Adds a macro for safely implementing the ListItem trait. As part of the
implementation of the macro, we also provide a HasListLinks trait
similar to the workqueue's HasWorkItem trait.

The HasListLinks trait is only necessary if you are implementing
ListItem using the impl_list_item macro.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 rust/kernel/list.rs                    |  3 ++
 rust/kernel/list/impl_list_item_mod.rs | 98 ++++++++++++++++++++++++++++++++++
 2 files changed, 101 insertions(+)

diff --git a/rust/kernel/list.rs b/rust/kernel/list.rs
index 76597c49fa56..7af5109500f2 100644
--- a/rust/kernel/list.rs
+++ b/rust/kernel/list.rs
@@ -8,6 +8,9 @@
 use crate::types::Opaque;
 use core::ptr;
 
+mod impl_list_item_mod;
+pub use self::impl_list_item_mod::{impl_has_list_links, impl_list_item, HasListLinks};
+
 mod arc;
 pub use self::arc::{
     impl_list_arc_safe, AtomicListArcTracker, ListArc, ListArcSafe, TryNewListArc,
diff --git a/rust/kernel/list/impl_list_item_mod.rs b/rust/kernel/list/impl_list_item_mod.rs
new file mode 100644
index 000000000000..9e2f6d6d4786
--- /dev/null
+++ b/rust/kernel/list/impl_list_item_mod.rs
@@ -0,0 +1,98 @@
+// SPDX-License-Identifier: GPL-2.0
+
+// Copyright (C) 2024 Google LLC.
+
+//! Helpers for implementing list traits safely.
+
+use crate::list::ListLinks;
+
+/// Declares that this type has a `ListLinks<ID>` field at a fixed offset.
+///
+/// This trait is only used to help implement `ListItem` safely. If `ListItem` is implemented
+/// manually, then this trait is not needed.
+///
+/// # Safety
+///
+/// All values of this type must have a `ListLinks<ID>` field at the given offset.
+pub unsafe trait HasListLinks<const ID: u64 = 0> {
+    /// The offset of the `ListLinks` field.
+    const OFFSET: usize;
+
+    /// Returns a pointer to the [`ListLinks<T, ID>`] field.
+    ///
+    /// # Safety
+    ///
+    /// The provided pointer must point at a valid struct of type `Self`.
+    ///
+    /// [`ListLinks<T, ID>`]: ListLinks
+    // We don't really need this method, but it's necessary for the implementation of
+    // `impl_has_work!` to be correct.
+    #[inline]
+    unsafe fn raw_get_list_links(ptr: *mut Self) -> *mut ListLinks<ID> {
+        // SAFETY: The caller promises that the pointer is valid.
+        unsafe { (ptr as *mut u8).add(Self::OFFSET) as *mut ListLinks<ID> }
+    }
+}
+
+/// Implements the [`HasListLinks`] trait for the given type.
+#[macro_export]
+macro_rules! impl_has_list_links {
+    ($(impl$(<$($implarg:ident),*>)?
+       HasListLinks$(<$id:tt>)?
+       for $self:ident $(<$($selfarg:ty),*>)?
+       { self$(.$field:ident)* }
+    )*) => {$(
+        // SAFETY: The implementation of `raw_get_list_links` only compiles if the field has the
+        // right type.
+        unsafe impl$(<$($implarg),*>)? $crate::list::HasListLinks$(<$id>)? for
+            $self $(<$($selfarg),*>)?
+        {
+            const OFFSET: usize = ::core::mem::offset_of!(Self, $($field).*) as usize;
+
+            #[inline]
+            unsafe fn raw_get_list_links(ptr: *mut Self) -> *mut $crate::list::ListLinks$(<$id>)? {
+                // SAFETY: The caller promises that the pointer is not dangling.
+                unsafe {
+                    ::core::ptr::addr_of_mut!((*ptr)$(.$field)*)
+                }
+            }
+        }
+    )*};
+}
+pub use impl_has_list_links;
+
+/// Implements the [`ListItem`] trait for the given type.
+///
+/// Assumes that the type implements [`HasListLinks`].
+///
+/// [`ListItem`]: crate::list::ListItem
+#[macro_export]
+macro_rules! impl_list_item {
+    (
+        impl$({$($generics:tt)*})? ListItem<$num:tt> for $t:ty {
+            using ListLinks;
+        } $($rest:tt)*
+    ) => {
+        unsafe impl$(<$($generics)*>)? ListItem<$num> for $t {
+            unsafe fn view_links(me: *const Self) -> *mut ListLinks<$num> {
+                unsafe {
+                    <Self as HasListLinks<$num>>::raw_get_list_links(me.cast_mut())
+                }
+            }
+
+            unsafe fn view_value(me: *mut ListLinks<$num>) -> *const Self {
+                let offset = <Self as HasListLinks<$num>>::OFFSET;
+                unsafe { (me as *const u8).sub(offset) as *const Self }
+            }
+
+            unsafe fn prepare_to_insert(me: *const Self) -> *mut ListLinks<$num> {
+                unsafe { Self::view_links(me) }
+            }
+
+            unsafe fn post_remove(me: *mut ListLinks<$num>) -> *const Self {
+                unsafe { Self::view_value(me) }
+            }
+        }
+    };
+}
+pub use impl_list_item;

-- 
2.44.0.478.gd926399ef9-goog

On 02.04.24 14:17, Alice Ryhl wrote:
> +/// Declares that this type has a `ListLinks<ID>` field at a fixed offset.
> +///
> +/// This trait is only used to help implement `ListItem` safely. If `ListItem` is implemented
> +/// manually, then this trait is not needed.
> +///
> +/// # Safety
> +///
> +/// All values of this type must have a `ListLinks<ID>` field at the given offset.
> +pub unsafe trait HasListLinks<const ID: u64 = 0> {
> +    /// The offset of the `ListLinks` field.
> +    const OFFSET: usize;
> +
> +    /// Returns a pointer to the [`ListLinks<T, ID>`] field.
> +    ///
> +    /// # Safety
> +    ///
> +    /// The provided pointer must point at a valid struct of type `Self`.
> +    ///
> +    /// [`ListLinks<T, ID>`]: ListLinks
> +    // We don't really need this method, but it's necessary for the implementation of
> +    // `impl_has_work!` to be correct.
> +    #[inline]
> +    unsafe fn raw_get_list_links(ptr: *mut Self) -> *mut ListLinks<ID> {
> +        // SAFETY: The caller promises that the pointer is valid.

The caller only provides a valid pointer, this also needs the
requirement from the trait that `OFFSET` is correct.

-- 
Cheers,
Benno

> +        unsafe { (ptr as *mut u8).add(Self::OFFSET) as *mut ListLinks<ID> }
> +    }
> +}