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Date:   Thu, 30 Mar 2023 12:06:43 +0100
From:   Gary Guo <gary@garyguo.net>
To:     y86-dev@protonmail.com
Cc:     Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        =?UTF-8?B?QmrDtnJu?= Roy Baron <bjorn3_gh@protonmail.com>,
        Alice Ryhl <alice@ryhl.io>, rust-for-linux@vger.kernel.org,
        linux-kernel@vger.kernel.org, patches@lists.linux.dev
Subject: Re: [PATCH v3 09/13] rust: init: add `Zeroable` trait and
 `init::zeroed` function
Message-ID: <20230330120643.11f92c38.gary@garyguo.net>
In-Reply-To: <20230329223239.138757-10-y86-dev@protonmail.com>
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On Wed, 29 Mar 2023 22:33:35 +0000
y86-dev@protonmail.com wrote:

> From: Benno Lossin <y86-dev@protonmail.com>
> 
> 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>

Reviewed-by: Gary Guo <gary@garyguo.net>

> ---
>  rust/kernel/init.rs | 65 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 65 insertions(+)
> 
> diff --git a/rust/kernel/init.rs b/rust/kernel/init.rs
> index 3358f14beffb..a923546696ce 100644
> --- a/rust/kernel/init.rs
> +++ b/rust/kernel/init.rs
> @@ -1254,3 +1254,68 @@ pub unsafe trait PinnedDrop: __internal::HasPinData {
>      /// automatically.
>      fn drop(self: Pin<&mut Self>, only_call_from_drop: __internal::OnlyCallFromDrop);
>  }
> +
> +/// Marker trait for types that can be initialized by writing just zeroes.
> +///
> +/// # Safety
> +///
> +/// The bit pattern consisting of only zeroes is a valid bit pattern for this type. In other words,
> +/// this is not UB:
> +///
> +/// ```rust,ignore
> +/// let val: Self = unsafe { core::mem::zeroed() };
> +/// ```
> +pub unsafe trait Zeroable {}
> +
> +/// Create a new zeroed T.
> +///
> +/// The returned initializer will write `0x00` to every byte of the given `slot`.
> +#[inline]
> +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(())
> +        })
> +    }
> +}
> +
> +macro_rules! impl_zeroable {
> +    ($($t:ty, )*) => {
> +        $(unsafe impl Zeroable for $t {})*
> +    };
> +}
> +
> +impl_zeroable! {
> +    // SAFETY: All primitives that are allowed to be zero.
> +    bool,
> +    char,
> +    u8, u16, u32, u64, u128, usize,
> +    i8, i16, i32, i64, i128, isize,
> +    f32, f64,
> +    // SAFETY: There is nothing to zero.
> +    core::marker::PhantomPinned, Infallible, (),
> +}
> +
> +// SAFETY: We are allowed to zero padding bytes.
> +unsafe impl<const N: usize, T: Zeroable> Zeroable for [T; N] {}
> +
> +// SAFETY: There is nothing to zero.
> +unsafe impl<T: ?Sized> Zeroable for PhantomData<T> {}
> +
> +// SAFETY: `null` pointer is valid.
> +unsafe impl<T: ?Sized> Zeroable for *mut T {}
> +unsafe impl<T: ?Sized> Zeroable for *const T {}
> +
> +macro_rules! impl_tuple_zeroable {
> +    ($(,)?) => {};
> +    ($first:ident, $($t:ident),* $(,)?) => {
> +        // SAFETY: All elements are zeroable and padding can be zero.
> +        unsafe impl<$first: Zeroable, $($t: Zeroable),*> Zeroable for ($first, $($t),*) {}
> +        impl_tuple_zeroable!($($t),* ,);
> +    }
> +}
> +
> +impl_tuple_zeroable!(A, B, C, D, E, F, G, H, I, J);
> --
> 2.39.2
> 
>