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From:   Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Date:   Sat, 17 Apr 2021 01:46:35 +0200
Message-ID: <CANiq72k3wmuqgPz+WR1=64vr--SFu971P+2Neq+Xe2TUSZFv0g@mail.gmail.com>
Subject: Re: [PATCH 04/13] Kbuild: Rust support
To:     Willy Tarreau <w@1wt.eu>
Cc:     Connor Kuehl <ckuehl@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Miguel Ojeda <ojeda@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        rust-for-linux@vger.kernel.org,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Geoffrey Thomas <geofft@ldpreload.com>,
        Finn Behrens <me@kloenk.de>,
        Adam Bratschi-Kaye <ark.email@gmail.com>,
        Wedson Almeida Filho <wedsonaf@google.com>,
        Michael Ellerman <mpe@ellerman.id.au>
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On Sat, Apr 17, 2021 at 12:04 AM Willy Tarreau <w@1wt.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