linux-kernel - Re: [PATCH] rust: kernel: drop repetition in offset

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Date:   Fri, 16 Dec 2022 18:26:57 +0000
From:   Wedson Almeida Filho <wedsonaf@...il.com>
To:     Wei Liu <wei.liu@...nel.org>
Cc:     rust-for-linux@...r.kernel.org,
        Linux Kernel List <linux-kernel@...r.kernel.org>,
        Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>
Subject: Re: [PATCH] rust: kernel: drop repetition in offset_of macro

On Fri, 16 Dec 2022 at 17:49, Wei Liu <wei.liu@...nel.org> wrote:
>
> It doesn't make sense to allow multiple fields to be specified in
> offset_of.

Why do you say it doesn't make sense?

Here's what I had in mind:
```
    struct Y {
        z: u32
    }
    struct X {
        y: Y
    }
    offset_of!(X, y.z)
```

Which is something very plausible.

> No functional change.
>
> Signed-off-by: Wei Liu <wei.liu@...nel.org>
> ---
> Cc: Miguel Ojeda <ojeda@...nel.org>
> Cc: Alex Gaynor <alex.gaynor@...il.com>
> Cc: Wedson Almeida Filho <wedsonaf@...il.com>
> Cc: Boqun Feng <boqun.feng@...il.com>
> Cc: Gary Guo <gary@...yguo.net>
> Cc: Björn Roy Baron <bjorn3_gh@...tonmail.com>
> ---
>  rust/kernel/lib.rs | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index 6a322effa60c..2f3601e4e27e 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -208,7 +208,7 @@ impl<'a> Drop for KParamGuard<'a> {
>  /// ```
>  #[macro_export]
>  macro_rules! offset_of {
> -    ($type:ty, $($f:tt)*) => {{
> +    ($type:ty, $f:tt) => {{
>          let tmp = core::mem::MaybeUninit::<$type>::uninit();
>          let outer = tmp.as_ptr();
>          // To avoid warnings when nesting `unsafe` blocks.
> @@ -216,12 +216,14 @@ macro_rules! offset_of {
>          // 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;
> +        let inner = unsafe { core::ptr::addr_of!((*outer).$f) } 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) }
> -    }}
> +        unsafe {
> +            inner.offset_from(outer as *const u8)
> +        }
> +    }};
>  }
>
>  /// Produces a pointer to an object from a pointer to one of its fields.
> --
> 2.35.1
>