Re: [PATCH v2 1/2] rust: kernel: add `drop_contents` to `BoxExt`

[Boqun Feng <boqun.feng@gmail.com>]

On Mon, Jul 08, 2024 at 08:53:38PM +0000, Benno Lossin wrote:
> 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.
> 
> Signed-off-by: Benno Lossin <benno.lossin@proton.me>
> Link: https://lore.kernel.org/rust-for-linux/20240418-b4-rbtree-v3-5-323e134390ce@google.com/ [1]
> ---
>  rust/kernel/alloc/box_ext.rs | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/rust/kernel/alloc/box_ext.rs b/rust/kernel/alloc/box_ext.rs
> index cdbb5ad166d9..6cf79f96d6c7 100644
> --- a/rust/kernel/alloc/box_ext.rs
> +++ b/rust/kernel/alloc/box_ext.rs
> @@ -5,6 +5,7 @@
>  use super::{AllocError, Flags};
>  use alloc::boxed::Box;
>  use core::mem::MaybeUninit;
> +use core::ptr;
>  use core::result::Result;
>  
>  /// Extensions to [`Box`].
> @@ -18,6 +19,18 @@ 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
> +    ///
> +    /// ```
> +    /// let value = Box::new([0; 32], flags::GFP_KERNEL)
> +    /// let value = value.drop_contents();
> +    /// // Now we can re-use `value`:
> +    /// Box::write(value, [1; 32]);
> +    /// ```
> +    fn drop_contents(self) -> Box<MaybeUninit<T>>;
>  }
>  
>  impl<T> BoxExt<T> for Box<T> {
> @@ -54,4 +67,12 @@ 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(self) -> Box<MaybeUninit<T>> {
> +        let ptr = Box::into_raw(self);
> +        // SAFETY: `ptr` is valid, because it came from `Box::into_raw`.
> +        unsafe { ptr::drop_in_place(ptr) };
> +        // SAFETY: `ptr` is valid, because it came from `Box::into_raw`.

Maybe add

	// And `ptr.cast()` is a valid pointer to `MaybeUninit<T>`,
	// since `MaybeUninit<T>` and `T` has the same layout.

?

> +        unsafe { Box::from_raw(ptr.cast()) }
> +    }

With the above or similar:

Reviewed-by: Boqun Feng <boqun.feng@gmail.com>

Regards,
Boqun

>  }
> -- 
> 2.44.0
> 
>