[PATCH RFC v6 01/26] rust: clist: Add support to interface with C linked lists

[Joel Fernandes <joelagnelf@nvidia.com>]

Add a new module `clist` for working with C's doubly circular linked
lists. Provide low-level iteration over list nodes.

Typed iteration over actual items is provided with a `clist_create`
macro to assist in creation of the `Clist` type.

Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
---
 MAINTAINERS            |   7 +
 rust/helpers/helpers.c |   1 +
 rust/helpers/list.c    |  12 ++
 rust/kernel/clist.rs   | 357 +++++++++++++++++++++++++++++++++++++++++
 rust/kernel/lib.rs     |   1 +
 5 files changed, 378 insertions(+)
 create mode 100644 rust/helpers/list.c
 create mode 100644 rust/kernel/clist.rs

diff --git a/MAINTAINERS b/MAINTAINERS
index 0d044a58cbfe..b76988c38045 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -22936,6 +22936,13 @@ F:	rust/kernel/init.rs
 F:	rust/pin-init/
 K:	\bpin-init\b|pin_init\b|PinInit
 
+RUST TO C LIST INTERFACES
+M:	Joel Fernandes <joelagnelf@nvidia.com>
+M:	Alexandre Courbot <acourbot@nvidia.com>
+L:	rust-for-linux@vger.kernel.org
+S:	Maintained
+F:	rust/kernel/clist.rs
+
 RXRPC SOCKETS (AF_RXRPC)
 M:	David Howells <dhowells@redhat.com>
 M:	Marc Dionne <marc.dionne@auristor.com>
diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
index 79c72762ad9c..634fa2386bbb 100644
--- a/rust/helpers/helpers.c
+++ b/rust/helpers/helpers.c
@@ -32,6 +32,7 @@
 #include "io.c"
 #include "jump_label.c"
 #include "kunit.c"
+#include "list.c"
 #include "maple_tree.c"
 #include "mm.c"
 #include "mutex.c"
diff --git a/rust/helpers/list.c b/rust/helpers/list.c
new file mode 100644
index 000000000000..6044979c7a2e
--- /dev/null
+++ b/rust/helpers/list.c
@@ -0,0 +1,12 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * Helpers for C Circular doubly linked list implementation.
+ */
+
+#include <linux/list.h>
+
+void rust_helper_list_add_tail(struct list_head *new, struct list_head *head)
+{
+	list_add_tail(new, head);
+}
diff --git a/rust/kernel/clist.rs b/rust/kernel/clist.rs
new file mode 100644
index 000000000000..91754ae721b9
--- /dev/null
+++ b/rust/kernel/clist.rs
@@ -0,0 +1,357 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! A C doubly circular intrusive linked list interface for rust code.
+//!
+//! # Examples
+//!
+//! ```
+//! use kernel::{
+//!     bindings,
+//!     clist::init_list_head,
+//!     clist_create,
+//!     types::Opaque, //
+//! };
+//! # // Create test list with values (0, 10, 20) - normally done by C code but it is
+//! # // emulated here for doctests using the C bindings.
+//! # use core::mem::MaybeUninit;
+//! #
+//! # /// C struct with embedded `list_head` (typically will be allocated by C code).
+//! # #[repr(C)]
+//! # pub(crate) struct SampleItemC {
+//! #     pub value: i32,
+//! #     pub link: bindings::list_head,
+//! # }
+//! #
+//! # let mut head = MaybeUninit::<bindings::list_head>::uninit();
+//! #
+//! # let head = head.as_mut_ptr();
+//! # // SAFETY: head and all the items are test objects allocated in this scope.
+//! # unsafe { init_list_head(head) };
+//! #
+//! # let mut items = [
+//! #     MaybeUninit::<SampleItemC>::uninit(),
+//! #     MaybeUninit::<SampleItemC>::uninit(),
+//! #     MaybeUninit::<SampleItemC>::uninit(),
+//! # ];
+//! #
+//! # for (i, item) in items.iter_mut().enumerate() {
+//! #     let ptr = item.as_mut_ptr();
+//! #     // SAFETY: pointers are to allocated test objects with a list_head field.
