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.

Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 rust/kernel/list.rs                    |   3 +
 rust/kernel/list/impl_list_item_mod.rs | 143 +++++++++++++++++++++++++++++++++
 2 files changed, 146 insertions(+)

diff --git a/rust/kernel/list.rs b/rust/kernel/list.rs
index 074ae863ff5a..670d53989b8f 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, AtomicTracker, 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..1bcb14774aeb
--- /dev/null
+++ b/rust/kernel/list/impl_list_item_mod.rs
@@ -0,0 +1,143 @@
+// 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. Use the [`impl_has_list_links!`] macro to implement
+/// this trait.
+///
+/// # Safety
+///
+/// All values of this type must have a `ListLinks<ID>` field at the given offset.
+///
+/// The behavior of `raw_get_list_links` must not be changed.
+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_list_links!` 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 implementer promises that the
+        // `OFFSET` constant is correct.
+        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.
+        //
+        // The behavior of `raw_get_list_links` is not changed since the `addr_of_mut!` macro is
+        // equivalent to the pointer offset operation in the trait definition.
+        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 $crate::list::ListLinks$(<$id>)? {
+                // SAFETY: The caller promises that the pointer is not dangling. We know that this
+                // expression doesn't follow any pointers, as the `offset_of!` invocation above
+                // would otherwise not compile.
+                unsafe { ::core::ptr::addr_of_mut!((*ptr)$(.$field)*) }
+            }
+        }
+    )*};
+}
+pub use impl_has_list_links;
+
+/// Implements the [`ListItem`] trait for the given type.
+///
+/// Requires that the type implements [`HasListLinks`]. Use the [`impl_has_list_links!`] macro to
+/// implement that trait.
+///
+/// [`ListItem`]: crate::list::ListItem
+#[macro_export]
+macro_rules! impl_list_item {
+    (
+        $(impl$({$($generics:tt)*})? ListItem<$num:tt> for $t:ty {
+            using ListLinks;
+        })*
+    ) => {$(
+        // SAFETY: See GUARANTEES comment on each method.
+        unsafe impl$(<$($generics)*>)? $crate::list::ListItem<$num> for $t {
+            // GUARANTEES:
+            // * This returns the same pointer as `prepare_to_insert` because `prepare_to_insert`
+            //   is implemented in terms of `view_links`.
+            // * By the type invariants of `ListLinks`, the `ListLinks` has two null pointers when
+            //   this value is not in a list.
+            unsafe fn view_links(me: *const Self) -> *mut $crate::list::ListLinks<$num> {
+                // SAFETY: The caller guarantees that `me` points at a valid value of type `Self`.
+                unsafe {
+                    <Self as $crate::list::HasListLinks<$num>>::raw_get_list_links(me.cast_mut())
+                }
+            }
+
+            // GUARANTEES:
+            // * `me` originates from the most recent call to `prepare_to_insert`, which just added
+            //   `offset` to the pointer passed to `prepare_to_insert`. This method subtracts
+            //   `offset` from `me` so it returns the pointer originally passed to
+            //   `prepare_to_insert`.
+            // * The pointer remains valid until the next call to `post_remove` because the caller
+            //   of the most recent call to `prepare_to_insert` promised to retain ownership of the
+            //   `ListArc` containing `Self` until the next call to `post_remove`. The value cannot
+            //   be destroyed while a `ListArc` reference exists.
+            unsafe fn view_value(me: *mut $crate::list::ListLinks<$num>) -> *const Self {
+                let offset = <Self as $crate::list::HasListLinks<$num>>::OFFSET;
+                // SAFETY: `me` originates from the most recent call to `prepare_to_insert`, so it
+                // points at the field at offset `offset` in a value of type `Self`. Thus,
+                // subtracting `offset` from `me` is still in-bounds of the allocation.
+                unsafe { (me as *const u8).sub(offset) as *const Self }
+            }
+
+            // GUARANTEES:
+            // This implementation of `ListItem` will not give out exclusive access to the same
+            // `ListLinks` several times because calls to `prepare_to_insert` and `post_remove`
+            // must alternate and exclusive access is given up when `post_remove` is called.
+            //
+            // Other invocations of `impl_list_item!` also cannot give out exclusive access to the
+            // same `ListLinks` because you can only implement `ListItem` once for each value of
+            // `ID`, and the `ListLinks` fields only work with the specified `ID`.
+            unsafe fn prepare_to_insert(me: *const Self) -> *mut $crate::list::ListLinks<$num> {
+                // SAFETY: The caller promises that `me` points at a valid value.
+                unsafe { <Self as $crate::list::ListItem<$num>>::view_links(me) }
+            }
+
+            // GUARANTEES:
+            // * `me` originates from the most recent call to `prepare_to_insert`, which just added
+            //   `offset` to the pointer passed to `prepare_to_insert`. This method subtracts
+            //   `offset` from `me` so it returns the pointer originally passed to
+            //   `prepare_to_insert`.
+            unsafe fn post_remove(me: *mut $crate::list::ListLinks<$num>) -> *const Self {
+                let offset = <Self as $crate::list::HasListLinks<$num>>::OFFSET;
+                // SAFETY: `me` originates from the most recent call to `prepare_to_insert`, so it
+                // points at the field at offset `offset` in a value of type `Self`. Thus,
+                // subtracting `offset` from `me` is still in-bounds of the allocation.
+                unsafe { (me as *const u8).sub(offset) as *const Self }
+            }
+        }
+    )*};
+}
+pub use impl_list_item;

-- 
2.46.0.76.ge559c4bf1a-goog