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

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Message-ID: <19db6213-297b-a33d-7331-3c1a5b72fad9@ryhl.io>
Date:   Fri, 31 Mar 2023 00:40:34 +0200
From:   Alice Ryhl <alice@...l.io>
To:     y86-dev@...tonmail.com, Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>
Cc:     rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
        patches@...ts.linux.dev
Subject: Re: [PATCH v3 09/13] rust: init: add `Zeroable` trait and
 `init::zeroed` function

On 3/30/23 00:33, y86-dev@...tonmail.com wrote:
> From: Benno Lossin <y86-dev@...tonmail.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@...tonmail.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