Re: Rust: how to implement file_operations for a misc device?

[Greg KH <greg@kroah.com>]

On Tue, Sep 23, 2025 at 06:47:25PM +0200, Fabio wrote:
> Hello,
> 
> brand-new kernel newbie here. I'm interested in the Rust side of the kernel.
> To get acquainted, I'm following the Linux Driver Development book, 3rd
> edition, also integrating it with modern documentation and best practices
> (or at least I hope so).
> 
> I'm developing the scull module on top of the
> `kernel::miscdevice::MiscDevice` trait. Where I'm stuck at is the lack of
> the `kernel::file::Operations` trait [1], that most online resources refer
> to. I need such trait to implement read and write operations for the misc
> device. AFAIU, this trait used to exist but it's no longer in the mainline
> codebase [2]. Also, `kernel::miscdevice::MiscdeviceVTable` is private inside
> the miscdevice module, so there's no way I can somehow extend it.
> 
> Is there any workaround for what I want to achieve, or is ioctl the only
> allowed interface to misc devices in Rust?

Please see the rust sample misc driver that is right now in linux-next,
it shows how to use the read and write iter functions, which should be
all that you need.

Specifically look at commit e5b0d7da941a ("samples: rust_misc_device:
Expand the sample to support read()ing from userspace"), which shows how
this can be done.

Yes, this is a bit different than the traditional "read/write"
callbacks, as documented in the device driver's book, but things have
moved on in the decades since we wrote that :)

The changes for this will be showing up in the next kernel release (i.e.
6.18).

hope this helps,

greg k-h

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