lkml.org 
[lkml]   [2023]   [May]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    Date
    From
    SubjectRe: [PATCH v1 2/7] rust: add offset_of! macro
    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

    \
     
     \ /
      Last update: 2023-05-23 17:49    [W:4.176 / U:0.224 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site