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

[Alice Ryhl]

Gary Guo <gary@xxxxxxxxxxx> writes:
> On Wed, 17 May 2023 20:31:14 +0000
> Alice Ryhl <aliceryhl@xxxxxxxxxx> wrote:
>> +#[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.

Added. I had to change `$type:ty` to `$type:path` to get that to
compile, since otherwise you can't use the token in a pattern. However,
I think it's fine - this is temporary anyway until the standard library
gets the macro.
 
>> +    }};
>> +}
>> 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.

Agreed. I mentioned that in the commit message.

Alice