linux-kernel - Re: [RFC PATCH v2 0/4] rust: miscdevice: abstraction for uring-cmd

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Message-Id: <CACB1415-0535-4A05-B904-5F388A1F7C08@collabora.com>
Date: Sun, 27 Jul 2025 14:17:05 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: Sidong Yang <sidong.yang@...iosa.ai>
Cc: Caleb Sander Mateos <csander@...estorage.com>,
 Benno Lossin <lossin@...nel.org>,
 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 0/4] rust: miscdevice: abstraction for uring-cmd

Hi Sidong,

> On 27 Jul 2025, at 12:03, Sidong Yang <sidong.yang@...iosa.ai> wrote:
> 
> This patch series implemens an abstraction for io-uring sqe and cmd and
> adds uring_cmd callback for miscdevice. Also there is an example that use
> uring_cmd in rust-miscdevice sample.
> 
> I received a email from kernel bot that `io_tw_state` is not FFI-safe.
> It seems that the struct has no field how can I fix this?

It’s not something that you introduced. Empty structs are problematic when
used in FFI, because  ZSTs are not defined in the C standard AFAIK, although
they are supported in some compilers. For example, this is not illegal nor UB
in GCC [0]. The docs say:

> The structure has size zero.

This aligns with Rust's treatment of ZSTs, which are also zero-sized, so I
don't think this will be a problem, but lets wait for others to chime in.

I'll review this tomorrow.

> 
> Changelog:
> v2:
> * use pinned &mut for IoUringCmd
> * add missing safety comments
> * use write_volatile for read uring_cmd in sample
> 
> Sidong Yang (4):
>  rust: bindings: add io_uring headers in bindings_helper.h
>  rust: io_uring: introduce rust abstraction for io-uring cmd
>  rust: miscdevice: add uring_cmd() for MiscDevice trait
>  samples: rust: rust_misc_device: add uring_cmd example
> 
> rust/bindings/bindings_helper.h  |   2 +
> rust/kernel/io_uring.rs          | 183 +++++++++++++++++++++++++++++++
> rust/kernel/lib.rs               |   1 +
> rust/kernel/miscdevice.rs        |  41 +++++++
> samples/rust/rust_misc_device.rs |  34 ++++++
> 5 files changed, 261 insertions(+)
> create mode 100644 rust/kernel/io_uring.rs
> 
> -- 
> 2.43.0
> 
> 


— Daniel

[0]: https://gcc.gnu.org/onlinedocs/gcc/Empty-Structures.html