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

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Message-ID: <20210417042405.GA13432@1wt.eu>
Date:   Sat, 17 Apr 2021 06:24:05 +0200
From:   Willy Tarreau <w@....eu>
To:     Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
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 01:46:35AM +0200, Miguel Ojeda wrote:
> 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.

Sure.

> 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;
>    }

Yes I mentioned this when it was my understanding that the composite
result returned was made both of a pointer and an error code, but Connor
explained that it was in fact more of a selector and a union.

> 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?

My concern was to know what field to look at to reliably detect an error
from the C side after a sequence doing C -> Rust -> C when the inner C
code uses NULL to mark an error and the upper C code uses NULL as a valid
value and needs to look at an error code instead to rebuild a result. But
if it's more:
     
     if (result.ok)
        return result.pointer;
     else
        return (void *)-result.error;
    
then it shouldn't be an issue.

Willy