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// SPDX-License-Identifier: GPL-2.0
//! Extensions to [`Box`] for fallible allocations.
use super::{AllocError, Flags};
use alloc::boxed::Box;
use core::{mem::MaybeUninit, ptr, result::Result};
/// Extensions to [`Box`].
pub trait BoxExt<T>: Sized {
/// Allocates a new box.
///
/// The allocation may fail, in which case an error is returned.
fn new(x: T, flags: Flags) -> Result<Self, AllocError>;
/// Allocates a new uninitialised box.
///
/// The allocation may fail, in which case an error is returned.
fn new_uninit(flags: Flags) -> Result<Box<MaybeUninit<T>>, AllocError>;
/// Drops the contents, but keeps the allocation.
///
/// # Examples
///
/// ```
/// use kernel::alloc::{flags, box_ext::BoxExt};
/// let value = Box::new([0; 32], flags::GFP_KERNEL)?;
/// assert_eq!(*value, [0; 32]);
/// let mut value = Box::drop_contents(value);
/// // Now we can re-use `value`:
/// value.write([1; 32]);
/// // SAFETY: We just wrote to it.
/// let value = unsafe { value.assume_init() };
/// assert_eq!(*value, [1; 32]);
/// # Ok::<(), Error>(())
/// ```
fn drop_contents(this: Self) -> Box<MaybeUninit<T>>;
}
impl<T> BoxExt<T> for Box<T> {
fn new(x: T, flags: Flags) -> Result<Self, AllocError> {
let mut b = <Self as BoxExt<_>>::new_uninit(flags)?;
b.write(x);
// SAFETY: We just wrote to it.
Ok(unsafe { b.assume_init() })
}
#[cfg(any(test, testlib))]
fn new_uninit(_flags: Flags) -> Result<Box<MaybeUninit<T>>, AllocError> {
Ok(Box::new_uninit())
}
#[cfg(not(any(test, testlib)))]
fn new_uninit(flags: Flags) -> Result<Box<MaybeUninit<T>>, AllocError> {
let ptr = if core::mem::size_of::<MaybeUninit<T>>() == 0 {
core::ptr::NonNull::<_>::dangling().as_ptr()
} else {
let layout = core::alloc::Layout::new::<MaybeUninit<T>>();
// SAFETY: Memory is being allocated (first arg is null). The only other source of
// safety issues is sleeping on atomic context, which is addressed by klint. Lastly,
// the type is not a SZT (checked above).
let ptr =
unsafe { super::allocator::krealloc_aligned(core::ptr::null_mut(), layout, flags) };
if ptr.is_null() {
return Err(AllocError);
}
ptr.cast::<MaybeUninit<T>>()
};
// SAFETY: For non-zero-sized types, we allocate above using the global allocator. For
// zero-sized types, we use `NonNull::dangling`.
Ok(unsafe { Box::from_raw(ptr) })
}
fn drop_contents(this: Self) -> Box<MaybeUninit<T>> {
let ptr = Box::into_raw(this);
// SAFETY: `ptr` is valid, because it came from `Box::into_raw`.
unsafe { ptr::drop_in_place(ptr) };
// CAST: `MaybeUninit<T>` is a transparent wrapper of `T`.
let ptr = ptr.cast::<MaybeUninit<T>>();
// SAFETY: `ptr` is valid for writes, because it came from `Box::into_raw` and it is valid for
// reads, since the pointer came from `Box::into_raw` and the type is `MaybeUninit<T>`.
unsafe { Box::from_raw(ptr) }
}
}
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