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Message-ID: <20240402-arc-for-list-v4-1-54db6440a9a9@google.com>
Date: Tue, 02 Apr 2024 13:07:42 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
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>
Cc: rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
Alice Ryhl <aliceryhl@...gle.com>
Subject: [PATCH v4 1/2] rust: sync: add `ArcBorrow::from_raw`
Allows access to a value in an `Arc` that is currently held as a raw
pointer due to use of `Arc::into_raw`, without destroying or otherwise
consuming that raw pointer.
This is a dependency of the linked list that Rust Binder uses. The
linked list uses this method when iterating over the linked list [1].
Reviewed-by: Benno Lossin <benno.lossin@...ton.me>
Reviewed-by: Boqun Feng <boqun.feng@...il.com>
Link: https://lore.kernel.org/r/20240402-linked-list-v1-6-b1c59ba7ae3b@google.com [1]
Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
---
rust/kernel/sync/arc.rs | 76 +++++++++++++++++++++++++++++++++++++------------
1 file changed, 58 insertions(+), 18 deletions(-)
diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index 7d4c4bf58388..53addb8876c2 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -137,6 +137,39 @@ struct ArcInner<T: ?Sized> {
data: T,
}
+impl<T: ?Sized> ArcInner<T> {
+ /// Converts a pointer to the contents of an [`Arc`] into a pointer to the [`ArcInner`].
+ ///
+ /// # Safety
+ ///
+ /// `ptr` must have been returned by a previous call to [`Arc::into_raw`], and the `Arc` must
+ /// not yet have been destroyed.
+ unsafe fn container_of(ptr: *const T) -> NonNull<ArcInner<T>> {
+ let refcount_layout = Layout::new::<bindings::refcount_t>();
+ // SAFETY: The caller guarantees that the pointer is valid.
+ let val_layout = Layout::for_value(unsafe { &*ptr });
+ // SAFETY: We're computing the layout of a real struct that existed when compiling this
+ // binary, so its layout is not so large that it can trigger arithmetic overflow.
+ let val_offset = unsafe { refcount_layout.extend(val_layout).unwrap_unchecked().1 };
+
+ // Pointer casts leave the metadata unchanged. This is okay because the metadata of `T` and
+ // `ArcInner<T>` is the same since `ArcInner` is a struct with `T` as its last field.
+ //
+ // This is documented at:
+ // <https://doc.rust-lang.org/std/ptr/trait.Pointee.html>.
+ let ptr = ptr as *const ArcInner<T>;
+
+ // SAFETY: The pointer is in-bounds of an allocation both before and after offsetting the
+ // pointer, since it originates from a previous call to `Arc::into_raw` on an `Arc` that is
+ // still valid.
+ let ptr = unsafe { ptr.byte_sub(val_offset) };
+
+ // SAFETY: The pointer can't be null since you can't have an `ArcInner<T>` value at the null
+ // address.
+ unsafe { NonNull::new_unchecked(ptr.cast_mut()) }
+ }
+}
+
// This is to allow [`Arc`] (and variants) to be used as the type of `self`.
impl<T: ?Sized> core::ops::Receiver for Arc<T> {}
@@ -232,27 +265,13 @@ pub fn into_raw(self) -> *const T {
/// `ptr` must have been returned by a previous call to [`Arc::into_raw`]. Additionally, it
/// must not be called more than once for each previous call to [`Arc::into_raw`].
pub unsafe fn from_raw(ptr: *const T) -> Self {
- let refcount_layout = Layout::new::<bindings::refcount_t>();
- // SAFETY: The caller guarantees that the pointer is valid.
- let val_layout = Layout::for_value(unsafe { &*ptr });
- // SAFETY: We're computing the layout of a real struct that existed when compiling this
- // binary, so its layout is not so large that it can trigger arithmetic overflow.
- let val_offset = unsafe { refcount_layout.extend(val_layout).unwrap_unchecked().1 };
-
- // Pointer casts leave the metadata unchanged. This is okay because the metadata of `T` and
- // `ArcInner<T>` is the same since `ArcInner` is a struct with `T` as its last field.
- //
- // This is documented at:
- // <https://doc.rust-lang.org/std/ptr/trait.Pointee.html>.
- let ptr = ptr as *const ArcInner<T>;
-
- // SAFETY: The pointer is in-bounds of an allocation both before and after offsetting the
- // pointer, since it originates from a previous call to `Arc::into_raw` and is still valid.
- let ptr = unsafe { ptr.byte_sub(val_offset) };
+ // SAFETY: The caller promises that this pointer originates from a call to `into_raw` on an
+ // `Arc` that is still valid.
+ let ptr = unsafe { ArcInner::container_of(ptr) };
// SAFETY: By the safety requirements we know that `ptr` came from `Arc::into_raw`, so the
// reference count held then will be owned by the new `Arc` object.
- unsafe { Self::from_inner(NonNull::new_unchecked(ptr.cast_mut())) }
+ unsafe { Self::from_inner(ptr) }
}
/// Returns an [`ArcBorrow`] from the given [`Arc`].
@@ -453,6 +472,27 @@ unsafe fn new(inner: NonNull<ArcInner<T>>) -> Self {
_p: PhantomData,
}
}
+
+ /// Creates an [`ArcBorrow`] to an [`Arc`] that has previously been deconstructed with
+ /// [`Arc::into_raw`].
+ ///
+ /// # Safety
+ ///
+ /// * The provided pointer must originate from a call to [`Arc::into_raw`].
+ /// * For the duration of the lifetime annotated on this `ArcBorrow`, the reference count must
+ /// not hit zero.
+ /// * For the duration of the lifetime annotated on this `ArcBorrow`, there must not be a
+ /// [`UniqueArc`] reference to this value.
+ pub unsafe fn from_raw(ptr: *const T) -> Self {
+ // SAFETY: The caller promises that this pointer originates from a call to `into_raw` on an
+ // `Arc` that is still valid.
+ let ptr = unsafe { ArcInner::container_of(ptr) };
+
+ // SAFETY: The caller promises that the value remains valid since the reference count must
+ // not hit zero, and no mutable reference will be created since that would involve a
+ // `UniqueArc`.
+ unsafe { Self::new(ptr) }
+ }
}
impl<T: ?Sized> From<ArcBorrow<'_, T>> for Arc<T> {
--
2.44.0.478.gd926399ef9-goog
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