rust: types: implement `ForeignOwnable` for `Box<T>`

This allows us to hand ownership of Rust dynamically allocated
objects to the C side of the kernel.

Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
Reviewed-by: Alice Ferrazzi <alice.ferrazzi@miraclelinux.com>
Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/types.rs

@@ -2,6 +2,7 @@
 
 //! Kernel types.
 
+use alloc::boxed::Box;
 use core::{
     cell::UnsafeCell,
     mem::MaybeUninit,
@@ -62,6 +63,28 @@ pub trait ForeignOwnable: Sized {
     unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self;
 }
 
+impl<T: 'static> ForeignOwnable for Box<T> {
+    type Borrowed<'a> = &'a T;
+
+    fn into_foreign(self) -> *const core::ffi::c_void {
+        Box::into_raw(self) as _
+    }
+
+    unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> &'a T {
+        // SAFETY: The safety requirements for this function ensure that the object is still alive,
+        // so it is safe to dereference the raw pointer.
+        // The safety requirements of `from_foreign` also ensure that the object remains alive for
+        // the lifetime of the returned value.
+        unsafe { &*ptr.cast() }
+    }
+
+    unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self {
+        // SAFETY: The safety requirements of this function ensure that `ptr` comes from a previous
+        // call to `Self::into_foreign`.
+        unsafe { Box::from_raw(ptr as _) }
+    }
+}
+
 /// Runs a cleanup function/closure when dropped.
 ///
 /// The [`ScopeGuard::dismiss`] function prevents the cleanup function from running.