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Date:   Tue, 4 Apr 2023 14:24:15 +0100
From:   Gary Guo <gary@garyguo.net>
To:     Benno Lossin <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,
        Alice Ryhl <aliceryhl@google.com>,
        Andreas Hindborg <a.hindborg@samsung.com>
Subject: Re: [PATCH v5 11/15] rust: init: add `Zeroable` trait and
 `init::zeroed` function
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On Mon, 03 Apr 2023 16:05:40 +0000
Benno Lossin <y86-dev@protonmail.com> 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: Gary Guo <gary@garyguo.net>

> ---
>  rust/kernel/init.rs | 94 ++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 92 insertions(+), 2 deletions(-)
> 
> diff --git a/rust/kernel/init.rs b/rust/kernel/init.rs
> index 1343cafdd760..09bba4e86ace 100644
> --- a/rust/kernel/init.rs
> +++ b/rust/kernel/init.rs
> @@ -195,8 +195,14 @@ use crate::{
>  };
>  use alloc::boxed::Box;
>  use core::{
> -    alloc::AllocError, cell::Cell, convert::Infallible, marker::PhantomData, mem::MaybeUninit,
> -    pin::Pin, ptr,
> +    alloc::AllocError,
> +    cell::Cell,
> +    convert::Infallible,
> +    marker::PhantomData,
> +    mem::MaybeUninit,
> +    num::*,
> +    pin::Pin,
> +    ptr::{self, NonNull},
>  };
> 
>  #[doc(hidden)]
> @@ -1329,3 +1335,87 @@ 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 {
> +    ($($({$($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,

Just a note: if `Pointee` trait is stabilised, this could be changed to

impl<T: ?Sized> Zeroable for *const T where <T as Pointee>::Metadata: Zeroable {}

> +
> +    // SAFETY: `T` is `Zeroable`.
> +    {<const N: usize, T: Zeroable>} [T; N], {<T: Zeroable>} Wrapping<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
> 
>