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

[Arnd Bergmann]

On Thu, Jan 28, 2021 at 3:17 PM Alessio Balsini <balsini@xxxxxxxxxxx> 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