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

Gary Guo
2023-06-01T17:17:56Z

On Thu, 1 Jun 2023 13:49:40 +0000
Alice Ryhl <aliceryhl@google.com> 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: Gary Guo <gary@garyguo.net>

> ---
> 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) \