rust: add UnsafePinned type

`UnsafePinned<T>` is useful for cases where a value might be shared with
C code but not directly used by it. In particular this is added for
storing additional data in the `MiscDeviceRegistration` which will be
shared between `fops->open` and the containing struct.

Similar to `Opaque` but guarantees that the value is always initialized
and that the inner value is dropped when `UnsafePinned` is dropped.

This was originally proposed for the IRQ abstractions [0] and is also
useful for other where the inner data may be aliased, but is always
valid and automatic `Drop` is desired.

Since then the `UnsafePinned` type was added to upstream Rust [1] by Sky
as a unstable feature, therefore this patch implements the subset of the
upstream API for the `UnsafePinned` type required for additional data in
`MiscDeviceRegistration` and in the implementation of the `Opaque` type.

Some differences to the upstream type definition are required in the
kernel implementation, because upstream type uses some compiler changes
to opt out of certain optimizations, this is documented in the
documentation and a comment on the `UnsafePinned` type.

The documentation on is based on the upstream rust documentation with
minor modifications for the kernel implementation.

Link: https://lore.kernel.org/rust-for-linux/CAH5fLgiOASgjoYKFz6kWwzLaH07DqP2ph+3YyCDh2+gYqGpABA@mail.gmail.com [0]
Link: rust-lang/rust#137043 [1]
Suggested-by: Alice Ryhl <[email protected]>
Reviewed-by: Gerald Wisböck <[email protected]>
Reviewed-by: Alice Ryhl <[email protected]>
Co-developed-by: Sky <[email protected]>
Signed-off-by: Sky <[email protected]>
Signed-off-by: Christian Schrefl <[email protected]>

Author: onestacked

rust/kernel/types.rs

@@ -269,6 +269,9 @@ impl<T, F: FnOnce(T)> Drop for ScopeGuard<T, F> {
 ///
 /// [`Opaque<T>`] is meant to be used with FFI objects that are never interpreted by Rust code.
 ///
+/// In cases where the contained data is only used by Rust, is not allowed to be
+/// uninitialized and automatic [`Drop`] is desired [`UnsafePinned`] should be used instead.
+///
 /// 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:
 ///
@@ -594,3 +597,6 @@ pub type NotThreadSafe = PhantomData<*mut ()>;
 /// [`NotThreadSafe`]: type@NotThreadSafe
 #[allow(non_upper_case_globals)]
 pub const NotThreadSafe: NotThreadSafe = PhantomData;
+
+mod unsafe_pinned;
+pub use unsafe_pinned::UnsafePinned;

rust/kernel/types/unsafe_pinned.rs

@@ -0,0 +1,100 @@
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+//! The contents of this file partially come from the Rust standard library, hosted in
+//! the <https://github.com/rust-lang/rust> repository, licensed under
+//! "Apache-2.0 OR MIT" and adapted for kernel use. For copyright details,
+//! see <https://github.com/rust-lang/rust/blob/master/COPYRIGHT>.
+//!
+//! This file provides a implementation / polyfill of a subset of the upstream
+//! rust `UnsafePinned` type. For details on the difference to the upstream
+//! implementation see the comment on the [`UnsafePinned`] struct definition.
+
+use core::{cell::UnsafeCell, marker::PhantomPinned};
+use pin_init::{cast_pin_init, PinInit, Wrapper};
+
+/// This type provides a way to entirely opt-out of typical aliasing rules;
+/// specifically, `&mut UnsafePinned<T>` is not guaranteed to be a unique pointer.
+/// This also subsumes the effects of `UnsafeCell`, i.e., `&UnsafePinned<T>` may point to data
+/// that is being mutated.
+///
+/// However, even if you define your type like `pub struct Wrapper(UnsafePinned<...>)`, it is still
+/// very risky to have an `&mut Wrapper` that aliases anything else. Many functions that work
+/// generically on `&mut T` assume that the memory that stores `T` is uniquely owned (such as
+/// `mem::swap`). In other words, while having aliasing with `&mut Wrapper` is not immediate
+/// Undefined Behavior, it is still unsound to expose such a mutable reference to code you do not
+/// control! Techniques such as pinning via [`Pin`](core::pin::Pin) are needed to ensure soundness.
+///
+/// Similar to [`UnsafeCell`], [`UnsafePinned`] will not usually show up in
+/// the public API of a library. It is an internal implementation detail of libraries that need to
+/// support aliasing mutable references.
+///
+/// This type blocks niches the same way [`UnsafeCell`] does.
+///
+/// # Kernel implementation notes
+///
+/// This implementation works because of the "`!Unpin` hack" in rustc, which allows (some kinds of)
+/// mutual aliasing of `!Unpin` types. This hack might be removed at some point, after which only
+/// the `core::pin::UnsafePinned` type will allow this behavior. In order to simplify the migration
+/// to future rust versions only this polyfill of this type should be used when this behavior is
+/// required.
+//
+// As opposed to the upstream Rust type this contains a `PhantomPinned` to ensure the struct always
+// is `!Unpin` the upstream type has a explicit impl `impl !Unpin for UnsafePinned`.
+#[repr(transparent)]
+pub struct UnsafePinned<T: ?Sized> {
+    _ph: PhantomPinned,
+    value: UnsafeCell<T>,
+}
+
+impl<T> UnsafePinned<T> {
+    /// Constructs a new instance of [`UnsafePinned`] which will wrap the specified value.
+    ///
+    /// All access to the inner value through `&UnsafePinned<T>` or `&mut UnsafePinned<T>` or
+    /// `Pin<&mut UnsafePinned<T>>` requires `unsafe` code.
+    #[inline(always)]
+    #[must_use]
+    pub const fn new(value: T) -> Self {
+        UnsafePinned {
+            value: UnsafeCell::new(value),
+            _ph: PhantomPinned,
+        }
+    }
+}
+impl<T: ?Sized> UnsafePinned<T> {
+    /// Get mutable access to the contents of a shared `UnsafePinned`.
+    ///
+    /// This can be cast to a pointer of any kind. When creating references, you must uphold the
+    /// aliasing rules; see [`UnsafeCell`] for more discussion and caveats.
+    ///
+    /// [`UnsafeCell`]: core::cell::UnsafeCell#aliasing-rules
+    ///
+    /// ```rust,no_run
+    /// use kernel::types::UnsafePinned;
+    ///
+    /// unsafe {
+    ///     let mut x = UnsafePinned::new(0);
+    ///     let ptr = x.get();
+    ///     UnsafePinned::raw_get_mut(&raw mut x).write(1);
+    ///     assert_eq!(ptr.read(), 1);
+    /// }
+    /// ```
+    #[inline(always)]
+    #[must_use]
+    pub const fn get(&self) -> *mut T {
+        self.value.get()
+    }
+
+    /// Gets a mutable pointer to the wrapped value.
+    #[inline(always)]
+    #[must_use]
+    pub const fn raw_get_mut(this: *mut Self) -> *mut T {
+        this as *mut T
+    }
+}
+
+impl<T> Wrapper<T> for UnsafePinned<T> {
+    fn pin_init<E>(init: impl PinInit<T, E>) -> impl PinInit<Self, E> {
+        // SAFETY: `UnsafePinned<T>` has a compatible layout to `T`.
+        unsafe { cast_pin_init(init) }
+    }
+}