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]>
Co-developed-by: Sky <[email protected]>
Signed-off-by: Sky <[email protected]>
Signed-off-by: Christian Schrefl <[email protected]>

Author: onestacked

init/Kconfig

@@ -140,6 +140,9 @@ config LD_CAN_USE_KEEP_IN_OVERLAY
 config RUSTC_HAS_COERCE_POINTEE
 	def_bool RUSTC_VERSION >= 108400
 
+config RUSTC_HAS_UNSAFE_PINNED
+	def_bool RUSTC_VERSION >= 108800
+
 config PAHOLE_VERSION
 	int
 	default $(shell,$(srctree)/scripts/pahole-version.sh $(PAHOLE))

rust/kernel/lib.rs

@@ -17,6 +17,7 @@
 #![cfg_attr(not(CONFIG_RUSTC_HAS_COERCE_POINTEE), feature(coerce_unsized))]
 #![cfg_attr(not(CONFIG_RUSTC_HAS_COERCE_POINTEE), feature(dispatch_from_dyn))]
 #![cfg_attr(not(CONFIG_RUSTC_HAS_COERCE_POINTEE), feature(unsize))]
+#![cfg_attr(CONFIG_RUSTC_HAS_UNSAFE_PINNED, feature(unsafe_pinned))]
 #![feature(inline_const)]
 #![feature(lint_reasons)]
 // Stable in Rust 1.82

rust/kernel/types.rs

@@ -253,6 +253,9 @@ 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:
 ///
@@ -578,3 +581,6 @@ 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 +1,115 @@
+// 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.
+
+use core::{cell::UnsafeCell, marker::PhantomPinned};
+use pin_init::{cast_pin_init, PinInit, Wrapper};
+
+/// This type provides a way to opt-out of typical aliasing rules;
+/// specifically, `&mut UnsafePinned<T>` is not guaranteed to be a unique pointer.
+///
+/// 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.
+///
+/// Further note that this does *not* lift the requirement that shared references must be read-only!
+/// Use [`UnsafeCell`] for that.
+///
+/// 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.
+///
+/// In order to disable niche optimizations this implementation uses [`UnsafeCell`] internally,
+/// the upstream version however will not. So the fact that [`UnsafePinned`] contains an
+/// [`UnsafeCell`] must not be relied on (Other than the niche blocking).
+// As opposed to the upstream Rust type this contains a `PhantomPinned`` and `UnsafeCell<T>`
+// - `PhantomPinned` to avoid needing a `impl<T> !Unpin for UnsafePinned<T>`
+//      Required to use the `!Unpin hack`.
+// - `UnsafeCell<T>` instead of T to disallow niche optimizations,
+//     which is handled in the compiler in upstream Rust
+#[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 read-only access to the contents of a shared `UnsafePinned`.
+    ///
+    /// Note that `&UnsafePinned<T>` is read-only if `&T` is read-only. This means that if there is
+    /// mutation of the `T`, future reads from the `*const T` returned here are UB! Use
+    /// [`UnsafeCell`] if you also need interior mutability.
+    ///
+    /// [`UnsafeCell`]: core::cell::UnsafeCell
+    ///
+    /// ```rust,no_build
+    /// use kernel::types::UnsafePinned;
+    ///
+    /// unsafe {
+    ///     let mut x = UnsafePinned::new(0);
+    ///     let ptr = x.get(); // read-only pointer, assumes immutability
+    ///     x.get_mut_unchecked().write(1);
+    ///     ptr.read(); // UB!
+    /// }
+    /// ```
+    ///
+    /// Note that the `get_mut_unchecked` function used by this example is
+    /// currently not implemented in the kernel implementation.
+    #[inline(always)]
+    #[must_use]
+    pub const fn get(&self) -> *const T {
+        self.value.get()
+    }
+
+    /// Gets a mutable pointer to the wrapped value.
+    ///
+    /// The difference from `get_mut_pinned` and `get_mut_unchecked` is that this function
+    /// accepts a raw pointer, which is useful to avoid the creation of temporary references.
+    ///
+    /// These functions mentioned here are currently not implemented in the kernel.
+    #[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) }
+    }
+}