[PATCH v5 09/26] rust: alloc: implement kernel `Box`

Danilo Krummrich
2024-08-12T18:25:04Z

`Box` provides the simplest way to allocate memory for a generic type
with one of the kernel's allocators, e.g. `Kmalloc`, `Vmalloc` or
`KVmalloc`.

In contrast to Rust's `Box` type, the kernel `Box` type considers the
kernel's GFP flags for all appropriate functions, always reports
allocation failures through `Result<_, AllocError>` and remains
independent from unstable features.

Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
rust/kernel/alloc.rs | 6 +
rust/kernel/alloc/kbox.rs | 423 ++++++++++++++++++++++++++++++++++++++
rust/kernel/prelude.rs | 2 +-
3 files changed, 430 insertions(+), 1 deletion(-)
create mode 100644 rust/kernel/alloc/kbox.rs

diff --git a/rust/kernel/alloc.rs b/rust/kernel/alloc.rs
index 295107777a12..ed46b69204d0 100644
--- a/rust/kernel/alloc.rs
+++ b/rust/kernel/alloc.rs
@@ -5,6 +5,7 @@
#[cfg(not(any(test, testlib)))]
pub mod allocator;
pub mod box_ext;
+pub mod kbox;
pub mod vec_ext;

#[cfg(any(test, testlib))]
@@ -13,6 +14,11 @@
#[cfg(any(test, testlib))]
pub use self::allocator_test as allocator;

