[PATCH v3 05/10] rust: list: add macro for implementing ListItem

Alice Ryhl
2024-07-23T08:23:08Z

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 | 139 +++++++++++++++++++++++++++++++++
2 files changed, 142 insertions(+)

diff --git a/rust/kernel/list.rs b/rust/kernel/list.rs
index 63e6f6a47964..8fb7d2c13580 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..9b1947371c63
--- /dev/null
+++ b/rust/kernel/list/impl_list_item_mod.rs
@@ -0,0 +1,139 @@
+// 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.
+///
+/// The implementation 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_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 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 implementation 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 $crate::list::ListLinks$(<$id>)? {
+ // SAFETY: The caller promises that the pointer is not dangling. We know that this
+ // expression doesn't follow any pointers, as the `offset_of!` invocation above
+ // would otherwise not compile.
+ unsafe { ::core::ptr::addr_of_mut!((*ptr)$(.$field)*) }
+ }
+ }
+ )*};
+}
+pub use impl_has_list_links;
+
+/// Implements the [`ListItem`] trait for the given type.
+///
+/// Assumes that the type implements [`HasListLinks`].
+///
+/// [`ListItem`]: crate::list::ListItem
+#[macro_export]
+macro_rules! impl_list_item {
+ (
+ impl$({$($generics:tt)*})? ListItem<$num:tt> for $t:ty {
+ using ListLinks;
+ } $($rest:tt)*
+ ) => {
+ // 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:
+ // The first guarantee of `view_value` is exactly what `post_remove` guarantees.
+ unsafe fn post_remove(me: *mut $crate::list::ListLinks<$num>) -> *const Self {
+ // SAFETY: This violates the safety requirement that `post_remove` has not been
+ // called since the most recent call to `prepare_to_insert`, but that is okay
+ // because the concrete implementation of `view_value` above does not rely on that
+ // requirement anywhere except for its second guarantee, and we don't need its
+ // second guarantee.
+ unsafe { <Self as $crate::list::ListItem<$num>>::view_value(me) }
+ }
+ }
+ };
+}
+pub use impl_list_item;

--
2.45.2.1089.g2a221341d9-goog