rust: init: add `Zeroable` trait and `init::zeroed` function

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 <benno.lossin@proton.me>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>
Link: https://lore.kernel.org/r/20230408122429.1103522-12-y86-dev@protonmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

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)]
@@ -1323,3 +1329,90 @@ 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>() -> 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 $t {})*
+    };
+}
+
+impl_zeroable! {
+    // SAFETY: All primitives that are allowed to be zero.
+    bool,
+    char,
+    u8, u16, u32, u64, u128, usize,
+    i8, i16, i32, i64, i128, isize,
+    f32, f64,
+
+    // SAFETY: These are ZSTs, there is nothing to zero.
+    {<T: ?Sized>} PhantomData<T>, core::marker::PhantomPinned, Infallible, (),
+
+    // SAFETY: Type is allowed to take any value, including all zeros.
+    {<T>} MaybeUninit<T>,
+
+    // SAFETY: All zeros is equivalent to `None` (option layout optimization guarantee).
+    Option<NonZeroU8>, Option<NonZeroU16>, Option<NonZeroU32>, Option<NonZeroU64>,
+    Option<NonZeroU128>, Option<NonZeroUsize>,
+    Option<NonZeroI8>, Option<NonZeroI16>, Option<NonZeroI32>, Option<NonZeroI64>,
+    Option<NonZeroI128>, Option<NonZeroIsize>,
+
+    // SAFETY: All zeros is equivalent to `None` (option layout optimization guarantee).
+    //
+    // In this case we are allowed to use `T: ?Sized`, since all zeros is the `None` variant.
+    {<T: ?Sized>} Option<NonNull<T>>,
+    {<T: ?Sized>} Option<Box<T>>,
+
+    // SAFETY: `null` pointer is valid.
+    //
+    // We cannot use `T: ?Sized`, since the VTABLE pointer part of fat pointers is not allowed to be
+    // null.
+    //
+    // When `Pointee` gets stabilized, we could use
+    // `T: ?Sized where <T as Pointee>::Metadata: Zeroable`
+    {<T>} *mut T, {<T>} *const T,
+
+    // SAFETY: `null` pointer is valid and the metadata part of these fat pointers is allowed to be
+    // zero.
+    {<T>} *mut [T], {<T>} *const [T], *mut str, *const str,
+
+    // SAFETY: `T` is `Zeroable`.
+    {<const N: usize, T: Zeroable>} [T; N], {<T: Zeroable>} Wrapping<T>,
+}
+
+macro_rules! impl_tuple_zeroable {
+    ($(,)?) => {};
+    ($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);