Re: [PATCH] rust: Add `container_of` and `offset_of` macros

[Martin Rodriguez Reboredo <yakoyoku@gmail.com>]

On 2/17/24 12:32, Maíra Canal wrote:
> From: Asahi Lina <lina@asahilina.net>
> 
> Add Rust counterparts to these C macros. `container_of` is useful for C
> struct subtyping, to recover the original pointer to the container
> structure. `offset_of` is useful for struct-relative addressing.
> 
> Co-authored-by: Miguel Ojeda <ojeda@kernel.org>
> Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
> Co-authored:by: Léo Lanteri Thauvin <leseulartichaut@gmail.com>
> Signed-off-by: Léo Lanteri Thauvin <leseulartichaut@gmail.com>
> Co-authored-by: Wedson Almeida Filho <wedsonaf@google.com>
> Signed-off-by: Wedson Almeida Filho <wedsonaf@google.com>
> Signed-off-by: Asahi Lina <lina@asahilina.net>
> Signed-off-by: Maíra Canal <mcanal@igalia.com>

A handful of authors? So then, it would be sensible to list their
contributions to the patch, though, it could be done when merged.

> [...]
> +/// Calculates the offset of a field from the beginning of the struct it belongs to.
> +///
> +/// # Examples
> +///
> +/// ```rust
> +/// use kernel::prelude::*;
> +/// use kernel::offset_of;
> +/// struct Test {
> +///     a: u64,
> +///     b: u32,
> +/// }
> +///
> +/// assert_eq!(offset_of!(Test, b), 8);

Would've added a check for field `a`, but that's my personal preference.

> +/// ```
> +#[macro_export]
> +macro_rules! offset_of {
> +    ($type:ty, $($f:tt)*) => {{
> +        let tmp = core::mem::MaybeUninit::<$type>::uninit();
> +        let outer = tmp.as_ptr();
> +        // To avoid warnings when nesting `unsafe` blocks.
> +        #[allow(unused_unsafe)]
> +        // SAFETY: The pointer is valid and aligned, just not initialised; `addr_of` ensures that
> +        // we don't actually read from `outer` (which would be UB) nor create an intermediate
> +        // reference.
> +        let inner = unsafe { core::ptr::addr_of!((*outer).$($f)*) } as *const u8;

Curiously enough, you can get the offset of a member of a field struct
per what `addr_of!` does, i.e. if we have a type `Foo` that contains a
struct field `Bar` which has member `qux`, then
`offset_of!(Foo, bar.qux)` will give out an offset too. I don't know if
this applies for `container_of!`. Just saying.

> [...]

Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>