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Diffstat (limited to 'rust/kernel/types.rs')
| -rw-r--r-- | rust/kernel/types.rs | 435 |
1 files changed, 421 insertions, 14 deletions
diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs index e84e51ec9716..9c5e7dbf1632 100644 --- a/rust/kernel/types.rs +++ b/rust/kernel/types.rs @@ -2,36 +2,443 @@ //! Kernel types. -use core::{cell::UnsafeCell, mem::MaybeUninit}; +use crate::ffi::c_void; +use core::{ + cell::UnsafeCell, + marker::{PhantomData, PhantomPinned}, + mem::MaybeUninit, + ops::{Deref, DerefMut}, +}; +use pin_init::{PinInit, Wrapper, Zeroable}; + +pub use crate::sync::aref::{ARef, AlwaysRefCounted}; + +/// 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. +/// +/// # Safety +/// +/// - Implementations must satisfy the guarantees of [`Self::into_foreign`]. +pub unsafe trait ForeignOwnable: Sized { + /// The alignment of pointers returned by `into_foreign`. + const FOREIGN_ALIGN: usize; + + /// Type used to immutably borrow a value that is currently foreign-owned. + type Borrowed<'a>; + + /// Type used to mutably borrow a value that is currently foreign-owned. + type BorrowedMut<'a>; + + /// Converts a Rust-owned object to a foreign-owned one. + /// + /// The foreign representation is a pointer to void. Aside from the guarantees listed below, + /// there are no other guarantees for this pointer. For example, it might be invalid, dangling + /// or pointing to uninitialized memory. Using it in any way except for [`from_foreign`], + /// [`try_from_foreign`], [`borrow`], or [`borrow_mut`] can result in undefined behavior. + /// + /// # Guarantees + /// + /// - Minimum alignment of returned pointer is [`Self::FOREIGN_ALIGN`]. + /// - The returned pointer is not null. + /// + /// [`from_foreign`]: Self::from_foreign + /// [`try_from_foreign`]: Self::try_from_foreign + /// [`borrow`]: Self::borrow + /// [`borrow_mut`]: Self::borrow_mut + fn into_foreign(self) -> *mut c_void; + + /// Converts a foreign-owned object back to a Rust-owned one. + /// + /// # Safety + /// + /// The provided pointer must have been returned by a previous call to [`into_foreign`], and it + /// must not be passed to `from_foreign` more than once. + /// + /// [`into_foreign`]: Self::into_foreign + unsafe fn from_foreign(ptr: *mut c_void) -> Self; + + /// Tries to convert a foreign-owned object back to a Rust-owned one. + /// + /// A convenience wrapper over [`ForeignOwnable::from_foreign`] that returns [`None`] if `ptr` + /// is null. + /// + /// # Safety + /// + /// `ptr` must either be null or satisfy the safety requirements for [`from_foreign`]. + /// + /// [`from_foreign`]: Self::from_foreign + unsafe fn try_from_foreign(ptr: *mut c_void) -> Option<Self> { + if ptr.is_null() { + None + } else { + // SAFETY: Since `ptr` is not null here, then `ptr` satisfies the safety requirements + // of `from_foreign` given the safety requirements of this function. + unsafe { Some(Self::from_foreign(ptr)) } + } + } + + /// Borrows a foreign-owned object immutably. + /// + /// This method provides a way to access a foreign-owned value from Rust immutably. It provides + /// you with exactly the same abilities as an `&Self` when the value is Rust-owned. + /// + /// # Safety + /// + /// The provided pointer must have been returned by a previous call to [`into_foreign`], and if + /// the pointer is ever passed to [`from_foreign`], then that call must happen after the end of + /// the lifetime `'a`. + /// + /// [`into_foreign`]: Self::into_foreign + /// [`from_foreign`]: Self::from_foreign + unsafe fn borrow<'a>(ptr: *mut c_void) -> Self::Borrowed<'a>; + + /// Borrows a foreign-owned object mutably. + /// + /// This method provides a way to access