+pub use self::kbox::Box;
+pub use self::kbox::KBox;
+pub use self::kbox::KVBox;
+pub use self::kbox::VBox;
+
/// Indicates an allocation error.
#[derive(Copy, Clone, PartialEq, Eq, Debug)]
pub struct AllocError;
diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
new file mode 100644
index 000000000000..67bdfc0712d2
--- /dev/null
+++ b/rust/kernel/alloc/kbox.rs
@@ -0,0 +1,423 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Implementation of [`Box`].
+
+use super::{AllocError, Allocator, Flags};
+use core::fmt;
+use core::marker::PhantomData;
+use core::mem::ManuallyDrop;
+use core::mem::MaybeUninit;
+use core::ops::{Deref, DerefMut};
+use core::pin::Pin;
+use core::ptr::NonNull;
+use core::result::Result;
+
+use crate::init::{InPlaceInit, Init, PinInit};
+use crate::types::ForeignOwnable;
+
+/// The kernel's [`Box`] type - a heap allocation for a single value of type `T`.
+///
+/// This is the kernel's version of the Rust stdlib's `Box`. There are a couple of differences,
+/// for example no `noalias` attribute is emitted and partially moving out of a `Box` is not
+/// supported.
+///
+/// `Box` works with any of the kernel's allocators, e.g. [`super::allocator::Kmalloc`],
+/// [`super::allocator::Vmalloc`] or [`super::allocator::KVmalloc`]. There are aliases for `Box`
+/// with these allocators ([`KBox`], [`VBox`], [`KVBox`]).
+///
+/// When dropping a [`Box`], the value is also dropped and the heap memory is automatically freed.
+///
+/// # Examples
+///
+/// ```
+/// let b = KBox::<u64>::new(24_u64, GFP_KERNEL)?;
+///
+/// assert_eq!(*b, 24_u64);
+///
+/// # Ok::<(), Error>(())
+/// ```
+///
+/// ```
+/// # use kernel::bindings;
+///
+/// const SIZE: usize = bindings::KMALLOC_MAX_SIZE as usize + 1;
+/// struct Huge([u8; SIZE]);
+///
+/// assert!(KBox::<Huge>::new_uninit(GFP_KERNEL | __GFP_NOWARN).is_err());
+/// ```
+///
+/// ```
+/// # use kernel::bindings;
+///
+/// const SIZE: usize = bindings::KMALLOC_MAX_SIZE as usize + 1;
+/// struct Huge([u8; SIZE]);
+///
+/// assert!(KVBox::<Huge>::new_uninit(GFP_KERNEL).is_ok());
+/// ```
+///
+/// # Invariants
+///
+/// The [`Box`]' pointer always properly aligned and either points to memory allocated with `A` or,
+/// for zero-sized types, is a dangling pointer.
+pub struct Box<T: ?Sized, A: Allocator>(NonNull<T>, PhantomData);
+
+/// Type alias for `Box` with a `Kmalloc` allocator.
+///
+/// # Examples
+///
+/// ```
+/// let b = KBox::new(24_u64, GFP_KERNEL)?;
+///
+/// assert_eq!(*b, 24_u64);
+///
+/// # Ok::<(), Error>(())
+/// ```
+pub type KBox<T> = Box<T, super::allocator::Kmalloc>;
+
+/// Type alias for `Box` with a `Vmalloc` allocator.
+///
+/// # Examples
+///
+/// ```
+/// let b = VBox::new(24_u64, GFP_KERNEL)?;
+///
+/// assert_eq!(*b, 24_u64);
+///
+/// # Ok::<(), Error>(())
+/// ```
+pub type VBox<T> = Box<T, super::allocator::Vmalloc>;
+
+/// Type alias for `Box` with a `KVmalloc` allocator.
+///
+/// # Examples
+///
+/// ```
+/// let b = KVBox::new(24_u64, GFP_KERNEL)?;
+///
+/// assert_eq!(*b, 24_u64);
+///
+/// # Ok::<(), Error>(())
+/// ```
+pub type KVBox<T> = Box<T, super::allocator::KVmalloc>;
+
+// SAFETY: `Box` is `Send` if `T` is `Send` because the data referenced by `self.0` is unaliased.
+unsafe impl<T, A> Send for Box<T, A>
+where
+ T: Send + ?Sized,
+ A: Allocator,
+{
+}
+
+// SAFETY: `Box` is `Sync` if `T` is `Sync` because the data referenced by `self.0` is unaliased.
+unsafe impl<T, A> Sync for Box<T, A>
+where
+ T: Send + ?Sized,
+ A: Allocator,
+{
+}
+
+impl<T, A> Box<T, A>
+where
+ T: ?Sized,
+ A: Allocator,
+{
+ /// Creates a new `Box<T, A>` from a raw pointer.
+ ///
+ /// # Safety
+ ///
+ /// `raw` must point to valid memory, previously be allocated with `A`, and provide at least
+ /// the size of type `T`. For ZSTs `raw` must be a dangling pointer.
+ #[inline]
+ pub const unsafe fn from_raw(raw: *mut T) -> Self {
