kernel/
types.rs

1// SPDX-License-Identifier: GPL-2.0
2
3//! Kernel types.
4
5use crate::ffi::c_void;
6use core::{
7    cell::UnsafeCell,
8    marker::{PhantomData, PhantomPinned},
9    mem::MaybeUninit,
10    ops::{Deref, DerefMut},
11};
12use pin_init::{PinInit, Wrapper, Zeroable};
13
14pub use crate::sync::aref::{ARef, AlwaysRefCounted};
15
16/// Used to transfer ownership to and from foreign (non-Rust) languages.
17///
18/// Ownership is transferred from Rust to a foreign language by calling [`Self::into_foreign`] and
19/// later may be transferred back to Rust by calling [`Self::from_foreign`].
20///
21/// This trait is meant to be used in cases when Rust objects are stored in C objects and
22/// eventually "freed" back to Rust.
23///
24/// # Safety
25///
26/// - Implementations must satisfy the guarantees of [`Self::into_foreign`].
27pub unsafe trait ForeignOwnable: Sized {
28    /// The alignment of pointers returned by `into_foreign`.
29    const FOREIGN_ALIGN: usize;
30
31    /// Type used to immutably borrow a value that is currently foreign-owned.
32    type Borrowed<'a>;
33
34    /// Type used to mutably borrow a value that is currently foreign-owned.
35    type BorrowedMut<'a>;
36
37    /// Converts a Rust-owned object to a foreign-owned one.
38    ///
39    /// The foreign representation is a pointer to void. Aside from the guarantees listed below,
40    /// there are no other guarantees for this pointer. For example, it might be invalid, dangling
41    /// or pointing to uninitialized memory. Using it in any way except for [`from_foreign`],
42    /// [`try_from_foreign`], [`borrow`], or [`borrow_mut`] can result in undefined behavior.
43    ///
44    /// # Guarantees
45    ///
46    /// - Minimum alignment of returned pointer is [`Self::FOREIGN_ALIGN`].
47    /// - The returned pointer is not null.
48    ///
49    /// [`from_foreign`]: Self::from_foreign
50    /// [`try_from_foreign`]: Self::try_from_foreign
51    /// [`borrow`]: Self::borrow
52    /// [`borrow_mut`]: Self::borrow_mut
53    fn into_foreign(self) -> *mut c_void;
54
55    /// Converts a foreign-owned object back to a Rust-owned one.
56    ///
57    /// # Safety
58    ///
59    /// The provided pointer must have been returned by a previous call to [`into_foreign`], and it
60    /// must not be passed to `from_foreign` more than once.
61    ///
62    /// [`into_foreign`]: Self::into_foreign
63    unsafe fn from_foreign(ptr: *mut c_void) -> Self;
64
65    /// Tries to convert a foreign-owned object back to a Rust-owned one.
66    ///
67    /// A convenience wrapper over [`ForeignOwnable::from_foreign`] that returns [`None`] if `ptr`
68    /// is null.
69    ///
70    /// # Safety
71    ///
72    /// `ptr` must either be null or satisfy the safety requirements for [`from_foreign`].
73    ///
74    /// [`from_foreign`]: Self::from_foreign
75    unsafe fn try_from_foreign(ptr: *mut c_void) -> Option<Self> {
76        if ptr.is_null() {
77            None
78        } else {
79            // SAFETY: Since `ptr` is not null here, then `ptr` satisfies the safety requirements
80            // of `from_foreign` given the safety requirements of this function.
81            unsafe { Some(Self::from_foreign(ptr)) }
82        }
83    }
84
85    /// Borrows a foreign-owned object immutably.
86    ///
87    /// This method provides a way to access a foreign-owned value from Rust immutably. It provides
88    /// you with exactly the same abilities as an `&Self` when the value is Rust-owned.
89    ///
90    /// # Safety
91    ///
92    /// The provided pointer must have been returned by a previous call to [`into_foreign`], and if
93    /// the pointer is ever passed to [`from_foreign`], then that call must happen after the end of
94    /// the lifetime `'a`.
95    ///
96    /// [`into_foreign`]: Self::into_foreign
97    /// [`from_foreign`]: Self::from_foreign
98    unsafe fn borrow<'a>(ptr: *mut c_void) -> Self::Borrowed<'a>;
99
100    /// Borrows a foreign-owned object mutably.
101    ///
102    /// This method provides a way to access a foreign-owned value from Rust mutably. It provides
103    /// you with exactly the same abilities as an `&mut Self` when the value is Rust-owned, except
104    /// that the address of the object must not be changed.
