linux-kernel - Re: [PATCH RESEND V12 2/8] fuse: 32-bit user space ioctl compat for fuse device

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Message-ID: <CAK8P3a2K2FzPvqBYL9W=Yut58SFXyetXwU4Fz50G5O3TsS0pPQ@mail.gmail.com>
Date:   Tue, 16 Mar 2021 19:57:09 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Alessio Balsini <balsini@...roid.com>
Cc:     qxy <qxy65535@...il.com>, Miklos Szeredi <miklos@...redi.hu>,
        Akilesh Kailash <akailash@...gle.com>,
        Amir Goldstein <amir73il@...il.com>,
        Antonio SJ Musumeci <trapexit@...wn.link>,
        David Anderson <dvander@...gle.com>,
        Giuseppe Scrivano <gscrivan@...hat.com>,
        Jann Horn <jannh@...gle.com>, Jens Axboe <axboe@...nel.dk>,
        Martijn Coenen <maco@...roid.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Lawrence <paullawrence@...gle.com>,
        Peng Tao <bergwolf@...il.com>,
        Stefano Duo <duostefano93@...il.com>,
        Zimuzo Ezeozue <zezeozue@...gle.com>, wuyan <wu-yan@....com>,
        fuse-devel@...ts.sourceforge.net,
        Android Kernel Team <kernel-team@...roid.com>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RESEND V12 2/8] fuse: 32-bit user space ioctl compat for
 fuse device

On Thu, Jan 28, 2021 at 3:17 PM Alessio Balsini <balsini@...roid.com> wrote:
>
> Hi all,
>
> I'm more than happy to change the interface into something that is
> objectively better and accepted by everyone.
> I would really love to reach the point at which we have a "stable-ish"
> UAPI as soon as possible.

It's in the mainline kernel, so you already have a stable uapi and
cannot change that in any incompatible way!

> I've been thinking about a few possible approaches to fix the issue, yet
> to preserve its flexibility. These are mentioned below.
>
>
>   Solution 1: Size
>
> As mentioned in my previous email, one solution could be to introduce
> the "size" field to allow the structure to grow in the future.
>
> struct fuse_passthrough_out {
>     uint32_t        size;   // Size of this data structure
>     uint32_t        fd;
> };
>
> The problem here is that we are making the promise that all the upcoming
> fields are going to be maintained forever and at the offsets they were
> originally defined.
>
>
>   Solution 2: Version
>
> Another solution could be to s/size/version, where for every version of
> FUSE passthrough we reserve the right to modifying the fields over time,
> casting them to the right data structure according to the version.


Please read Documentation/driver-api/ioctl.rst for how to design
ioctls. Neither 'size' nor 'version' fields are appropriate here. If you
have a new behavior, you need a new command code.

      Arnd