Commit 01db99b2 authored by Benno Lossin's avatar Benno Lossin Committed by Miguel Ojeda
Browse files

rust: kernel: add `drop_contents` to `BoxExt` 

Sometimes (see [1]) it is necessary to drop the value inside of a
`Box<T>`, but retain the allocation. For example to reuse the allocation
in the future.

Introduce a new function `drop_contents` that turns a `Box<T>` into
`Box<MaybeUninit<T>>` by dropping the value.

Link: https://lore.kernel.org/rust-for-linux/20240418-b4-rbtree-v3-5-323e134390ce@google.com/

 [1]
Signed-off-by: default avatarBenno Lossin <benno.lossin@proton.me>
Reviewed-by: default avatarBoqun Feng <boqun.feng@gmail.com>
Reviewed-by: default avatarAlice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20240819112415.99810-1-benno.lossin@proton.me


Signed-off-by: default avatarMiguel Ojeda <ojeda@kernel.org>
parent 76501d19

rust/kernel/alloc/box_ext.rs

+30 −1
Original line number Diff line number Diff line
@@ -4,7 +4,7 @@ 


use super::{AllocError, Flags};

use alloc::boxed::Box;

use core::mem::MaybeUninit;

use core::{mem::MaybeUninit, ptr, result::Result};



/// Extensions to [`Box`].

pub trait BoxExt<T>: Sized {
@@ -17,6 +17,22 @@ pub trait BoxExt<T>: Sized { 
    ///

    /// 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 value = Box::drop_contents(value);

    /// // Now we can re-use `value`:

    /// let value = Box::write(value, [1; 32]);

    /// assert_eq!(*value, [1; 32]);

    /// # Ok::<(), Error>(())

    /// ```

    fn drop_contents(this: Self) -> Box<MaybeUninit<T>>;

}



impl<T> BoxExt<T> for Box<T> {
@@ -53,4 +69,17 @@ fn new_uninit(flags: Flags) -> Result<Box<MaybeUninit<T>>, AllocError> { 
        // 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) }

    }

}