105    ///
106    /// Note that for types like [`Arc`], an `&mut Arc<T>` only gives you immutable access to the
107    /// inner value, so this method also only provides immutable access in that case.
108    ///
109    /// In the case of `Box<T>`, this method gives you the ability to modify the inner `T`, but it
110    /// does not let you change the box itself. That is, you cannot change which allocation the box
111    /// points at.
112    ///
113    /// # Safety
114    ///
115    /// The provided pointer must have been returned by a previous call to [`into_foreign`], and if
116    /// the pointer is ever passed to [`from_foreign`], then that call must happen after the end of
117    /// the lifetime `'a`.
118    ///
119    /// The lifetime `'a` must not overlap with the lifetime of any other call to [`borrow`] or
120    /// `borrow_mut` on the same object.
121    ///
122    /// [`into_foreign`]: Self::into_foreign
123    /// [`from_foreign`]: Self::from_foreign
124    /// [`borrow`]: Self::borrow
125    /// [`Arc`]: crate::sync::Arc
126    unsafe fn borrow_mut<'a>(ptr: *mut c_void) -> Self::BorrowedMut<'a>;
127}
128
129// SAFETY: The pointer returned by `into_foreign` comes from a well aligned
130// pointer to `()`.
131unsafe impl ForeignOwnable for () {
132    const FOREIGN_ALIGN: usize = core::mem::align_of::<()>();
133    type Borrowed<'a> = ();
134    type BorrowedMut<'a> = ();
135
136    fn into_foreign(self) -> *mut c_void {
137        core::ptr::NonNull::dangling().as_ptr()
138    }
139
140    unsafe fn from_foreign(_: *mut c_void) -> Self {}
141
142    unsafe fn borrow<'a>(_: *mut c_void) -> Self::Borrowed<'a> {}
143    unsafe fn borrow_mut<'a>(_: *mut c_void) -> Self::BorrowedMut<'a> {}
144}
145
146/// Runs a cleanup function/closure when dropped.
147///
148/// The [`ScopeGuard::dismiss`] function prevents the cleanup function from running.
149///
150/// # Examples
151///
152/// In the example below, we have multiple exit paths and we want to log regardless of which one is
153/// taken:
154///
155/// ```
156/// # use kernel::types::ScopeGuard;
157/// fn example1(arg: bool) {
158///     let _log = ScopeGuard::new(|| pr_info!("example1 completed\n"));
159///
160///     if arg {
161///         return;
162///     }
163///
164///     pr_info!("Do something...\n");
165/// }
166///
167/// # example1(false);
168/// # example1(true);
169/// ```
170///
171/// In the example below, we want to log the same message on all early exits but a different one on
172/// the main exit path:
173///
174/// ```
175/// # use kernel::types::ScopeGuard;
176/// fn example2(arg: bool) {
177///     let log = ScopeGuard::new(|| pr_info!("example2 returned early\n"));
178///
179///     if arg {
180///         return;
181///     }
182///
183///     // (Other early returns...)
184///
185///     log.dismiss();
186///     pr_info!("example2 no early return\n");
187/// }
188///
189/// # example2(false);
190/// # example2(true);
191/// ```
192///
193/// In the example below, we need a mutable object (the vector) to be accessible within the log
194/// function, so we wrap it in the [`ScopeGuard`]:
195///
196/// ```
197/// # use kernel::types::ScopeGuard;
198/// fn example3(arg: bool) -> Result {
199///     let mut vec =
200///         ScopeGuard::new_with_data(KVec::new(), |v| pr_info!("vec had {} elements\n", v.len()));
201///
202///     vec.push(10u8, GFP_KERNEL)?;
203///     if arg {
204///         return Ok(());
205///     }
206///     vec.push(20u8, GFP_KERNEL)?;
207///     Ok(())
208/// }
209///
210/// # assert_eq!(example3(false), Ok(()));
211/// # assert_eq!(example3(true), Ok(()));
212/// ```
213///
214/// # Invariants
215///
216/// The value stored in the struct is nearly always `Some(_)`, except between
217/// [`ScopeGuard::dismiss`] and [`ScopeGuard::drop`]: in this case, it will be `None` as the value
218/// will have been returned to the caller. Since  [`ScopeGuard::dismiss`] consumes the guard,
219/// callers won't be able to use it anymore.
220pub struct ScopeGuard<T, F: FnOnce(T)>(Option<(T, F)>);
221
222impl<T, F: FnOnce(T)> ScopeGuard<T, F> {
223    /// Creates a new guarded object wrapping the given data and with the given cleanup function.
