rust/kernel/types.rs

// SPDX-License-Identifier: GPL-2.0

//! Kernel types.

use crate::init::{self, PinInit};
use alloc::boxed::Box;
use core::{
    cell::UnsafeCell,
    marker::PhantomData,
    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.
    /// Additionally, all instances (if any) of values returned by [`ForeignOwnable::borrow_mut`]
    /// for this object must have been dropped.
    unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> Self::Borrowed<'a>;

    /// Mutably 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.
    /// Additionally, all instances (if any) of values returned by [`ForeignOwnable::borrow`] and
    /// [`ForeignOwnable::borrow_mut`] for this object must have been dropped.
    unsafe fn borrow_mut(ptr: *const core::ffi::c_void) -> ScopeGuard<Self, fn(Self)> {
        // SAFETY: The safety requirements ensure that `ptr` came from a previous call to
        // `into_foreign`.
        ScopeGuard::new_with_data(unsafe { Self::from_foreign(ptr) }, |d| {
            d.into_foreign();
        })
    }

    /// 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`] and
    /// [`ForeignOwnable::borrow_mut`] 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::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::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::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>(MaybeUninit<UnsafeCell<T>>);

impl<T> Opaque<T> {
    /// Creates a new opaque value.
    pub const fn new(value: T) -> Self {
        Self(MaybeUninit::new(UnsafeCell::new(value)))
    }

    /// Creates an uninitialised value.
    pub const fn uninit() -> Self {
        Self(MaybeUninit::uninit())
    }

    /// 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::raw_get(self.0.as_ptr())
    }

    /// 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<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>,
}

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),
}