Re: [PATCH v6 3/4] rust: uaccess: add typed accessors for userspace pointers

[Benno Lossin]

On 18.04.24 10:59, Alice Ryhl wrote:
> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
> index 8fad61268465..9c57c6c75553 100644
> --- a/rust/kernel/types.rs
> +++ b/rust/kernel/types.rs
> @@ -409,3 +409,67 @@ pub enum Either<L, R> {
>      /// Constructs an instance of [`Either`] containing a value of type `R`.
>      Right(R),
>  }
> +
> +/// Types for which any bit pattern is valid.
> +///
> +/// Not all types are valid for all values. For example, a `bool` must be either zero or one, so
> +/// reading arbitrary bytes into something that contains a `bool` is not okay.
> +///
> +/// It's okay for the type to have padding, as initializing those bytes has no effect.
> +///
> +/// # Safety
> +///
> +/// All bit-patterns must be valid for this type. This type must not have interior mutability.

What is the reason for disallowing interior mutability here? I agree
that it is necessary for `AsBytes`, but I don't think we need it here.
For example it is fine to convert `u8` to `UnsafeCell<u8>`. Niches also
should not be a problem, since eg `Option<UnsafeCell<NonNull<u8>>>`
already fails the "All bit-patterns must be valid for this type".

-- 
Cheers,
Benno

> +pub unsafe trait FromBytes {}
> +
> +// SAFETY: All bit patterns are acceptable values of the types below.
> +unsafe impl FromBytes for u8 {}
> +unsafe impl FromBytes for u16 {}
> +unsafe impl FromBytes for u32 {}
> +unsafe impl FromBytes for u64 {}
> +unsafe impl FromBytes for usize {}
> +unsafe impl FromBytes for i8 {}
> +unsafe impl FromBytes for i16 {}
> +unsafe impl FromBytes for i32 {}
> +unsafe impl FromBytes for i64 {}
> +unsafe impl FromBytes for isize {}