Messages in this thread Patch in this message | | | Date | Wed, 17 May 2023 20:31:14 +0000 | Subject | [PATCH v1 2/7] rust: add offset_of! macro | From | Alice Ryhl <> |
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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 + } + }}; +} 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: $(src)/%.c FORCE # Compile Rust sources (.rs) # --------------------------------------------------------------------------- -rust_allowed_features := core_ffi_c,explicit_generic_args_with_impl_trait,new_uninit,pin_macro +rust_allowed_features := const_ptr_offset_from,const_refs_to_cell,core_ffi_c,explicit_generic_args_with_impl_trait,new_uninit,pin_macro rust_common_cmd = \ RUST_MODFILE=$(modfile) $(RUSTC_OR_CLIPPY) $(rust_flags) \ -- 2.40.1.606.ga4b1b128d6-goog
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