_ref().unwrap().0 + } +} + +impl<T, F: FnOnce(T)> DerefMut for ScopeGuard<T, F> { + fn deref_mut(&mut self) -> &mut T { + // The type invariants guarantee that `unwrap` will succeed. + &mut self.0.as_mut().unwrap().0 + } +} + +impl<T, F: FnOnce(T)> Drop for ScopeGuard<T, F> { + fn drop(&mut self) { + // Run the cleanup function if one is still present. + if let Some((data, cleanup)) = self.0.take() { + cleanup(data) + } + } +} + +/// Stores an opaque value. +/// +/// This is meant to be used with FFI objects that are never interpreted by Rust code. +#[repr(transparent)] +pub struct Opaque<T> { + value: UnsafeCell<MaybeUninit<T>>, + _pin: PhantomPinned, +} + +impl<T> Opaque<T> { + /// Creates a new opaque value. + pub const fn new(value: T) -> Self { + Self { + value: UnsafeCell::new(MaybeUninit::new(value)), + _pin: PhantomPinned, + } + } + + /// Creates an uninitialised value. + pub const fn uninit() -> Self { + Self { + value: UnsafeCell::new(MaybeUninit::uninit()), + _pin: PhantomPinned, + } + } + + /// Creates a pin-initializer from the given initializer closure. + /// + /// The returned initializer calls the given closure with the pointer to the inner `T` of this + /// `Opaque`. Since this memory is uninitialized, the closure is not allowed to read from it. + /// + /// This function is safe, because the `T` inside of an `Opaque` is allowed to be + /// uninitialized. Additionally, access to the inner `T` requires `unsafe`, so the caller needs + /// to verify at that point that the inner value is valid. + pub fn ffi_init(init_func: impl FnOnce(*mut T)) -> impl PinInit<Self> { + // SAFETY: We contain a `MaybeUninit`, so it is OK for the `init_func` to not fully + // initialize the `T`. + unsafe { + init::pin_init_from_closure::<_, ::core::convert::Infallible>(move |slot| { + init_func(Self::raw_get(slot)); + Ok(()) + }) + } + } + + /// Returns a raw pointer to the opaque data. + pub fn get(&self) -> *mut T { + UnsafeCell::get(&self.value).cast::<T>() + } + + /// Gets the value behind `this`. + /// + /// This function is useful to get access to the value without creating intermediate + /// references. + pub const fn raw_get(this: *const Self) -> *mut T { + UnsafeCell::raw_get(this.cast::<UnsafeCell<MaybeUninit<T>>>()).cast::<T>() + } +} + +/// Types that are _always_ reference counted. +/// +/// It allows such types to define their own custom ref increment and decrement functions. +/// Additionally, it allows users to convert from a shared reference `&T` to an owned reference +/// [`ARef<T>`]. +/// +/// This is usually implemented by wrappers to existing structures on the C side of the code. For +/// Rust code, the recommendation is to use [`Arc`](crate::sync::Arc) to create reference-counted +/// instances of a type. +/// +/// # Safety +/// +/// Implementers must ensure that increments to the reference count keep the object alive in memory +/// at least until matching decrements are performed. +/// +/// Implementers must also ensure that all instances are reference-counted. (Otherwise they +/// won't be able to honour the requirement that [`AlwaysRefCounted::inc_ref`] keep the object +/// alive.) +pub unsafe trait AlwaysRefCounted { + /// Increments the reference count on the object. + fn inc_ref(&self); + + /// Decrements the reference count on the object. + /// + /// Frees the object when the count reaches zero. + /// + /// # Safety + /// + /// Callers must ensure that there was a previous matching increment to the reference count, + /// and that the object is no longer used after its reference count is decremented (as it may + /// result in the object being freed), unless the caller owns another increment on the refcount + /// (e.g., it calls [`AlwaysRefCounted::inc_ref`] twice, then calls + /// [`AlwaysRefCounted::dec_ref`] once). + unsafe fn dec_ref(obj: NonNull<Self>); +} + +/// An owned reference to an always-reference-counted object. +/// +/// The object's reference count is automatically decremented when an instance of [`ARef`] is +/// dropped. It is also automatically incremented when a new instance is created via +/// [`ARef::clone`]. +/// +/// # Invariants +/// +/// The pointer stored in `ptr` is non-null and valid for the lifetime of the [`ARef`] instance. In +/// particular, the [`ARef`] instance owns an increment on the underlying object's reference count. +pub struct ARef<T: AlwaysRefCounted> { + ptr: NonNull<T>, + _p: PhantomData<T>, +} + +// SAFETY: It is safe to send `ARef<T>` to another thread when the underlying `T` is `Sync` because +// it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally, it needs +// `T` to be `Send` because any thread that has an `ARef<T>` may ultimately access `T` using a +// mutable reference, for example, when the reference count reaches zero and `T` is dropped. +unsafe impl<T: AlwaysRefCounted + Sync + Send> Send for ARef<T> {} + +// SAFETY: It is safe to send `&ARef<T>` to another thread when the underlying `T` is `Sync` +// because it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally, +// it needs `T` to be `Send` because any thread that has a `&ARef<T>` may clone it and get an +// `ARef<T>` on that thread, so the thread may ultimately access `T` using a mutable reference, for +// example, when the reference count reaches zero and `T` is dropped. +unsafe impl<T: AlwaysRefCounted + Sync + Send> Sync for ARef<T> {} + +impl<T: AlwaysRefCounted> ARef<T> { + /// Creates a new instance of [`ARef`]. + /// + /// It takes over an increment of the reference count on the underlying object. + /// + /// # Safety + /// + /// Callers must ensure that the reference count was incremented at least once, and that they + /// are properly relinquishing one increment. That is, if there is only one increment, callers + /// must not use the underlying object anymore -- it is only safe to do so via the newly + /// created [`ARef`]. + pub unsafe fn from_raw(ptr: NonNull<T>) -> Self { + // INVARIANT: The safety requirements guarantee that the new instance now owns the + // increment on the refcount. + Self { + ptr, + _p: PhantomData, + } + } +} + +impl<T: AlwaysRefCounted> Clone for ARef<T> { + fn clone(&self) -> Self { + self.inc_ref(); + // SAFETY: We just incremented the refcount above. + unsafe { Self::from_raw(self.ptr) } + } +} + +impl<T: AlwaysRefCounted> Deref for ARef<T> { + type Target = T; + + fn deref(&self) -> &Self::Target { + // SAFETY: The type invariants guarantee that the object is valid. + unsafe { self.ptr.as_ref() } + } +} + +impl<T: AlwaysRefCounted> From<&T> for ARef<T> { + fn from(b: &T) -> Self { + b.inc_ref(); + // SAFETY: We just incremented the refcount above. + unsafe { Self::from_raw(NonNull::from(b)) } + } +} + +impl<T: AlwaysRefCounted> Drop for ARef<T> { + fn drop(&mut self) { + // SAFETY: The type invariants guarantee that the `ARef` owns the reference we're about to + // decrement. + unsafe { T::dec_ref(self.ptr) }; + } +} + +/// A sum type that always holds either a value of type `L` or `R`. +pub enum Either<L, R> { + /// Constructs an instance of [`Either`] containing a value of type `L`. + Left(L), + + /// Constructs an instance of [`Either`] containing a value of type `R`. + Right(R), +} |