linux-kernel - Re: [RFC PATCH v2 2/4] rust: io_uring: introduce rust abstraction for io-uring cmd

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Message-Id: <DBVDWWHX8UY7.TG5OHXBZM2OX@kernel.org>
Date: Wed, 06 Aug 2025 15:38:24 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Daniel Almeida" <daniel.almeida@...labora.com>
Cc: "Sidong Yang" <sidong.yang@...iosa.ai>, "Caleb Sander Mateos"
 <csander@...estorage.com>, "Miguel Ojeda" <ojeda@...nel.org>, "Arnd
 Bergmann" <arnd@...db.de>, "Jens Axboe" <axboe@...nel.dk>, "Greg
 Kroah-Hartman" <gregkh@...uxfoundation.org>,
 <rust-for-linux@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
 <io-uring@...r.kernel.org>
Subject: Re: [RFC PATCH v2 2/4] rust: io_uring: introduce rust abstraction
 for io-uring cmd

On Wed Aug 6, 2025 at 2:38 PM CEST, Daniel Almeida wrote:
> Hi Benno,
>
>> On 2 Aug 2025, at 07:52, Benno Lossin <lossin@...nel.org> wrote:
>> 
>> On Fri Aug 1, 2025 at 3:48 PM CEST, Daniel Almeida wrote:
>>>> On 27 Jul 2025, at 12:03, Sidong Yang <sidong.yang@...iosa.ai> wrote:
>>>> +    #[inline]
>>>> +    pub fn pdu(&mut self) -> &mut MaybeUninit<[u8; 32]> {
>>> 
>>> Why MaybeUninit? Also, this is a question for others, but I don’t think
>>> that `u8`s can ever be uninitialized as all byte values are valid for `u8`.
>> 
>> `u8` can be uninitialized. Uninitialized doesn't just mean "can take any
>> bit pattern", but also "is known to the compiler as being
>> uninitialized". The docs of `MaybeUninit` explain it like this:
>> 
>>    Moreover, uninitialized memory is special in that it does not have a
>>    fixed value (“fixed” meaning “it won’t change without being written
>>    to”). Reading the same uninitialized byte multiple times can give
>>    different results.
>> 
>> But the return type probably should be `&mut [MaybeUninit<u8>; 32]`
>> instead.
>
>
> Right, but I guess the question then is why would we ever need to use
> MaybeUninit here anyways.
>
> It's a reference to a C array. Just treat that as initialized.

AFAIK C uninitialized memory also is considered uninitialized in Rust.
So if this array is not properly initialized on the C side, this would
be the correct type. If it is initialized, then just use `&mut [u8; 32]`.

---
Cheers,
Benno