+//! #     unsafe {
+//! #         (*ptr).value = i as i32 * 10;
+//! #         // addr_of_mut!() computes address of link directly as link is uninitialized.
+//! #         init_list_head(core::ptr::addr_of_mut!((*ptr).link));
+//! #         bindings::list_add_tail(&mut (*ptr).link, head);
+//! #     }
+//! # }
+//!
+//! // Rust wrapper for the C struct.
+//! // The list item struct in this example is defined in C code as:
+//! //   struct SampleItemC {
+//! //       int value;
+//! //       struct list_head link;
+//! //   };
+//! //
+//! #[repr(transparent)]
+//! pub(crate) struct Item(Opaque<SampleItemC>);
+//!
+//! impl Item {
+//!     pub(crate) fn value(&self) -> i32 {
+//!         // SAFETY: [`Item`] has same layout as [`SampleItemC`].
+//!         unsafe { (*self.0.get()).value }
+//!     }
+//! }
+//!
+//! // Create typed [`CList`] from sentinel head.
+//! // SAFETY: head is valid, items are [`SampleItemC`] with embedded `link` field.
+//! let list = unsafe { clist_create!(head, Item, SampleItemC, link) };
+//!
+//! // Iterate directly over typed items.
+//! let mut found_0 = false;
+//! let mut found_10 = false;
+//! let mut found_20 = false;
+//!
+//! for item in list.iter() {
+//!     let val = item.value();
+//!     if val == 0 { found_0 = true; }
+//!     if val == 10 { found_10 = true; }
+//!     if val == 20 { found_20 = true; }
+//! }
+//!
+//! assert!(found_0 && found_10 && found_20);
+//! ```
+
+use core::{
+    iter::FusedIterator,
+    marker::PhantomData, //
+};
+
+use crate::{
+    bindings,
+    types::Opaque, //
+};
+
+use pin_init::PinInit;
+
+/// Initialize a `list_head` object to point to itself.
+///
+/// # Safety
+///
+/// `list` must be a valid pointer to a `list_head` object.
+#[inline]
+pub unsafe fn init_list_head(list: *mut bindings::list_head) {
+    // SAFETY: Caller guarantees `list` is a valid pointer to a `list_head`.
+    unsafe {
+        (*list).next = list;
+        (*list).prev = list;
+    }
+}
+
+/// Wraps a `list_head` object for use in intrusive linked lists.
+///
+/// # Invariants
+///
+/// - [`CListHead`] represents an allocated and valid `list_head` structure.
+/// - Once a [`CListHead`] is created in Rust, it will not be modified by non-Rust code.
+/// - All `list_head` for individual items are not modified for the lifetime of [`CListHead`].
+#[repr(transparent)]
+pub struct CListHead(Opaque<bindings::list_head>);
+
+impl CListHead {
+    /// Create a `&CListHead` reference from a raw `list_head` pointer.
+    ///
+    /// # Safety
+    ///
+    /// - `ptr` must be a valid pointer to an allocated and initialized `list_head` structure.
+    /// - `ptr` must remain valid and unmodified for the lifetime `'a`.
+    #[inline]
+    pub unsafe fn from_raw<'a>(ptr: *mut bindings::list_head) -> &'a Self {
+        // SAFETY:
+        // - [`CListHead`] has same layout as `list_head`.
+        // - `ptr` is valid and unmodified for 'a.
+        unsafe { &*ptr.cast() }
+    }
+
+    /// Get the raw `list_head` pointer.
+    #[inline]
+    pub fn as_raw(&self) -> *mut bindings::list_head {
+        self.0.get()
+    }
+
+    /// Get the next [`CListHead`] in the list.
+    #[inline]
+    pub fn next(&self) -> &Self {
+        let raw = self.as_raw();
+        // SAFETY:
+        // - `self.as_raw()` is valid per type invariants.
+        // - The `next` pointer is guaranteed to be non-NULL.
+        unsafe { Self::from_raw((*raw).next) }
+    }
+
+    /// Get the previous [`CListHead`] in the list.
