Re: [PATCH v5 11/15] rust: init: add `Zeroable` trait and `init::zeroed` function

Alice Ryhl
2023-04-03T17:58:11Z

On 4/3/23 18:05, Benno Lossin wrote:
> Add the `Zeroable` trait which marks types that can be initialized by
> writing `0x00` to every byte of the type. Also add the `init::zeroed`
> function that creates an initializer for a `Zeroable` type that writes
> `0x00` to every byte.
>
> Signed-off-by: Benno Lossin <y86-dev@protonmail.com>
> Cc: Gary Guo <gary@garyguo.net>
> Cc: Alice Ryhl <aliceryhl@google.com>
> Cc: Andreas Hindborg <a.hindborg@samsung.com>

Reviewed-by: Alice Ryhl <aliceryhl@google.com>

I have two minor suggestions - take them or leave them.

> +pub fn zeroed<T: Zeroable + Unpin>() -> impl Init<T> {
> + // SAFETY: Because `T: Zeroable`, all bytes zero is a valid bit pattern for `T`
> + // and because we write all zeroes, the memory is initialized.
> + unsafe {
> + init_from_closure(|slot: *mut T| {
> + slot.write_bytes(0, 1);
> + Ok(())
> + })
> + }
> +}

You don't need `T: Unpin` here.

> +macro_rules! impl_zeroable {
> + ($($({$($generics:tt)*})? $t:ty, )*) => {
> + $(unsafe impl$($($generics)*)? Zeroable for $t {})*
> + };
> +}
> +
> +impl_zeroable! {
> + // SAFETY: All primitives that are allowed to be zero.
> + bool,
> + char,
> + u8, u16, u32, u64, u128, usize,
> + i8, i16, i32, i64, i128, isize,
> + f32, f64,
> +
> + // SAFETY: These are ZSTs, there is nothing to zero.
> + {<T: ?Sized>} PhantomData<T>, core::marker::PhantomPinned, Infallible, (),
> +
> + // SAFETY: Type is allowed to take any value, including all zeros.
> + {<T>} MaybeUninit<T>,
> +
> + // SAFETY: All zeros is equivalent to `None` (option layout optimization guarantee).
> + Option<NonZeroU8>, Option<NonZeroU16>, Option<NonZeroU32>, Option<NonZeroU64>,
> + Option<NonZeroU128>, Option<NonZeroUsize>,
> + Option<NonZeroI8>, Option<NonZeroI16>, Option<NonZeroI32>, Option<NonZeroI64>,
> + Option<NonZeroI128>, Option<NonZeroIsize>,
> +
> + // SAFETY: All zeros is equivalent to `None` (option layout optimization guarantee).
> + //
> + // In this case we are allowed to use `T: ?Sized`, since all zeros is the `None` variant.
> + {<T: ?Sized>} Option<NonNull<T>>,
> + {<T: ?Sized>} Option<Box<T>>,
> +
> + // SAFETY: `null` pointer is valid.
> + //
> + // We cannot use `T: ?Sized`, since the VTABLE pointer part of fat pointers is not allowed to be
> + // null.
> + {<T>} *mut T, {<T>} *const T,
> +
> + // SAFETY: `null` pointer is valid and the metadata part of these fat pointers is allowed to be
> + // zero.
> + {<T>} *mut [T], {<T>} *const [T], *mut str, *const str,
> +
> + // SAFETY: `T` is `Zeroable`.
> + {<const N: usize, T: Zeroable>} [T; N], {<T: Zeroable>} Wrapping<T>,
> +}

Arguably, it would make sense to just expand this macro. The code
doesn't become that much longer, and it removes the need to understand
the macro.