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Date:   Fri, 31 Mar 2023 07:09:30 +0000
To:     Alice Ryhl <alice@ryhl.io>, Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
        =?utf-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>
From:   Benno Lossin <y86-dev@protonmail.com>
Cc:     rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
        patches@lists.linux.dev
Subject: Re: [PATCH v3 09/13] rust: init: add `Zeroable` trait and `init::zeroed` function
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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/8391afa876ba2e99dffb0c991cc7fa7=
75287d106/src/zeroable.rs#L56-L65

Wow I forgot about fat pointers completely! Good catch!

--
Cheers,
Benno