Re: [PATCH 09/22] rust: pin-init: move impl `Zeroable` for `Opaque` and `Option<KBox<T>>` into the kernel crate

[Andreas Hindborg <a.hindborg@kernel.org>]

"Benno Lossin" <benno.lossin@proton.me> writes:

> In order to make pin-init a standalone crate, move kernel-specific code
> directly into the kernel crate. Since `Opaque<T>` and `KBox<T>` are part
> of the kernel, move their `Zeroable` implementation into the kernel
> crate.
>
> Signed-off-by: Benno Lossin <benno.lossin@proton.me>
> ---
>  rust/kernel/alloc/kbox.rs | 8 +++++++-
>  rust/kernel/types.rs      | 5 ++++-
>  rust/pin-init/src/lib.rs  | 8 +-------
>  3 files changed, 12 insertions(+), 9 deletions(-)
>
> diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
> index 39a3ea7542da..9861433559dc 100644
> --- a/rust/kernel/alloc/kbox.rs
> +++ b/rust/kernel/alloc/kbox.rs
> @@ -15,7 +15,7 @@
>  use core::ptr::NonNull;
>  use core::result::Result;
>
> -use crate::init::{InPlaceWrite, Init, PinInit};
> +use crate::init::{InPlaceWrite, Init, PinInit, Zeroable};
>  use crate::init_ext::InPlaceInit;
>  use crate::types::ForeignOwnable;
>
> @@ -100,6 +100,12 @@
>  /// ```
>  pub type KVBox<T> = Box<T, super::allocator::KVmalloc>;
>
> +// 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 and there
> +// is no problem with a VTABLE pointer being null.
> +unsafe impl<T: ?Sized, A: Allocator> Zeroable for Option<Box<T, A>> {}

Could you elaborate the statement related to vtable pointers? How does
that come into play for `Option<Box<_>>`? Is it for fat pointers to
trait objects?

Otherwise LGTM.


Best regards,
Andreas Hindborg