On 11/29/25 1:30 PM, Joel Fernandes wrote: > Add a new module `clist` for working with C's doubly circular linked > lists. Provide low-level iteration over list_head nodes and high-level > iteration over typed list items. ... > > MAINTAINERS | 7 + > rust/helpers/helpers.c | 1 + > rust/helpers/list.c | 12 ++ > rust/kernel/clist.rs | 349 +++++++++++++++++++++++++++++++++++++++++ > rust/kernel/lib.rs | 1 + > 5 files changed, 370 insertions(+) > create mode 100644 rust/helpers/list.c > create mode 100644 rust/kernel/clist.rs
Hi Joel, This is sufficiently tricky that I think it needs some code to exercise it. Lately I'm not sure what to recommend, there are several choices, each with trade-offs: kunit, samples/rust, or even new DRM Rust code. Maybe the last one is especially nice, because it doesn't really have many downsides. Rather than wait for any of that, I wrote a quick samples/rust/rust_clist.rs and used it to sanity check my review findings, which are below. > > diff --git a/MAINTAINERS b/MAINTAINERS > index 5f7aa6a8a9a1..fb2ff877b6d1 100644 > --- a/MAINTAINERS > +++ b/MAINTAINERS > @@ -22666,6 +22666,13 @@ 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 <[email protected]> > +M: Alexandre Courbot <[email protected]> > +L: [email protected] > +S: Maintained > +F: rust/kernel/clist.rs > + > RXRPC SOCKETS (AF_RXRPC) > M: David Howells <[email protected]> > M: Marc Dionne <[email protected]> > 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. s/Circular/circular/ ...but: > + */ > + > +#include <linux/list.h> > + > +void rust_helper_list_add_tail(struct list_head *new, struct list_head *head) > +{ > + list_add_tail(new, head); > +} This is, so far, not used. Let's remove it, until/unless you have some code in this patch(set) to use it, please. > diff --git a/rust/kernel/clist.rs b/rust/kernel/clist.rs > new file mode 100644 > index 000000000000..361a6132299b > --- /dev/null > +++ b/rust/kernel/clist.rs > @@ -0,0 +1,349 @@ > +// 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(); > +//! # > +//! # // SAFETY: head and all the items are test objects allocated in this > scope. > +//! # unsafe { init_list_head(head.as_mut_ptr()) }; > +//! # // SAFETY: head is a test object allocated in this scope. > +//! # let mut head = unsafe { head.assume_init() }; This is a bug that leads to a corrupted list. I have the test code (and the kernel hang/crash) to prove it. The problem is that any move after init_list_head() invalidates the list: the next/prev pointers still point to the old address. The fix requires two-step initialization, like this, for example: //! # // Two-step init: create uninit first (can be moved), then init after. //! # let mut head = MaybeUninit::<bindings::list_head>::uninit(); //! # let mut items = [ //! # MaybeUninit::<SampleItemC>::uninit(), //! # MaybeUninit::<SampleItemC>::uninit(), //! # MaybeUninit::<SampleItemC>::uninit(), //! # ]; //! # //! # // Step 2: Now init after they're in their final location //! # // SAFETY: head is in its final stack location. //! # unsafe { init_list_head(head.as_mut_ptr()) }; > +//! # 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, &mut 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!(&mut 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, // > +}; > + > +/// 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. > +/// > +/// # Safety > +/// > +/// - All `list_head` nodes must not be modified by C code 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 } > + } > +} > + > +// 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 > C 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, > + exhausted: bool, > +} > + > +impl<'a> Iterator for ClistHeadIter<'a> { > + type Item = &'a ClistHead; > + > + #[inline] > + fn next(&mut self) -> Option<Self::Item> { > + if self.exhausted { > + return None; > + } > + > + // Advance to