Re: [PATCH v2 4/9] rust: list: add macro for implementing ListItem

[Benno Lossin]

On 06.05.24 11:53, Alice Ryhl wrote:
> 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@xxxxxxxxxx>
> ---
>  rust/kernel/list.rs                    |  3 ++
>  rust/kernel/list/impl_list_item_mod.rs | 99 ++++++++++++++++++++++++++++++++++
>  2 files changed, 102 insertions(+)
> 
> diff --git a/rust/kernel/list.rs b/rust/kernel/list.rs
> index b5cfbb96a392..f2eca542e090 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..3ff483be89d1
> --- /dev/null
> +++ b/rust/kernel/list/impl_list_item_mod.rs
> @@ -0,0 +1,99 @@
> +// 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.

You could also check for correctness within the expansion of
`impl_has_list_links` (by creating an invisible function there), so you
don't need this function. It still could be useful for `dyn Trait`
though.

> +    #[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.
> +        unsafe impl$(<$($implarg),*>)? $crate::list::HasListLinks$(<$id>)? for
> +            $self $(<$($selfarg),*>)?
> +        {
> +            const OFFSET: usize = ::core::mem::offset_of!(Self, $($field).*) as usize;

AFAIK only a single field is supported on stable.

> +
> +            #[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)*)

Note that this runs into the `Box` issue, but `offset_of` will fail to
compile in that case, so it is fine.
Do we want to note this in a comment?

> +                }
> +            }
> +        }
> +    )*};
> +}
> +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)*})? $crate::list::ListItem<$num:tt> for $t:ty {
> +            using ListLinks;
> +        } $($rest:tt)*
> +    ) => {
> +        unsafe impl$(<$($generics)*>)? $crate::list::ListItem<$num> for $t {

Missing SAFETY comment.

---
Cheers,
Benno

> +            unsafe fn view_links(me: *const Self) -> *mut $crate::list::ListLinks<$num> {
> +                unsafe {
> +                    <Self as $crate::list::HasListLinks<$num>>::raw_get_list_links(me.cast_mut())
> +                }
> +            }
> +
> +            unsafe fn view_value(me: *mut $crate::list::ListLinks<$num>) -> *const Self {
> +                let offset = <Self as $crate::list::HasListLinks<$num>>::OFFSET;
> +                unsafe { (me as *const u8).sub(offset) as *const Self }
> +            }
> +
> +            unsafe fn prepare_to_insert(me: *const Self) -> *mut $crate::list::ListLinks<$num> {
> +                unsafe { <Self as $crate::list::ListItem<$num>>::view_links(me) }
> +            }
> +
> +            unsafe fn post_remove(me: *mut $crate::list::ListLinks<$num>) -> *const Self {
> +                unsafe { <Self as $crate::list::ListItem<$num>>::view_value(me) }
> +            }
> +        }
> +    };
> +}
> +pub use impl_list_item;
> 
> --
> 2.45.0.rc1.225.g2a3ae87e7f-goog
>