+ // INVARIANT: Validity of `raw` is guaranteed by the safety preconditions of this function.
+ // SAFETY: By the safety preconditions of this function, `raw` is not a NULL pointer.
+ Self(unsafe { NonNull::new_unchecked(raw) }, PhantomData::)
+ }
+
+ /// Consumes the `Box<T, A>` and returns a raw pointer.
+ ///
+ /// This will not run the destructor of `T` and for non-ZSTs the allocation will stay alive
+ /// indefinitely. Use [`Box::from_raw`] to recover the [`Box`], drop the value and free the
+ /// allocation, if any.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// let x = KBox::new(24, GFP_KERNEL)?;
+ /// let ptr = KBox::into_raw(x);
+ /// let x = unsafe { KBox::from_raw(ptr) };
+ ///
+ /// assert_eq!(*x, 24);
+ ///
+ /// # Ok::<(), Error>(())
+ /// ```
+ #[inline]
+ pub fn into_raw(b: Self) -> *mut T {
+ let b = ManuallyDrop::new(b);
+
+ b.0.as_ptr()
+ }
+
+ /// Consumes and leaks the `Box<T, A>` and returns a mutable reference.
+ ///
+ /// See [Box::into_raw] for more details.
+ #[inline]
+ pub fn leak<'a>(b: Self) -> &'a mut T
+ where
+ T: 'a,
+ {
+ // SAFETY: `Box::into_raw` always returns a properly aligned and dereferenceable pointer
+ // which points to an initialized instance of `T`.
+ unsafe { &mut *Box::into_raw(b) }
+ }
+
+ /// Converts a `Box<T, A>` into a `Pin<Box<T, A>>`. If `T` does not implement [`Unpin`], then
+ /// `*b` will be pinned in memory and can't be moved.
+ ///
+ /// This moves `b` into `Pin` without moving `*b` or allocating and copying any memory.
+ #[inline]
+ pub fn into_pin(b: Self) -> Pin<Self> {
+ // SAFETY: The value wrapped inside a `Pin<Box<T, A>>` cannot be moved or replaced as long
+ // as `T` does not implement `Unpin`.
+ unsafe { Pin::new_unchecked(b) }
+ }
+}
+
+impl<T, A> Box<MaybeUninit<T>, A>
+where
+ A: Allocator,
+{
+ /// Converts a `Box<MaybeUninit<T>, A>` to a `Box<T, A>`.
+ ///
+ /// # Safety
+ ///
+ /// Callers must ensure that the value inside of `b` is in an initialized state. It is
+ /// undefined behavior to call this function while the value inside of `b` is not yet fully
+ /// initialized.
+ pub unsafe fn assume_init(b: Self) -> Box<T, A> {
+ let raw = Self::into_raw(b);
+
+ // SAFETY: `raw` comes from a previous call to `Box::into_raw`. By the safety requirements
+ // of this function, the value inside the `Box` is in an initialized state. Hence, it is
+ // safe to reconstruct the `Box` as `Box<T, A>`.
+ unsafe { Box::from_raw(raw as *mut T) }
+ }
+
+ /// Writes the value and converts to `Box<T, A>`.
+ pub fn write(mut b: Self, value: T) -> Box<T, A> {
+ (*b).write(value);
+ // SAFETY: We've just initialized `boxed`'s value.
+ unsafe { Self::assume_init(b) }
+ }
+}
+
+impl<T, A> Box<T, A>
+where
+ A: Allocator,
+{
+ fn is_zst() -> bool {
+ core::mem::size_of::<T>() == 0
+ }
+
+ /// Creates a new `Box<T, A>` and initializes its contents with `x`.
+ ///
+ /// New memory is allocated with `a`. The allocation may fail, in which case an error is
+ /// returned. For ZSTs no memory is allocated.
+ pub fn new(x: T, flags: Flags) -> Result<Self, AllocError> {
+ let b = Self::new_uninit(flags)?;
+ Ok(Box::write(b, x))
+ }
+
+ /// Creates a new `Box<T, A>` with uninitialized contents.
+ ///
+ /// New memory is allocated with `a`. The allocation may fail, in which case an error is
+ /// returned. For ZSTs no memory is allocated.
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// let b = KBox::<u64>::new_uninit(GFP_KERNEL)?;
+ /// let b = KBox::write(b, 24);
+ ///
+ /// assert_eq!(*b, 24_u64);
+ ///
+ /// # Ok::<(), Error>(())
+ /// ```
+ pub fn new_uninit(flags: Flags) -> Result<Box<MaybeUninit<T>, A>, AllocError> {
+ let ptr = if Self::is_zst() {
+ NonNull::dangling()
+ } else {
+ let layout = core::alloc::Layout::new::<MaybeUninit<T>>();
+ let ptr = A::alloc(layout, flags)?;
+
+ ptr.cast()
+ };
+
+ Ok(Box(ptr, PhantomData::))
+ }
+
+ /// Constructs a new `Pin<Box<T, A>>`. If `T` does not implement [`Unpin`], then `x` will be
