linux-kernel - Re: [PATCH 04/13] Kbuild: Rust support

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Message-ID: <CANiq72k3wmuqgPz+WR1=64vr--SFu971P+2Neq+Xe2TUSZFv0g@mail.gmail.com>
Date:   Sat, 17 Apr 2021 01:46:35 +0200
From:   Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:     Willy Tarreau <w@....eu>
Cc:     Connor Kuehl <ckuehl@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Miguel Ojeda <ojeda@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        rust-for-linux@...r.kernel.org,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Geoffrey Thomas <geofft@...reload.com>,
        Finn Behrens <me@...enk.de>,
        Adam Bratschi-Kaye <ark.email@...il.com>,
        Wedson Almeida Filho <wedsonaf@...gle.com>,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH 04/13] Kbuild: Rust support

On Sat, Apr 17, 2021 at 12:04 AM Willy Tarreau <w@....eu> wrote:
>
> But my point remains that the point of extreme care is at the interface
> with the rest of the kernel because there is a change of semantics
> there.
>
> Sure but as I said most often (due to API or ABI inheritance), both
> are already exclusive and stored as ranges. Returning 1..4095 for
> errno or a pointer including NULL for a success doesn't shock me at
> all.

At the point of the interface we definitely need to take care of
converting properly, but for Rust-to-Rust code (i.e. the ones using
`Result` etc.), that would not be a concern.

Just to ensure I understood your concern, for instance, in this case
you mentioned:

   result.status = foo_alloc();
   if (!result.status) {
       result.error = -ENOMEM;
       return result;
   }

Is your concern is that the caller would mix up the `status` with the
`error`, basically bubbling up the `status` as an `int` and forgetting
about the `error`, and then someone else later understanding that
`int` as a non-error because it is non-negative?

If yes: in C, yes, that could be a concern (if done with raw `int`s).
In Rust, if you get an `Err(ENOMEM)` from somebody, you cannot easily
conflate it with another type and return it by mistake because it is
more strictly typed than C.

Cheers,
Miguel