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

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

On 23.07.24 10:22, Alice Ryhl wrote:
> 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.

Can you reference the `impl_has_list_links!` macro here to guide the
user to the safe version?

> +///
> +/// # 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.

Stale comment (this has nothing to do with `impl_has_work!`).

> +    #[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`].

I would write "Requires", since "Assumes" sounds as if it isn't checked.

Can you also reference the `impl_has_list_links!` macro here to guide
the user to the safe version?

> +///
> +/// [`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.

I don't like the "this isn't correct, but if you look closely at the
implementations, it's fine". Do you think it would be better if you just
copy paste the impl of `view_value`?

---
Cheers,
Benno

> +                unsafe { <Self as $crate::list::ListItem<$num>>::view_value(me) }
> +            }
> +        }
> +    };
> +}
> +pub use impl_list_item;
> 
> --
> 2.45.2.1089.g2a221341d9-goog
>