[PATCH v3 1/2] rust: kernel: add `drop_contents` to `BoxExt`

Benno Lossin
2024-08-03T14:16:57Z

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 | 26 +++++++++++++++++++++++++-
1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/alloc/box_ext.rs b/rust/kernel/alloc/box_ext.rs
index 829cb1c1cf9e..076d5de5f47d 100644
--- a/rust/kernel/alloc/box_ext.rs
+++ b/rust/kernel/alloc/box_ext.rs
@@ -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 +69,12 @@ 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(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) };
+ // SAFETY: `ptr` is valid, because it came from `Box::into_raw`.
+ unsafe { Box::from_raw(ptr.cast()) }
+ }
}
--
2.45.2