Re: [PATCH v1 2/7] rust: add offset_of! macro

[Gary Guo <gary@garyguo.net>]

On Wed, 17 May 2023 20:31:14 +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 two unstable features that are 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 features 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>
> ---
>  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 c718524056a6..cdf9fe999328 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -14,6 +14,8 @@
>  #![no_std]
>  #![feature(allocator_api)]
>  #![feature(coerce_unsized)]
> +#![feature(const_ptr_offset_from)]
> +#![feature(const_refs_to_cell)]
>  #![feature(core_ffi_c)]
>  #![feature(dispatch_from_dyn)]
>  #![feature(explicit_generic_args_with_impl_trait)]
> @@ -102,3 +104,36 @@ 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:ty, $field:ident) => {{
> +        let tmp = ::core::mem::MaybeUninit::<$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
> +        }

This has no protection against using `Deref`. The memoffset crate has a 

let $type { $field: _, .. };

line to ensure that the field is a direct member of type and deref is
not happening.

> +    }};
> +}
> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> index 9f94fc83f086..ec583d13dde2 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 := core_ffi_c,explicit_generic_args_with_impl_trait,new_uninit,pin_macro
> +rust_allowed_features := const_ptr_offset_from,const_refs_to_cell,core_ffi_c,explicit_generic_args_with_impl_trait,new_uninit,pin_macro

Side note: once we bump our compiler to 1.71, we should switch to the
built-in `offset_of!` macro and get rid of these unstable features.

Best,
Gary