From: Andreas Hindborg <nmi@metaspace.dk>
To: Gary Guo <gary@garyguo.net>
Cc: "Alice Ryhl" <aliceryhl@google.com>,
rust-for-linux@vger.kernel.org, "Miguel Ojeda" <ojeda@kernel.org>,
"Wedson Almeida Filho" <wedsonaf@gmail.com>,
"Tejun Heo" <tj@kernel.org>,
"Lai Jiangshan" <jiangshanlai@gmail.com>,
"Alex Gaynor" <alex.gaynor@gmail.com>,
"Boqun Feng" <boqun.feng@gmail.com>,
"Björn Roy Baron" <bjorn3_gh@protonmail.com>,
"Benno Lossin" <benno.lossin@proton.me>,
linux-kernel@vger.kernel.org, patches@lists.linux.dev
Subject: Re: [PATCH v1 2/7] rust: add offset_of! macro
Date: Tue, 30 May 2023 10:40:03 +0200 [thread overview]
Message-ID: <87pm6i9pqh.fsf@metaspace.dk> (raw)
In-Reply-To: <20230523164818.0a44fec8.gary@garyguo.net>
Gary Guo <gary@garyguo.net> writes:
> 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.
Good catch 👍
>
>> + }};
>> +}
>> 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
next prev parent reply other threads:[~2023-05-30 8:41 UTC|newest]
Thread overview: 51+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-05-17 20:31 [PATCH v1 0/7] Bindings for the workqueue Alice Ryhl
2023-05-17 20:31 ` [PATCH v1 1/7] rust: workqueue: add low-level workqueue bindings Alice Ryhl
2023-05-18 14:51 ` Martin Rodriguez Reboredo
2023-05-19 9:40 ` Alice Ryhl
2023-05-19 12:04 ` Martin Rodriguez Reboredo
2023-05-23 10:03 ` Alice Ryhl
2023-05-30 8:26 ` Andreas Hindborg
2023-05-17 20:31 ` [PATCH v1 2/7] rust: add offset_of! macro Alice Ryhl
2023-05-18 14:51 ` Martin Rodriguez Reboredo
2023-05-23 15:48 ` Gary Guo
2023-05-24 12:26 ` Alice Ryhl
2023-05-30 8:40 ` Andreas Hindborg [this message]
2023-05-17 20:31 ` [PATCH v1 3/7] rust: sync: add `Arc::{from_raw, into_raw}` Alice Ryhl
2023-05-18 14:51 ` Martin Rodriguez Reboredo
2023-05-23 15:43 ` Gary Guo
2023-05-24 11:19 ` Alice Ryhl
2023-05-24 10:20 ` Andreas Hindborg
2023-05-24 11:11 ` Alice Ryhl
2023-05-25 7:45 ` Andreas Hindborg
2023-05-25 16:32 ` Gary Guo
2023-05-30 7:23 ` Andreas Hindborg
2023-05-17 20:31 ` [PATCH v1 4/7] rust: workqueue: define built-in queues Alice Ryhl
2023-05-18 14:52 ` Martin Rodriguez Reboredo
2023-05-25 11:40 ` Andreas Hindborg
2023-05-31 14:02 ` Alice Ryhl
2023-06-02 10:23 ` Andreas Hindborg (Samsung)
2023-05-17 20:31 ` [PATCH v1 5/7] rust: workqueue: add helper for defining work_struct fields Alice Ryhl
2023-05-18 23:18 ` Martin Rodriguez Reboredo
2023-05-24 14:50 ` Benno Lossin
2023-05-30 8:44 ` Andreas Hindborg
2023-05-31 9:00 ` Alice Ryhl
2023-05-31 10:18 ` Andreas Hindborg
2023-05-17 20:31 ` [PATCH v1 6/7] rust: workqueue: add safe API to workqueue Alice Ryhl
2023-05-19 0:17 ` Martin Rodriguez Reboredo
2023-05-23 11:07 ` Alice Ryhl
2023-05-30 7:19 ` Andreas Hindborg
2023-05-30 13:23 ` Martin Rodriguez Reboredo
2023-05-30 14:13 ` Miguel Ojeda
2023-05-24 14:51 ` Benno Lossin
2023-05-31 9:07 ` Alice Ryhl
2023-05-30 8:51 ` Andreas Hindborg
2023-05-31 14:07 ` Alice Ryhl
2023-05-17 20:31 ` [PATCH v1 7/7] rust: workqueue: add `try_spawn` helper method Alice Ryhl
2023-05-19 0:22 ` Martin Rodriguez Reboredo
2023-05-24 14:52 ` Benno Lossin
2023-05-31 14:03 ` Alice Ryhl
2023-05-17 21:48 ` [PATCH v1 0/7] Bindings for the workqueue Tejun Heo
2023-05-17 22:22 ` Alice Ryhl
2023-05-23 14:08 ` Andreas Hindborg
2023-05-23 14:14 ` Andreas Hindborg
2023-05-24 12:33 ` Alice Ryhl
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