[PATCH v3 09/13] rust: init: add `Zeroable` trait and `init::zeroed` function

[PATCH v3 09/13] rust: init: add `Zeroable` trait and `init::zeroed` function

[y86-dev]

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>
---
 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

Re: [PATCH v3 09/13] rust: init: add `Zeroable` trait and `init::zeroed` function

[Gary Guo]

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
> 
> 

Re: [PATCH v3 09/13] rust: init: add `Zeroable` trait and `init::zeroed` function

[Alice Ryhl]

On 3/30/23 00:33, 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: Alice Ryhl <aliceryhl@google.com>

> +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, (),
> +}

Here are some other types it might make sense to add:

  * Option<NonNull<T>>
  * Option<Box<T>>
  * Option<NonZeroU32>
  * PhantomData<T>
  * MaybeUninit<T>

> +
> +// SAFETY: We are allowed to zero padding bytes.
> +unsafe impl<const N: usize, T: Zeroable> Zeroable for [T; N] {}

There are no padding bytes in [T; N].

Re: [PATCH v3 09/13] rust: init: add `Zeroable` trait and `init::zeroed` function

[Alice Ryhl]

On 3/30/23 00:33, 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>
> ---
> +// SAFETY: `null` pointer is valid.
> +unsafe impl<T: ?Sized> Zeroable for *mut T {}
> +unsafe impl<T: ?Sized> Zeroable for *const T {}

Actually, I just realized that this is not ok for unsized types. When T 
is unsized, the raw pointer is a fat pointer with a vtable, and the 
vtable part is not necessarily zeroable.

However, it would be ok to do it for `*const [T]` since the fat part of 
the pointer is just the length in this case, and a length of zero is fine.

See more here:
https://github.com/Lokathor/bytemuck/blob/8391afa876ba2e99dffb0c991cc7fa775287d106/src/zeroable.rs#L56-L65

Re: [PATCH v3 09/13] rust: init: add `Zeroable` trait and `init::zeroed` function

[Gary Guo]

On Fri, 31 Mar 2023 00:40:34 +0200
Alice Ryhl <alice@ryhl.io> wrote:

> On 3/30/23 00:33, 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>
> > ---
> > +// SAFETY: `null` pointer is valid.
> > +unsafe impl<T: ?Sized> Zeroable for *mut T {}
> > +unsafe impl<T: ?Sized> Zeroable for *const T {}  
> 
> Actually, I just realized that this is not ok for unsized types. When T 
> is unsized, the raw pointer is a fat pointer with a vtable, and the 
> vtable part is not necessarily zeroable.
> 
> However, it would be ok to do it for `*const [T]` since the fat part of 
> the pointer is just the length in this case, and a length of zero is fine.
> 
> See more here:
> https://github.com/Lokathor/bytemuck/blob/8391afa876ba2e99dffb0c991cc7fa775287d106/src/zeroable.rs#L56-L65

Good catch. vtable completely slipped my mind when I am reviewing this
code.

Vtable is not *not necessary zeroable*, but actually never zeroable.
Although currently not yet formally specified, the compiler has always
assumed vtable part of fat pointers to be non-null, well aligned and
dereferenceable.

Best,
Gary

Re: [PATCH v3 09/13] rust: init: add `Zeroable` trait and `init::zeroed` function

[Benno Lossin]

On 31.03.23 00:40, Alice Ryhl wrote:
> On 3/30/23 00:33, 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>
>> ---
>> +// SAFETY: `null` pointer is valid.
>> +unsafe impl<T: ?Sized> Zeroable for *mut T {}
>> +unsafe impl<T: ?Sized> Zeroable for *const T {}
>
> Actually, I just realized that this is not ok for unsized types. When T
> is unsized, the raw pointer is a fat pointer with a vtable, and the
> vtable part is not necessarily zeroable.
>
> However, it would be ok to do it for `*const [T]` since the fat part of
> the pointer is just the length in this case, and a length of zero is fine.
>
> See more here:
> https://github.com/Lokathor/bytemuck/blob/8391afa876ba2e99dffb0c991cc7fa775287d106/src/zeroable.rs#L56-L65

Wow I forgot about fat pointers completely! Good catch!

--
Cheers,
Benno