linux-kernel - Re: [PATCH 2/5] rust: device: Add a minimal RawDevice trait

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Date:   Fri, 24 Feb 2023 20:22:05 +0100
From:   Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:     Asahi Lina <lina@...hilina.net>
Cc:     Robin Murphy <robin.murphy@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Will Deacon <will@...nel.org>, Joerg Roedel <joro@...tes.org>,
        Hector Martin <marcan@...can.st>,
        Sven Peter <sven@...npeter.dev>, Arnd Bergmann <arnd@...db.de>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        Neal Gompa <neal@...pa.dev>, rust-for-linux@...r.kernel.org,
        linux-kernel@...r.kernel.org, asahi@...ts.linux.dev
Subject: Re: [PATCH 2/5] rust: device: Add a minimal RawDevice trait

On Fri, Feb 24, 2023 at 5:23 PM Asahi Lina <lina@...hilina.net> wrote:
>
> So I guess what I'm saying is that at the end of the day, if we can't
> get an interface to be 100% safe and sound and usable, that's probably
> okay. We're still getting a lot of safe mileage out of the other 99%! ^^

We talked a few times about what approach to take for things where a
fully safe API is not feasible. There have been differing opinions in
the past.

One approach would be requiring a "global `unsafe`" so to speak, once,
somewhere in the API -- it does not improve things much, but at least
it would make the user acknowledge the pitfalls of that particular
API/feature/subsystem/hardware/... e.g. for module unloading, one
could ask for an `unsafe` inside the `module!` macro invocation (like
`unsafe unloadable: true,`). This would allow for `// SAFETY: ...`
comments.

Another approach would be declaring some of those "external entities"
outside the scope of Rust's safety guarantees, like it is done for
e.g. `/proc/self/mem` in userspace Rust [1]. They would be documented
wherever relevant, and perhaps we could have an "acknowledged
soundness holes" list.

Having said that, of course, what we definitely don't want to allow is
for subsystems to provide unsound safe APIs for no reason.

[1] https://doc.rust-lang.org/stable/std/os/unix/io/index.html#procselfmem-and-similar-os-features

Cheers,
Miguel