Re: [PATCH v4 05/10] rust: list: add macro for implementing ListItem

[Benno Lossin <benno.lossin@proton.me>]

On 06.08.24 15:58, 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@google.com>
> ---
>  rust/kernel/list.rs                    |   3 +
>  rust/kernel/list/impl_list_item_mod.rs | 143 +++++++++++++++++++++++++++++++++
>  2 files changed, 146 insertions(+)

I found one safety documentation nit below, with that fixed

Reviewed-by: Benno Lossin <benno.lossin@proton.me>

> 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..30886e64b8ef
> --- /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 implementation of `raw_get_list_links` must not be changed.

You require here that the implementation must not be changed, but I
would argue that the implementation is changed in the
`impl_has_list_links!` macro below.

How about 's/implementation/behavior/` in the line above?

---
Cheers,
Benno

> +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 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;