+    #[inline]
+    pub fn prev(&self) -> &Self {
+        let raw = self.as_raw();
+        // SAFETY:
+        // - self.as_raw() is valid per type invariants.
+        // - The `prev` pointer is guaranteed to be non-NULL.
+        unsafe { Self::from_raw((*raw).prev) }
+    }
+
+    /// Check if this node is linked in a list (not isolated).
+    #[inline]
+    pub fn is_linked(&self) -> bool {
+        let raw = self.as_raw();
+        // SAFETY: self.as_raw() is valid per type invariants.
+        unsafe { (*raw).next != raw && (*raw).prev != raw }
+    }
+
+    /// Fallible pin-initializer that initializes and then calls user closure.
+    ///
+    /// Initializes the list head first, then passes `&CListHead` to the closure.
+    /// This hides the raw FFI pointer from the user.
+    pub fn try_init<E>(
+        init_func: impl FnOnce(&CListHead) -> Result<(), E>,
+    ) -> impl PinInit<Self, E> {
+        // SAFETY: init_list_head initializes the list_head to point to itself.
+        // After initialization, we create a reference to pass to the closure.
+        unsafe {
+            pin_init::pin_init_from_closure(move |slot: *mut Self| {
+                init_list_head(slot.cast());
+                // SAFETY: slot is now initialized, safe to create reference.
+                init_func(&*slot)
+            })
+        }
+    }
+}
+
+// SAFETY: [`CListHead`] can be sent to any thread.
+unsafe impl Send for CListHead {}
+
+// SAFETY: [`CListHead`] can be shared among threads as it is not modified
+// by non-Rust code per type invariants.
+unsafe impl Sync for CListHead {}
+
+impl PartialEq for CListHead {
+    fn eq(&self, other: &Self) -> bool {
+        self.as_raw() == other.as_raw()
+    }
+}
+
+impl Eq for CListHead {}
+
+/// Low-level iterator over `list_head` nodes.
+///
+/// An iterator used to iterate over a C intrusive linked list (`list_head`). Caller has to
+/// perform conversion of returned [`CListHead`] to an item (using `container_of` macro or similar).
+///
+/// # Invariants
+///
+/// [`CListHeadIter`] is iterating over an allocated, initialized and valid list.
+struct CListHeadIter<'a> {
+    current_head: &'a CListHead,
+    list_head: &'a CListHead,
+}
+
+impl<'a> Iterator for CListHeadIter<'a> {
+    type Item = &'a CListHead;
+
+    #[inline]
+    fn next(&mut self) -> Option<Self::Item> {
+        // Advance to next node.
+        let next = self.current_head.next();
+
+        // Check if we've circled back to the sentinel head.
+        if next == self.list_head {
+            None
+        } else {
+            self.current_head = next;
+            Some(self.current_head)
+        }
+    }
+}
+
+impl<'a> FusedIterator for CListHeadIter<'a> {}
+
+/// A typed C linked list with a sentinel head.
+///
+/// A sentinel head represents the entire linked list and can be used for
+/// iteration over items of type `T`, it is not associated with a specific item.
+///
+/// The const generic `OFFSET` specifies the byte offset of the `list_head` field within
+/// the struct that `T` wraps.
+///
+/// # Invariants
+///
+/// - `head` is an allocated and valid C `list_head` structure that is the list's sentinel.
+/// - `OFFSET` is the byte offset of the `list_head` field within the struct that `T` wraps.
+/// - All the list's `list_head` nodes are allocated and have valid next/prev pointers.
+/// - The underlying `list_head` (and entire list) is not modified for the lifetime `'a`.
+pub struct CList<'a, T, const OFFSET: usize> {
+    head: &'a CListHead,
+    _phantom: PhantomData<&'a T>,
+}
+
+impl<'a, T, const OFFSET: usize> CList<'a, T, OFFSET> {
+    /// Create a typed [`CList`] from a raw sentinel `list_head` pointer.
+    ///
+    /// # Safety
+    ///
+    /// - `ptr` must be a valid pointer to an allocated and initialized `list_head` structure
+    ///   representing a list sentinel.
+    /// - `ptr` must remain valid and unmodified for the lifetime `'a`.
