Messages in this thread Patch in this message | | | Date | Tue, 23 May 2023 16:48:18 +0100 | From | Gary Guo <> | Subject | Re: [PATCH v1 2/7] rust: add offset_of! macro |
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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
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