a foreign-owned value from Rust mutably. It provides + /// you with exactly the same abilities as an `&mut Self` when the value is Rust-owned, except + /// that the address of the object must not be changed. + /// + /// Note that for types like [`Arc`], an `&mut Arc<T>` only gives you immutable access to the + /// inner value, so this method also only provides immutable access in that case. + /// + /// In the case of `Box<T>`, this method gives you the ability to modify the inner `T`, but it + /// does not let you change the box itself. That is, you cannot change which allocation the box + /// points at. + /// + /// # Safety + /// + /// The provided pointer must have been returned by a previous call to [`into_foreign`], and if + /// the pointer is ever passed to [`from_foreign`], then that call must happen after the end of + /// the lifetime `'a`. + /// + /// The lifetime `'a` must not overlap with the lifetime of any other call to [`borrow`] or + /// `borrow_mut` on the same object. + /// + /// [`into_foreign`]: Self::into_foreign + /// [`from_foreign`]: Self::from_foreign + /// [`borrow`]: Self::borrow + /// [`Arc`]: crate::sync::Arc + unsafe fn borrow_mut<'a>(ptr: *mut c_void) -> Self::BorrowedMut<'a>; +} + +// SAFETY: The pointer returned by `into_foreign` comes from a well aligned +// pointer to `()`. +unsafe impl ForeignOwnable for () { + const FOREIGN_ALIGN: usize = core::mem::align_of::<()>(); + type Borrowed<'a> = (); + type BorrowedMut<'a> = (); + + fn into_foreign(self) -> *mut c_void { + core::ptr::NonNull::dangling().as_ptr() + } + + unsafe fn from_foreign(_: *mut c_void) -> Self {} + + unsafe fn borrow<'a>(_: *mut c_void) -> Self::Borrowed<'a> {} + unsafe fn borrow_mut<'a>(_: *mut c_void) -> Self::BorrowedMut<'a> {} +} + +/// 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(KVec::new(), |v| pr_info!("vec had {} elements\n", v.len())); +/// +/// vec.push(10u8, GFP_KERNEL)?; +/// if arg { +/// return Ok(()); +/// } +/// vec.push(20u8, GFP_KERNEL)?; +/// 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. +/// [`Opaque<T>`] is meant to be used with FFI objects that are never interpreted by Rust code. +/// +/// It is used to wrap structs from the C side, like for example `Opaque<bindings::mutex>`. +/// It gets rid of all the usual assumptions that Rust has for a value: +/// +/// * The value is allowed to be uninitialized (for example have invalid bit patterns: `3` for a +/// [`bool`]). +/// * The value is allowed to be mutated, when a `&Opaque<T>` exists on the Rust side. +/// * No uniqueness for mutable references: it is fine to have multiple `&mut Opaque<T>` point to +/// the same value. +/// * The value is not allowed to be shared with other threads (i.e. it is `!Sync`). +/// +/// This has to be used for all values that the C side has access to, because it can't be ensured +/// that the C side is adhering to the usual constraints that Rust needs. +/// +/// Using [`Opaque<T>`] allows to continue to use references on the Rust side even for values shared +/// with C. +/// +/// # Examples +/// +/// ``` +/// use kernel::types::Opaque; +/// # // Emulate a C struct binding which is from C, maybe uninitialized or not, only the C side +/// # // knows. +/// # mod bindings { +/// # pub struct Foo { +/// # pub val: u8, +/// # } +/// # } +/// +/// // `foo.val` is assumed to be handled on the C side, so we use `Opaque` to wrap it. +/// pub struct Foo { +/// foo: Opaque<bindings::Foo>, +/// } +/// +/// impl Foo { +/// pub fn get_val(&self) -> u8 { +/// let ptr = Opaque::get(&self.foo); +/// +/// // SAFETY: `Self` is valid from C side. +/// unsafe { (*ptr).val } +/// } +/// } +/// +/// // Create an instance of `Foo` with the `Opaque` wrapper. +/// let foo = Foo { +/// foo: Opaque::new(bindings::Foo { val: 0xdb }), +/// }; +/// +/// assert_eq!