linux-kernel - Re: [PATCH v2 2/2] rust: miscdevice: add base miscdevice abstraction

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Message-Id: <50b1c868-3cab-4310-ba4f-2a0a24debaa9@app.fastmail.com>
Date: Wed, 02 Oct 2024 13:24:57 +0000
From: "Arnd Bergmann" <arnd@...db.de>
To: "Alice Ryhl" <aliceryhl@...gle.com>
Cc: "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
 "Miguel Ojeda" <ojeda@...nel.org>,
 "Alexander Viro" <viro@...iv.linux.org.uk>,
 "Christian Brauner" <brauner@...nel.org>, "Jan Kara" <jack@...e.cz>,
 "Boqun Feng" <boqun.feng@...il.com>, "Gary Guo" <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,
 "Benno Lossin" <benno.lossin@...ton.me>,
 "Andreas Hindborg" <a.hindborg@...nel.org>,
 "Trevor Gross" <tmgross@...ch.edu>, rust-for-linux@...r.kernel.org,
 linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] rust: miscdevice: add base miscdevice abstraction

On Wed, Oct 2, 2024, at 12:58, Alice Ryhl wrote:
> On Wed, Oct 2, 2024 at 2:48 PM Arnd Bergmann <arnd@...db.de> wrote:
> A quick sketch.
>
> One option is to do something along these lines:

This does seem promising, at least if I read your sketch
correctly. I'd probably need a more concrete example to
understand better how this would be used in a driver.

> struct IoctlParams {
>     pub cmd: u32,
>     pub arg: usize,
> }
>
> impl IoctlParams {
>     fn user_slice(&self) -> IoctlUser {
>         let userslice = UserSlice::new(self.arg, _IOC_SIZE(self.cmd));
>         match _IOC_DIR(self.cmd) {
>             _IOC_READ => IoctlParams::Read(userslice.reader()),
>             _IOC_WRITE => IoctlParams::Write(userslice.writer()),
>             _IOC_READ|_IOC_WRITE => IoctlParams::WriteRead(userslice),
>             _ => unreachable!(),

Does the unreachable() here mean that something bad happens
if userspace passes something other than one of the three,
or are the 'cmd' values here in-kernel constants that are
always valid?

> enum IoctlUser {
>     Read(UserSliceReader),
>     Write(UserSliceWriter),
>     WriteRead(UserSlice),
> }
>
> Then ioctl implementations can use a match statement like this:
>
> match ioc_params.user_slice() {
>     IoctlUser::Read(slice) => {},
>     IoctlUser::Write(slice) => {},
>     IoctlUser::WriteRead(slice) => {},
> }
>
> Where each branch of the match handles that case.

This is where I fail to see how that would fit in. If there
is a match statement in a driver, I would assume that it would
always match on the entire cmd code, but never have a command
that could with more than one _IOC_DIR type.

      Arnd