224    pub fn new_with_data(data: T, cleanup_func: F) -> Self {
225        // INVARIANT: The struct is being initialised with `Some(_)`.
226        Self(Some((data, cleanup_func)))
227    }
228
229    /// Prevents the cleanup function from running and returns the guarded data.
230    pub fn dismiss(mut self) -> T {
231        // INVARIANT: This is the exception case in the invariant; it is not visible to callers
232        // because this function consumes `self`.
233        self.0.take().unwrap().0
234    }
235}
236
237impl ScopeGuard<(), fn(())> {
238    /// Creates a new guarded object with the given cleanup function.
239    pub fn new(cleanup: impl FnOnce()) -> ScopeGuard<(), impl FnOnce(())> {
240        ScopeGuard::new_with_data((), move |()| cleanup())
241    }
242}
243
244impl<T, F: FnOnce(T)> Deref for ScopeGuard<T, F> {
245    type Target = T;
246
247    fn deref(&self) -> &T {
248        // The type invariants guarantee that `unwrap` will succeed.
249        &self.0.as_ref().unwrap().0
250    }
251}
252
253impl<T, F: FnOnce(T)> DerefMut for ScopeGuard<T, F> {
254    fn deref_mut(&mut self) -> &mut T {
255        // The type invariants guarantee that `unwrap` will succeed.
256        &mut self.0.as_mut().unwrap().0
257    }
258}
259
260impl<T, F: FnOnce(T)> Drop for ScopeGuard<T, F> {
261    fn drop(&mut self) {
262        // Run the cleanup function if one is still present.
263        if let Some((data, cleanup)) = self.0.take() {
264            cleanup(data)
265        }
266    }
267}
268
269/// Stores an opaque value.
270///
271/// [`Opaque<T>`] is meant to be used with FFI objects that are never interpreted by Rust code.
272///
273/// It is used to wrap structs from the C side, like for example `Opaque<bindings::mutex>`.
274/// It gets rid of all the usual assumptions that Rust has for a value:
275///
276/// * The value is allowed to be uninitialized (for example have invalid bit patterns: `3` for a
277///   [`bool`]).
278/// * The value is allowed to be mutated, when a `&Opaque<T>` exists on the Rust side.
279/// * No uniqueness for mutable references: it is fine to have multiple `&mut Opaque<T>` point to
280///   the same value.
281/// * The value is not allowed to be shared with other threads (i.e. it is `!Sync`).
282///
283/// This has to be used for all values that the C side has access to, because it can't be ensured
284/// that the C side is adhering to the usual constraints that Rust needs.
285///
286/// Using [`Opaque<T>`] allows to continue to use references on the Rust side even for values shared
287/// with C.
288///
289/// # Examples
290///
291/// ```
292/// # #![expect(unreachable_pub, clippy::disallowed_names)]
293/// use kernel::types::Opaque;
294/// # // Emulate a C struct binding which is from C, maybe uninitialized or not, only the C side
295/// # // knows.
296/// # mod bindings {
297/// #     pub struct Foo {
298/// #         pub val: u8,
299/// #     }
300/// # }
301///
302/// // `foo.val` is assumed to be handled on the C side, so we use `Opaque` to wrap it.
303/// pub struct Foo {
304///     foo: Opaque<bindings::Foo>,
305/// }
306///
307/// impl Foo {
308///     pub fn get_val(&self) -> u8 {
309///         let ptr = Opaque::get(&self.foo);
310///
311///         // SAFETY: `Self` is valid from C side.
312///         unsafe { (*ptr).val }
313///     }
314/// }
315///
316/// // Create an instance of `Foo` with the `Opaque` wrapper.
317/// let foo = Foo {
318///     foo: Opaque::new(bindings::Foo { val: 0xdb }),
319/// };
320///
321/// assert_eq!(foo.get_val(), 0xdb);
322/// ```
323#[repr(transparent)]
324pub struct Opaque<T> {
325    value: UnsafeCell<MaybeUninit<T>>,
326    _pin: PhantomPinned,
327}
328
329// SAFETY: `Opaque<T>` allows the inner value to be any bit pattern, including all zeros.
330unsafe impl<T> Zeroable for Opaque<T> {}
331
332impl<T> Opaque<T> {
333    /// Creates a new opaque value.
334    pub const fn new(value: T) -> Self {
335        Self {
336            value: UnsafeCell::new(MaybeUninit::new(value)),
337            _pin: PhantomPinned,
338        }
339    }
340
341    /// Creates an uninitialised value.