+    /// - The list must contain items where the `list_head` field is at byte offset `OFFSET`.
+    /// - `T` must be `#[repr(transparent)]` over the C struct.
+    #[inline]
+    pub unsafe fn from_raw(ptr: *mut bindings::list_head) -> Self {
+        Self {
+            // SAFETY: Caller guarantees `ptr` is a valid, sentinel `list_head` object.
+            head: unsafe { CListHead::from_raw(ptr) },
+            _phantom: PhantomData,
+        }
+    }
+
+    /// Get the raw sentinel `list_head` pointer.
+    #[inline]
+    pub fn as_raw(&self) -> *mut bindings::list_head {
+        self.head.as_raw()
+    }
+
+    /// Check if the list is empty.
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        let raw = self.as_raw();
+        // SAFETY: self.as_raw() is valid per type invariants.
+        unsafe { (*raw).next == raw }
+    }
+
+    /// Create an iterator over typed items.
+    #[inline]
+    pub fn iter(&self) -> CListIter<'a, T, OFFSET> {
+        CListIter {
+            head_iter: CListHeadIter {
+                current_head: self.head,
+                list_head: self.head,
+            },
+            _phantom: PhantomData,
+        }
+    }
+}
+
+/// High-level iterator over typed list items.
+pub struct CListIter<'a, T, const OFFSET: usize> {
+    head_iter: CListHeadIter<'a>,
+    _phantom: PhantomData<&'a T>,
+}
+
+impl<'a, T, const OFFSET: usize> Iterator for CListIter<'a, T, OFFSET> {
+    type Item = &'a T;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let head = self.head_iter.next()?;
+
+        // Convert to item using OFFSET.
+        // SAFETY: `item_ptr` calculation from `OFFSET` (calculated using offset_of!)
+        // is valid per invariants.
+        Some(unsafe { &*head.as_raw().byte_sub(OFFSET).cast::<T>() })
+    }
+}
+
+impl<'a, T, const OFFSET: usize> FusedIterator for CListIter<'a, T, OFFSET> {}
+
+/// Create a C doubly-circular linked list interface [`CList`] from a raw `list_head` pointer.
+///
+/// This macro creates a [`CList<T, OFFSET>`] that can iterate over items of type `$rust_type`
+/// linked via the `$field` field in the underlying C struct `$c_type`.
+///
+/// # Arguments
+///
+/// - `$head`: Raw pointer to the sentinel `list_head` object (`*mut bindings::list_head`).
+/// - `$rust_type`: Each item's rust wrapper type.
+/// - `$c_type`: Each item's C struct type that contains the embedded `list_head`.
+/// - `$field`: The name of the `list_head` field within the C struct.
+///
+/// # Safety
+///
+/// The caller must ensure:
+/// - `$head` is a valid, initialized sentinel `list_head` pointing to a list that remains
+///   unmodified for the lifetime of the rust [`CList`].
+/// - The list contains items of type `$c_type` linked via an embedded `$field`.
+/// - `$rust_type` is `#[repr(transparent)]` over `$c_type` or has compatible layout.
+/// - The macro is called from an unsafe block.
+///
+/// # Examples
+///
+/// Refer to the examples in the [`crate::clist`] module documentation.
+#[macro_export]
+macro_rules! clist_create {
+    ($head:expr, $rust_type:ty, $c_type:ty, $($field:tt).+) => {{
+        // Compile-time check that field path is a list_head.
+        let _: fn(*const $c_type) -> *const $crate::bindings::list_head =
+            |p| ::core::ptr::addr_of!((*p).$($field).+);
+
+        // Calculate offset and create `CList`.
+        const OFFSET: usize = ::core::mem::offset_of!($c_type, $($field).+);
+        $crate::clist::CList::<$rust_type, OFFSET>::from_raw($head)
+    }};
+}
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index f812cf120042..cd7e6a1055b0 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -75,6 +75,7 @@
 pub mod bug;
 #[doc(hidden)]
 pub mod build_assert;
+pub mod clist;
 pub mod clk;
 #[cfg(CONFIG_CONFIGFS_FS)]
 pub mod configfs;
-- 
2.34.1