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

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Message-Id: <949A27C5-1535-48D1-BE7E-F7E366A49A52@collabora.com>
Date: Wed, 6 Aug 2025 09:38:25 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: Benno Lossin <lossin@...nel.org>
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

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.

— Daniel