linux-kernel - [PATCH v4 1/2] rust: kernel: add `drop

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Message-ID: <20240819112415.99810-1-benno.lossin@proton.me>
Date: Mon, 19 Aug 2024 11:24:35 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Wedson Almeida Filho <wedsonaf@...il.com>, Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...sung.com>, Alice Ryhl <aliceryhl@...gle.com>
Cc: rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH v4 1/2] 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.

Signed-off-by: Benno Lossin <benno.lossin@...ton.me>
Link: https://lore.kernel.org/rust-for-linux/20240418-b4-rbtree-v3-5-323e134390ce@google.com/ [1]
---
 rust/kernel/alloc/box_ext.rs | 31 ++++++++++++++++++++++++++++++-
 1 file changed, 30 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/alloc/box_ext.rs b/rust/kernel/alloc/box_ext.rs
index 829cb1c1cf9e..b68ade26a42d 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 +69,17 @@ 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) }
+    }
 }

base-commit: c4d7f546dd9aa9780716cdb07416ca97264dce43
-- 
2.45.2