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

[Gary Guo]

On Mon, 3 Apr 2023 19:56:29 +0200
Alice Ryhl <alice@xxxxxxx> wrote:

> 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@xxxxxxxxxxxxxx>
> > Cc: Gary Guo <gary@xxxxxxxxxxx>
> > Cc: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> > Cc: Andreas Hindborg <a.hindborg@xxxxxxxxxxx>  
> 
> Reviewed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> 
> 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.

I think it makes sense to use macro for non-generic types (primitives
and `NonZero`s. It'll be quite verbose if these are just expanded.

For the generic ones, I have no preference.