next node. > + self.current_head = self.current_head.next(); > + > + // Check if we've circled back to the sentinel head. > + if self.current_head == self.list_head { > + self.exhausted = true; > + return None; > + } > + > + 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. > +/// > +/// # 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 C > struct that `T` wraps. > +/// > +/// # Safety > +/// > +/// - All the list's `list_head` nodes must be allocated and have valid > next/prev pointers. > +/// - The underlying `list_head` (and entire list) must not be modified by C > for the > +/// lifetime 'a of `Clist`. > +pub struct Clist<'a, T> { > + head: &'a ClistHead, > + offset: usize, > + _phantom: PhantomData<&'a T>, > +} This discards build-time (const generic) information, and demotes it to runtime (.offset), without any real benefit. I believe it's better to keep it as a const generic, like this: pub struct Clist<'a, T, const OFFSET: usize> { head: &'a ClistHead, _phantom: PhantomData<&'a T>, } > + > +impl<'a, T> Clist<'a, T> { And here, the above becomes: impl<'a, T, const OFFSET: usize> Clist<'a, T, OFFSET> { ...etc. I've tested all of this locally, and the diffs look nice and all my tests still pass, of course. > + /// Create a typed `Clist` from a raw sentinel `list_head` pointer. > + /// > + /// The const generic `OFFSET` specifies the byte offset of the > `list_head` field within > + /// the C struct that `T` wraps. > + /// > + /// # 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_and_offset<const OFFSET: usize>(ptr: *mut > bindings::list_head) -> Self { > + Self { > + // SAFETY: Caller guarantees `ptr` is a valid, sentinel > `list_head` object. > + head: unsafe { ClistHead::from_raw(ptr) }, > + offset: OFFSET, > + _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> { > + ClistIter { > + head_iter: ClistHeadIter { > + current_head: self.head, > + list_head: self.head, > + exhausted: false, > + }, > + offset: self.offset, > + _phantom: PhantomData, > + } > + } > +} > + > +/// High-level iterator over typed list items. > +pub struct ClistIter<'a, T> { > + head_iter: ClistHeadIter<'a>, > + offset: usize, > + _phantom: PhantomData<&'a T>, > +} > + > +impl<'a, T> Iterator for ClistIter<'a, T> { > + 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().cast::<u8>().sub(self.offset).cast::<T>() }) > + } > +} > + > +impl<'a, T> FusedIterator for ClistIter<'a, T> {} > + > +/// Create a C doubly-circular linked list interface `Clist` from a raw > `list_head` pointer. > +/// > +/// This macro creates a `Clist<T>` 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:ident) => { > + $crate::clist::Clist::<$rust_type>::from_raw_and_offset::< > + { ::core::mem::offset_of!($c_type, $field) }, > + >($head) > + }; > +} Unlike the corresponding C container_of() macro, this one here has no compile-time verification that the field is actually a list_head. How about this, to add that check: --- a/rust/kernel/clist.rs +++ b/rust/kernel/clist.rs macro_rules! clist_create { - ($head:expr, $rust_type:ty, $c_type:ty, $field:ident) => { - $crate::clist::Clist::<$rust_type>::from_raw_and_offset::< + ($head:expr, $rust_type:ty, $c_type:ty, $field:ident) => {{ + // Compile-time check: $field must be a list_head. + const _: () = { + let _check: fn(*const $c_type) -> *const $crate::bindings::list_head = + |p| unsafe { ::core::ptr::addr_of!((*p).$field) }; + }; + $crate::clist::Clist::<$rust_type, { ::core::mem::offset_of!($c_type, $field) }>::from_raw( $head, ) - }; + }}; } > diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs > index c2eea9a2a345..b69cc5ed3b59 100644 > --- a/rust/kernel/lib.rs > +++ b/rust/kernel/lib.rs > @@ -72,6 +72,7 @@ > pub mod bug; > #[doc(hidden)] > pub mod build_assert; > +pub mod clist; > pub mod clk; > #[cfg(CONFIG_CONFIGFS_FS)] > pub mod configfs; thanks, -- John Hubbard