+ /// pinned in memory and can't be moved.
+ #[inline]
+ pub fn pin(x: T, flags: Flags) -> Result<Pin<Box<T, A>>, AllocError>
+ where
+ A: 'static,
+ {
+ Ok(Self::new(x, flags)?.into())
+ }
+}
+
+impl<T, A> From<Box<T, A>> for Pin<Box<T, A>>
+where
+ T: ?Sized,
+ A: Allocator,
+{
+ /// Converts a `Box<T, A>` into a `Pin<Box<T, A>>`. If `T` does not implement [`Unpin`], then
+ /// `*b` will be pinned in memory and can't be moved.
+ ///
+ /// See [`Box::into_pin`] for more details.
+ fn from(b: Box<T, A>) -> Self {
+ Box::into_pin(b)
+ }
+}
+
+impl<T, A> InPlaceInit<T> for Box<T, A>
+where
+ A: Allocator + 'static,
+{
+ #[inline]
+ fn try_pin_init<E>(init: impl PinInit<T, E>, flags: Flags) -> Result<Pin<Self>, E>
+ where
+ E: From<AllocError>,
+ {
+ let mut this = Box::<_, A>::new_uninit(flags)?;
+ let slot = this.as_mut_ptr();
+ // SAFETY: When init errors/panics, slot will get deallocated but not dropped,
+ // slot is valid and will not be moved, because we pin it later.
+ unsafe { init.__pinned_init(slot)? };
+ // SAFETY: All fields have been initialized.
+ Ok(unsafe { Box::assume_init(this) }.into())
+ }
+
+ #[inline]
+ fn try_init<E>(init: impl Init<T, E>, flags: Flags) -> Result<Self, E>
+ where
+ E: From<AllocError>,
+ {
+ let mut this = Box::<_, A>::new_uninit(flags)?;
+ let slot = this.as_mut_ptr();
+ // SAFETY: When init errors/panics, slot will get deallocated but not dropped,
+ // slot is valid.
+ unsafe { init.__init(slot)? };
+ // SAFETY: All fields have been initialized.
+ Ok(unsafe { Box::assume_init(this) })
+ }
+}
+
+impl<T: 'static, A> ForeignOwnable for Box<T, A>
+where
+ A: Allocator,
+{
+ 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<T: 'static, A> ForeignOwnable for Pin<Box<T, A>>
+where
+ A: Allocator,
+{
+ type Borrowed<'a> = Pin<&'a T>;
+
+ fn into_foreign(self) -> *const core::ffi::c_void {
+ // SAFETY: We are still treating the box as pinned.
+ Box::into_raw(unsafe { Pin::into_inner_unchecked(self) }) as _
+ }
+
+ unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> Pin<&'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.
+ let r = unsafe { &*ptr.cast() };
+
+ // SAFETY: This pointer originates from a `Pin<Box<T>>`.
+ unsafe { Pin::new_unchecked(r) }
+ }
+
+ 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 { Pin::new_unchecked(Box::from_raw(ptr as _)) }
+ }
+}
+
+impl<T, A> Deref for Box<T, A>
+where
+ T: ?Sized,
+ A: Allocator,
+{
+ type Target = T;
+
+ fn deref(&self) -> &T {
+ // SAFETY: `self.0` is always properly aligned, dereferenceable and points to an initialized
+ // instance of `T`.
+ unsafe { self.0.as_ref() }
+ }
+}
+
+impl<T, A> DerefMut for Box<T, A>
+where
+ T: ?Sized,
+ A: Allocator,
+{
+ fn deref_mut(&mut self) -> &mut T {
+ // SAFETY: `self.0` is always properly aligned, dereferenceable and points to an initialized
+ // instance of `T`.
+ unsafe { self.0.as_mut() }
+ }
+}
+
+impl<T, A> fmt::Debug for Box<T, A>
+where
+ T: ?Sized + fmt::Debug,
+ A: Allocator,
+{
+ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+ fmt::Debug::fmt(&**self, f)
+ }
+}
+
+impl<T, A> Drop for Box<T, A>
+where
+ T: ?Sized,
+ A: Allocator,
+{
+ fn drop(&mut self) {
+ let size = core::mem::size_of_val::<T>(self);
+
+ // SAFETY: We need to drop `self.0` in place, before we free the backing memory.
+ unsafe { core::ptr::drop_in_place(self.0.as_ptr()) };
+
+ if size != 0 {
+ // SAFETY: `ptr` was previously allocated with `A`.
+ unsafe { A::free(self.0.cast()) };
+ }
+ }
+}
diff --git a/rust/kernel/prelude.rs b/rust/kernel/prelude.rs
index b37a0b3180fb..39f9331a48e2 100644
--- a/rust/kernel/prelude.rs
+++ b/rust/kernel/prelude.rs
@@ -14,7 +14,7 @@
#[doc(no_inline)]
pub use core::pin::Pin;

-pub use crate::alloc::{box_ext::BoxExt, flags::*, vec_ext::VecExt};
+pub use crate::alloc::{box_ext::BoxExt, flags::*, vec_ext::VecExt, KBox, KVBox, VBox};

#[doc(no_inline)]
pub use alloc::{boxed::Box, vec::Vec};
--
2.45.2