rust: implement ForeignOwnable for Pin<Box<T>>

We already implement ForeignOwnable for Box<T>, but it may be useful to
store pinned data in a ForeignOwnable container. This patch makes that
possible.

This will be used together with upcoming miscdev abstractions, which
Binder will use when binderfs is disabled.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20240730-foreign-ownable-pin-box-v1-1-b1d70cdae541@google.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/types.rs

@@ -9,6 +9,7 @@ use core::{
     marker::{PhantomData, PhantomPinned},
     mem::MaybeUninit,
     ops::{Deref, DerefMut},
+    pin::Pin,
     ptr::NonNull,
 };
 
@@ -89,6 +90,32 @@ impl<T: 'static> ForeignOwnable for Box<T> {
     }
 }
 
+impl<T: 'static> ForeignOwnable for Pin<Box<T>> {
+    type Borrowed<'a> = Pin<&'a T>;
+
+    fn into_foreign(self) -> *const core::ffi::c_void {
+        // SAFETY: We are still treating the box as pinned.
+        Box::into_raw(unsafe { Pin::into_inner_unchecked(self) }) as _
+    }
+
+    unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> Pin<&'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.
+        let r = unsafe { &*ptr.cast() };
+
+        // SAFETY: This pointer originates from a `Pin<Box<T>>`.
+        unsafe { Pin::new_unchecked(r) }
+    }
+
+    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 { Pin::new_unchecked(Box::from_raw(ptr as _)) }
+    }
+}
+
 impl ForeignOwnable for () {
     type Borrowed<'a> = ();