[PATCH v3 09/25] rust: alloc: implement kernel `Box`

Danilo Krummrich
2024-08-01T00:07:42Z

`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 | 326 ++++++++++++++++++++++++++++++++++++++
rust/kernel/init.rs | 35 +++-
rust/kernel/prelude.rs | 2 +-
rust/kernel/types.rs | 26 +++
5 files changed, 393 insertions(+), 2 deletions(-)
create mode 100644 rust/kernel/alloc/kbox.rs

diff --git a/rust/kernel/alloc.rs b/rust/kernel/alloc.rs
index 90f4452b146a..4e99eef26156 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..7074f00e07bc
--- /dev/null
+++ b/rust/kernel/alloc/kbox.rs
@@ -0,0 +1,326 @@
+// 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;
+use core::result::Result;
+
+use crate::types::Unique;
+
+/// The kernel's [`Box`] type.
+///
+/// `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`.
+///
+/// For non-zero-sized values, a [`Box`] will use the given allocator `A` for its allocation. For
+/// the most common allocators the type aliases `KBox`, `VBox` and `KVBox` exist.
+///
+/// It is valid to convert both ways between a [`Box`] and a raw pointer allocated with any
+/// `Allocator`, given that the `Layout` used with the allocator is correct for the type.
+///
+/// For zero-sized values the [`Box`]' pointer must be `dangling_mut::<T>`; no memory is allocated.
+///
+/// So long as `T: Sized`, a `Box<T>` is guaranteed to be represented as a single pointer and is
+/// also ABI-compatible with C pointers (i.e. the C type `T*`).
+///
+/// # Invariants
+///
+/// The [`Box`]' pointer always properly aligned and either points to memory allocated with `A` or,
+/// for zero-sized types, is a dangling pointer.
+///
+/// # Examples
+///
+/// ```
+/// let b = KBox::<u64>::new(24_u64, GFP_KERNEL)?;
+///
+/// assert_eq!(*b, 24_u64);
+///
+/// # Ok::<(), Error>(())
+/// ```
+///
+/// ```
+/// struct Huge([u8; 1 << 24]);
+///
+/// assert!(KVBox::<Huge>::new_uninit(GFP_KERNEL).is_ok());
+/// ```
+pub struct Box<T: ?Sized, A: Allocator>(Unique<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>;
+
+impl<T, A> Box<T, A>
+where
+ T: ?Sized,
+ A: Allocator,
+{
+ /// Constructs a `Box<T, A>` from a raw pointer.
+ ///
+ /// # Safety
+ ///
+ /// `raw` must point to valid memory, previously allocated with `A`, and at least the size of
+ /// type `T`.
+ #[inline]
+ pub const unsafe fn from_raw_alloc(raw: *mut T, alloc: PhantomData) -> Self {
+ // SAFETY: Safe by the requirements of this function.
+ Box(unsafe { Unique::new_unchecked(raw) }, alloc)
+ }
+
+ /// Consumes the `Box<T, A>`, returning a wrapped raw pointer and `PhantomData` of the allocator
+ /// it was allocated with.
+ pub fn into_raw_alloc(b: Self) -> (*mut T, PhantomData) {
+ let b = ManuallyDrop::new(b);
+ let alloc = unsafe { ptr::read(&b.1) };
+ (b.0.as_ptr(), alloc)
+ }
+
+ /// Constructs a `Box<T, A>` from a raw pointer.
+ ///
+ /// # Safety
+ ///
+ /// `raw` must point to valid memory, previously allocated with `A`, and at least the size of
+ /// type `T`.
+ #[inline]
+ pub const unsafe fn from_raw(raw: *mut T) -> Self {
+ // SAFETY: Validity of `raw` is guaranteed by the safety preconditions of this function.
+ unsafe { Box::from_raw_alloc(raw, PhantomData::) }
+ }
+
+ /// Consumes the `Box<T>`, returning a wrapped raw pointer.
+ ///
+ /// # 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 {
+ Self::into_raw_alloc(b).0
+ }
+
+ /// Consumes and leaks the `Box<T>`, returning a mutable reference, &'a mut T.
+ #[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>` into a `Pin<Box<T>>`.
+ #[inline]
+ pub fn into_pin(b: Self) -> Pin<Self>
+ where
+ A: 'static,
+ {
+ // SAFETY: It's not possible to move or replace the insides of a `Pin<Box<T>>` when
+ // `T: !Unpin`, so it's safe to pin it directly without any additional requirements.
+ unsafe { Pin::new_unchecked(b) }
+ }
+}
+
+impl<T, A> Box<MaybeUninit<T>, A>
+where
+ A: Allocator,
+{
+ /// Converts to `Box<T, A>`.
+ ///
+ /// # Safety
+ ///
+ /// As with MaybeUninit::assume_init, it is up to the caller to guarantee that the value really
+ /// is in an initialized state. Calling this when the content is not yet fully initialized
+ /// causes immediate undefined behavior.
+ pub unsafe fn assume_init(b: Self) -> Box<T, A> {
+ let raw = Self::into_raw(b);
+ // SAFETY: Reconstruct the `Box<MaybeUninit<T>, A>` as Box<T, A> now that has been
+ // initialized. `raw` and `alloc` are safe by the invariants of `Box`.
+ 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
+ }
+
+ /// Allocates memory with the allocator `A` and then places `x` into it.
+ ///
+ /// This doesn’t actually allocate if T is zero-sized.
+ pub fn new(x: T, flags: Flags) -> Result<Self, AllocError> {
+ let b = Self::new_uninit(flags)?;
+ Ok(Box::write(b, x))
+ }
+
+ /// Constructs a new `Box<T, A>` with uninitialized contents.
+ ///
+ /// # 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() {
+ Unique::dangling()
+ } else {
+ let layout = core::alloc::Layout::new::<MaybeUninit<T>>();
+ let ptr = A::alloc(layout, flags)?;
+
+ ptr.cast().into()
+ };
+
+ 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 unable to 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,
+ A: 'static,
+{
+ /// Converts a `Box<T>` into a `Pin<Box<T>>`. If `T` does not implement [`Unpin`], then
+ /// `*boxed` will be pinned in memory and unable to be moved.
+ ///
+ /// This conversion does not allocate on the heap and happens in place.
+ ///
+ /// This is also available via [`Box::into_pin`].
+ ///
+ /// Constructing and pinning a `Box` with <Pin<Box\<T>>>::from([Box::new]\(x))
+ /// can also be written more concisely using [Box::pin]\(x).
+ /// This `From` implementation is useful if you already have a `Box<T>`, or you are
+ /// constructing a (pinned) `Box` in a different way than with [`Box::new`].
+ fn from(b: Box<T, A>) -> Self {
+ Box::into_pin(b)
+ }
+}
+
+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 ptr = self.0.as_ptr();
+
+ // SAFETY: We need to drop `self.0` in place, before we free the backing memory.
+ unsafe { core::ptr::drop_in_place(ptr) };
+
+ // SAFETY: `ptr` is always properly aligned, dereferenceable and points to an initialized
+ // instance of `T`.
+ if unsafe { core::mem::size_of_val(&*ptr) } != 0 {
+ // SAFETY: `ptr` was previously allocated with `A`.
+ unsafe { A::free(self.0.as_non_null().cast()) };
+ }
+ }
+}
diff --git a/rust/kernel/init.rs b/rust/kernel/init.rs
index 495c09ebe3a3..5fd7a0ffabd2 100644
--- a/rust/kernel/init.rs
+++ b/rust/kernel/init.rs
@@ -211,7 +211,7 @@
//! [`pin_init!`]: crate::pin_init!

