From 01db99b272318da75a1aa5a81f75adb9d32f676e Mon Sep 17 00:00:00 2001
From: Benno Lossin <benno.lossin@proton.me>
Date: Mon, 19 Aug 2024 11:24:35 +0000
Subject: 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: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20240819112415.99810-1-benno.lossin@proton.me
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
 rust/kernel/alloc/box_ext.rs | 31 ++++++++++++++++++++++++++++++-
 1 file changed, 30 insertions(+), 1 deletion(-)

(limited to 'rust/kernel/alloc')

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 @@ impl<T> BoxExt<T> for Box<T> {
         // 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) }
+    }
 }
-- 
cgit 


From ab309b6e084c70a29f9fa3cee797572bd2340901 Mon Sep 17 00:00:00 2001
From: Miguel Ojeda <ojeda@kernel.org>
Date: Wed, 4 Sep 2024 16:42:29 +0200
Subject: rust: avoid `box_uninit_write` feature

Like commit 0903b9e2a46c ("rust: alloc: eschew
`Box<MaybeUninit<T>>::write`"), but for the new `rbtree` and `alloc` code.

That is, `feature(new_uninit)` [1] got partially stabilized [2]
for Rust 1.82.0 (expected to be released on 2024-10-17), but it
did not include `Box<MaybeUninit<T>>::write`, which got split into
`feature(box_uninit_write)` [3].

To avoid relying on a new unstable feature, rewrite the `write` +
`assume_init` pair manually.

Link: https://github.com/rust-lang/rust/issues/63291 [1]
Link: https://github.com/rust-lang/rust/pull/129401 [2]
Link: https://github.com/rust-lang/rust/issues/129397 [3]
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Matt Gilbride <mattgilbride@google.com>
Link: https://lore.kernel.org/r/20240904144229.18592-1-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
 rust/kernel/alloc/box_ext.rs | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

(limited to 'rust/kernel/alloc')

diff --git a/rust/kernel/alloc/box_ext.rs b/rust/kernel/alloc/box_ext.rs
index b68ade26a42d..5b1550d620fd 100644
--- a/rust/kernel/alloc/box_ext.rs
+++ b/rust/kernel/alloc/box_ext.rs
@@ -26,9 +26,11 @@ pub trait BoxExt<T>: Sized {
     /// 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);
+    /// let mut value = Box::drop_contents(value);
     /// // Now we can re-use `value`:
-    /// let value = Box::write(value, [1; 32]);
+    /// value.write([1; 32]);
+    /// // SAFETY: We just wrote to it.
+    /// let value = unsafe { value.assume_init() };
     /// assert_eq!(*value, [1; 32]);
     /// # Ok::<(), Error>(())
     /// ```
-- 
cgit