linux-kernel - [PATCH v2] rust: alloc: allow coercion from `Box<T>` to `Box<dyn U>` if T implements U

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-Id: <20250411-box_trait_objs-v2-1-c5f31b8db847@nvidia.com>
Date: Fri, 11 Apr 2025 21:07:26 +0900
From: Alexandre Courbot <acourbot@...dia.com>
To: Danilo Krummrich <dakr@...nel.org>, Miguel Ojeda <ojeda@...nel.org>, 
 Alex Gaynor <alex.gaynor@...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@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>, 
 Trevor Gross <tmgross@...ch.edu>
Cc: rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org, 
 Alexandre Courbot <acourbot@...dia.com>
Subject: [PATCH v2] rust: alloc: allow coercion from `Box<T>` to `Box<dyn
 U>` if T implements U

This enables the creation of trait objects backed by a Box, similarly to
what can be done with the standard library.

Suggested-by: Benno Lossin <benno.lossin@...ton.me>
Signed-off-by: Alexandre Courbot <acourbot@...dia.com>
---
>From this discussion on Zulip [1].

Heavily inspired from the similar feature on `Arc`.

[1] https://rust-for-linux.zulipchat.com/#narrow/channel/291565-Help/topic/Trait.20objects.3F/with/510689662
---
Changes in v2:
- Use where clauses to improve readability.
- Fix build with rustc 1.78.
- Link to v1: https://lore.kernel.org/r/20250408-box_trait_objs-v1-1-58d8e78b0fb2@nvidia.com
---
 rust/kernel/alloc/kbox.rs | 30 +++++++++++++++++++++++++++++-
 1 file changed, 29 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
index b77d32f3a58bab5ec73c612bdaaba0d79bfdff65..b9a905cd4bd285782b0db284b6771aec03e0c10b 100644
--- a/rust/kernel/alloc/kbox.rs
+++ b/rust/kernel/alloc/kbox.rs
@@ -32,6 +32,8 @@
 ///
 /// When dropping a [`Box`], the value is also dropped and the heap memory is automatically freed.
 ///
+/// [`Box`]es can also be used to store trait objects by coercing their type.
+///
 /// # Examples
 ///
 /// ```
@@ -62,7 +64,33 @@
 /// `self.0` is always properly aligned and either points to memory allocated with `A` or, for
 /// zero-sized types, is a dangling, well aligned pointer.
 #[repr(transparent)]
-pub struct Box<T: ?Sized, A: Allocator>(NonNull<T>, PhantomData<A>);
+#[cfg_attr(CONFIG_RUSTC_HAS_COERCE_POINTEE, derive(core::marker::CoercePointee))]
+pub struct Box<#[cfg_attr(CONFIG_RUSTC_HAS_COERCE_POINTEE, pointee)] T: ?Sized, A: Allocator>(
+    NonNull<T>,
+    PhantomData<A>,
+);
+
+// This is to allow coercion from `Box<T, A>` to `Box<U, A>` if `T` can be converted to the
+// dynamically-sized type (DST) `U`.
+#[cfg(not(CONFIG_RUSTC_HAS_COERCE_POINTEE))]
+impl<T, U, A> core::ops::CoerceUnsized<Box<U, A>> for Box<T, A>
+where
+    T: ?Sized + core::marker::Unsize<U>,
+    U: ?Sized,
+    A: Allocator,
+{
+}
+
+// This is to allow `Box<U, A>` to be dispatched on when `Box<T, A>` can be coerced into `Box<U,
+// A>`.
+#[cfg(not(CONFIG_RUSTC_HAS_COERCE_POINTEE))]
+impl<T, U, A> core::ops::DispatchFromDyn<Box<U, A>> for Box<T, A>
+where
+    T: ?Sized + core::marker::Unsize<U>,
+    U: ?Sized,
+    A: Allocator,
+{
+}
 
 /// Type alias for [`Box`] with a [`Kmalloc`] allocator.
 ///

---
base-commit: 0af2f6be1b4281385b618cb86ad946eded089ac8
change-id: 20250408-box_trait_objs-02a700401f0b

Best regards,
-- 
Alexandre Courbot <acourbot@...dia.com>