(foo.get_val(), 0xdb); +/// ``` #[repr(transparent)] -pub struct Opaque<T>(MaybeUninit<UnsafeCell<T>>); +pub struct Opaque<T> { + value: UnsafeCell<MaybeUninit<T>>, + _pin: PhantomPinned, +} + +// SAFETY: `Opaque<T>` allows the inner value to be any bit pattern, including all zeros. +unsafe impl<T> Zeroable for Opaque<T> {} impl<T> Opaque<T> { /// Creates a new opaque value. pub const fn new(value: T) -> Self { - Self(MaybeUninit::new(UnsafeCell::new(value))) + Self { + value: UnsafeCell::new(MaybeUninit::new(value)), + _pin: PhantomPinned, + } } /// Creates an uninitialised value. pub const fn uninit() -> Self { - Self(MaybeUninit::uninit()) + Self { + value: UnsafeCell::new(MaybeUninit::uninit()), + _pin: PhantomPinned, + } + } + + /// Creates a new zeroed opaque value. + pub const fn zeroed() -> Self { + Self { + value: UnsafeCell::new(MaybeUninit::zeroed()), + _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 { + pin_init::pin_init_from_closure::<_, ::core::convert::Infallible>(move |slot| { + init_func(Self::cast_into(slot)); + Ok(()) + }) + } + } + + /// Creates a fallible 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 try_ffi_init<E>( + init_func: impl FnOnce(*mut T) -> Result<(), E>, + ) -> impl PinInit<Self, E> { + // SAFETY: We contain a `MaybeUninit`, so it is OK for the `init_func` to not fully + // initialize the `T`. + unsafe { + pin_init::pin_init_from_closure::<_, E>(move |slot| init_func(Self::cast_into(slot))) + } } /// Returns a raw pointer to the opaque data. - pub fn get(&self) -> *mut T { - UnsafeCell::raw_get(self.0.as_ptr()) + pub const fn get(&self) -> *mut T { + UnsafeCell::get(&self.value).cast::<T>() } -} -/// 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), + /// Gets the value behind `this`. + /// + /// This function is useful to get access to the value without creating intermediate + /// references. + pub const fn cast_into(this: *const Self) -> *mut T { + UnsafeCell::raw_get(this.cast::<UnsafeCell<MaybeUninit<T>>>()).cast::<T>() + } + + /// The opposite operation of [`Opaque::cast_into`]. + pub const fn cast_from(this: *const T) -> *const Self { + this.cast() + } +} - /// Constructs an instance of [`Either`] containing a value of type `R`. - Right(R), +impl<T> Wrapper<T> for Opaque<T> { + /// Create an opaque pin-initializer from the given pin-initializer. + fn pin_init<E>(slot: impl PinInit<T, E>) -> impl PinInit<Self, E> { + Self::try_ffi_init(|ptr: *mut T| { + // SAFETY: + // - `ptr` is a valid pointer to uninitialized memory, + // - `slot` is not accessed on error, + // - `slot` is pinned in memory. + unsafe { PinInit::<T, E>::__pinned_init(slot, ptr) } + }) + } } + +/// Zero-sized type to mark types not [`Send`]. +/// +/// Add this type as a field to your struct if your type should not be sent to a different task. +/// Since [`Send`] is an auto trait, adding a single field that is `!Send` will ensure that the +/// whole type is `!Send`. +/// +/// If a type is `!Send` it is impossible to give control over an instance of the type to another +/// task. This is useful to include in types that store or reference task-local information. A file +/// descriptor is an example of such task-local information. +/// +/// This type also makes the type `!Sync`, which prevents immutable access to the value from +/// several threads in parallel. +pub type NotThreadSafe = PhantomData<*mut ()>; + +/// Used to construct instances of type [`NotThreadSafe`] similar to how `PhantomData` is +/// constructed. +/// +/// [`NotThreadSafe`]: type@NotThreadSafe +#[allow(non_upper_case_globals)] +pub const NotThreadSafe: NotThreadSafe = PhantomData; |