Re: [PATCH v2 2/8] rust: add offset_of! macro

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

On 01.06.23 15:49, Alice Ryhl wrote:
> From: Wedson Almeida Filho <walmeida@microsoft.com>
> 
> This macro is used to compute the offset of a field in a struct.
> 
> This commit enables an unstable feature that is necessary for using
> the macro in a constant. However, this is not a problem as the macro
> will become available from the Rust standard library soon [1]. The
> unstable feature can be disabled again once that happens.
> 
> The macro in this patch does not support sub-fields. That is, you cannot
> write `offset_of!(MyStruct, field.sub_field)` to get the offset of
> `sub_field` with `field`'s type being a struct with a field called
> `sub_field`. This is because `field` might be a `Box<SubStruct>`, which
> means that you would be trying to compute the offset to something in an
> entirely different allocation. There's no easy way to fix the current
> macro to support subfields, but the version being added to the standard
> library should support it, so the limitation is temporary and not a big
> deal.
> 
> Link: https://github.com/rust-lang/rust/issues/106655 [1]
> Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
> Co-developed-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>

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

-- 
Cheers,
Benno

> ---
>   rust/kernel/lib.rs     | 35 +++++++++++++++++++++++++++++++++++
>   scripts/Makefile.build |  2 +-
>   2 files changed, 36 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index eaded02ffb01..7ea777b731e6 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -14,6 +14,7 @@
>   #![no_std]
>   #![feature(allocator_api)]
>   #![feature(coerce_unsized)]
> +#![feature(const_refs_to_cell)]
>   #![feature(dispatch_from_dyn)]
>   #![feature(new_uninit)]
>   #![feature(receiver_trait)]
> @@ -98,3 +99,37 @@ fn panic(info: &core::panic::PanicInfo<'_>) -> ! {
>       // instead of `!`. See <https://github.com/rust-lang/rust-bindgen/issues/2094>.
>       loop {}
>   }
> +
> +/// Calculates the offset of a field from the beginning of the struct it belongs to.
> +///
> +/// # Examples
> +///
> +/// ```
> +/// #[repr(C)]
> +/// struct Test {
> +///     a: u64,
> +///     b: u32,
> +/// }
> +///
> +/// assert_eq!(kernel::offset_of!(Test, b), 8);
> +/// ```
> +#[macro_export]
> +macro_rules! offset_of {
> +    ($type:path, $field:ident) => {{
> +        let $type { $field: _, .. };
> +        let tmp = ::core::mem::MaybeUninit::<$type>::uninit();
> +        let outer = tmp.as_ptr();
> +        // To avoid warnings when nesting `unsafe` blocks.
> +        #[allow(unused_unsafe)]
> +        // SAFETY: The pointer is valid and aligned, just not initialised; `addr_of` ensures that
> +        // we don't actually read from `outer` (which would be UB) nor create an intermediate
> +        // reference.
> +        let inner = unsafe { ::core::ptr::addr_of!((*outer).$field) } as *const u8;
> +        // To avoid warnings when nesting `unsafe` blocks.
> +        #[allow(unused_unsafe)]
> +        // SAFETY: The two pointers are within the same allocation block.
> +        unsafe {
> +            inner.offset_from(outer as *const u8) as usize
> +        }
> +    }};
> +}
> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> index 78175231c969..819510694769 100644
> --- a/scripts/Makefile.build
> +++ b/scripts/Makefile.build
> @@ -277,7 +277,7 @@ $(obj)/%.lst: $(src)/%.c FORCE
>   # Compile Rust sources (.rs)
>   # ---------------------------------------------------------------------------
> 
> -rust_allowed_features := new_uninit
> +rust_allowed_features := const_refs_to_cell,new_uninit
> 
>   rust_common_cmd = \
>   	RUST_MODFILE=$(modfile) $(RUSTC_OR_CLIPPY) $(rust_flags) \
> --
> 2.41.0.rc0.172.g3f132b7071-goog
>