342    pub const fn uninit() -> Self {
343        Self {
344            value: UnsafeCell::new(MaybeUninit::uninit()),
345            _pin: PhantomPinned,
346        }
347    }
348
349    /// Creates a new zeroed opaque value.
350    pub const fn zeroed() -> Self {
351        Self {
352            value: UnsafeCell::new(MaybeUninit::zeroed()),
353            _pin: PhantomPinned,
354        }
355    }
356
357    /// Creates a pin-initializer from the given initializer closure.
358    ///
359    /// The returned initializer calls the given closure with the pointer to the inner `T` of this
360    /// `Opaque`. Since this memory is uninitialized, the closure is not allowed to read from it.
361    ///
362    /// This function is safe, because the `T` inside of an `Opaque` is allowed to be
363    /// uninitialized. Additionally, access to the inner `T` requires `unsafe`, so the caller needs
364    /// to verify at that point that the inner value is valid.
365    pub fn ffi_init(init_func: impl FnOnce(*mut T)) -> impl PinInit<Self> {
366        // SAFETY: We contain a `MaybeUninit`, so it is OK for the `init_func` to not fully
367        // initialize the `T`.
368        unsafe {
369            pin_init::pin_init_from_closure::<_, ::core::convert::Infallible>(move |slot| {
370                init_func(Self::cast_into(slot));
371                Ok(())
372            })
373        }
374    }
375
376    /// Creates a fallible pin-initializer from the given initializer closure.
377    ///
378    /// The returned initializer calls the given closure with the pointer to the inner `T` of this
379    /// `Opaque`. Since this memory is uninitialized, the closure is not allowed to read from it.
380    ///
381    /// This function is safe, because the `T` inside of an `Opaque` is allowed to be
382    /// uninitialized. Additionally, access to the inner `T` requires `unsafe`, so the caller needs
383    /// to verify at that point that the inner value is valid.
384    pub fn try_ffi_init<E>(
385        init_func: impl FnOnce(*mut T) -> Result<(), E>,
386    ) -> impl PinInit<Self, E> {
387        // SAFETY: We contain a `MaybeUninit`, so it is OK for the `init_func` to not fully
388        // initialize the `T`.
389        unsafe {
390            pin_init::pin_init_from_closure::<_, E>(move |slot| init_func(Self::cast_into(slot)))
391        }
392    }
393
394    /// Returns a raw pointer to the opaque data.
395    pub const fn get(&self) -> *mut T {
396        UnsafeCell::get(&self.value).cast::<T>()
397    }
398
399    /// Gets the value behind `this`.
400    ///
401    /// This function is useful to get access to the value without creating intermediate
402    /// references.
403    pub const fn cast_into(this: *const Self) -> *mut T {
404        UnsafeCell::raw_get(this.cast::<UnsafeCell<MaybeUninit<T>>>()).cast::<T>()
405    }
406
407    /// The opposite operation of [`Opaque::cast_into`].
408    pub const fn cast_from(this: *const T) -> *const Self {
409        this.cast()
410    }
411}
412
413impl<T> Wrapper<T> for Opaque<T> {
414    /// Create an opaque pin-initializer from the given pin-initializer.
415    fn pin_init<E>(slot: impl PinInit<T, E>) -> impl PinInit<Self, E> {
416        Self::try_ffi_init(|ptr: *mut T| {
417            // SAFETY:
418            //   - `ptr` is a valid pointer to uninitialized memory,
419            //   - `slot` is not accessed on error,
420            //   - `slot` is pinned in memory.
421            unsafe { PinInit::<T, E>::__pinned_init(slot, ptr) }
422        })
423    }
424}
425
426/// Zero-sized type to mark types not [`Send`].
427///
428/// Add this type as a field to your struct if your type should not be sent to a different task.
429/// Since [`Send`] is an auto trait, adding a single field that is `!Send` will ensure that the
430/// whole type is `!Send`.
431///
432/// If a type is `!Send` it is impossible to give control over an instance of the type to another
433/// task. This is useful to include in types that store or reference task-local information. A file
434/// descriptor is an example of such task-local information.
435///
436/// This type also makes the type `!Sync`, which prevents immutable access to the value from
437/// several threads in parallel.
438pub type NotThreadSafe = PhantomData<*mut ()>;
439
440/// Used to construct instances of type [`NotThreadSafe`] similar to how `PhantomData` is
441/// constructed.
442///
443/// [`NotThreadSafe`]: type@NotThreadSafe
444#[allow(non_upper_case_globals)]
445pub const NotThreadSafe: NotThreadSafe = PhantomData;