use crate::{
- alloc::{box_ext::BoxExt, AllocError, Flags},
+ alloc::{box_ext::BoxExt, AllocError, Allocator, Flags},
error::{self, Error},
sync::UniqueArc,
types::{Opaque, ScopeGuard},
@@ -1178,6 +1178,39 @@ fn try_init<E>(init: impl Init<T, E>, flags: Flags) -> Result<Self, E>
}
}

+impl<T, A> InPlaceInit<T> for crate::alloc::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 = crate::alloc::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 { crate::alloc::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 = crate::alloc::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 { crate::alloc::Box::assume_init(this) })
+ }
+}
+
impl<T> InPlaceInit<T> for UniqueArc<T> {
#[inline]
fn try_pin_init<E>(init: impl PinInit<T, E>, flags: Flags) -> Result<Pin<Self>, E>
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};
diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
index 7cf89067b5fc..a60d59c377bd 100644
--- a/rust/kernel/types.rs
+++ b/rust/kernel/types.rs
@@ -2,6 +2,7 @@

//! Kernel types.

+use crate::alloc::Allocator;
use crate::init::{self, PinInit};
use alloc::boxed::Box;
use core::{
@@ -89,6 +90,31 @@ unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self {
}
}

+impl<T: 'static, A> ForeignOwnable for crate::alloc::Box<T, A>
+where
+ A: Allocator,
+{
+ type Borrowed<'a> = &'a T;
+
+ fn into_foreign(self) -> *const core::ffi::c_void {
+ crate::alloc::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 { crate::alloc::Box::from_raw(ptr as _) }
+ }
+}
+
impl ForeignOwnable for () {
type Borrowed<'a> = ();

--
2.45.2