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

[Alessio Balsini <balsini@android.com>]

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.

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.


  Solution 3: Type

Using an enumerator to define the data structure content and purpose is
the most flexible solution I can think of.  This would for example allow
us to substitute FUSE_DEV_IOC_PASSTHROUGH_OPEN with the generic
FUSE_DEV_IOC_PASSTHROUGH and having a single ioctl for any eventually
upcoming passthrough requests.

enum fuse_passthrough_type {
    FUSE_PASSTHROUGH_OPEN
};

struct fuse_passthrough_out {
    uint32_t type; /* as defined by enum fuse_passthrough_type */
    union {
        uint32_t fd;
    };
};

This last is my favorite, as regardless the minimal logic required to
detect the size and content of the struct (not required now as we only
have a single option), it would also allow to do some kind of interface
versioning (e.g., in case we want to implement
FUSE_PASSTHROUGH_OPEN_V2).

What do you think?

Thanks,
Alessio

P.S.
Sorry if you received a duplicate email. I first sent this in reply to an email
without realizing it was a private message.

On Thu, Jan 28, 2021 at 11:01:59AM +0800, qxy wrote:
> Hi Alessio,
> 
> I have received a failure from the Mail Delivery System for times and feel
> really sorry if you have already received the duplicate message...
> 
> Thank you for your reply.
> I think it's wonderful to remove *vec from the data structure fields since
> we consider that it is not a good idea to use pointer when there is a need
> for cross-platform.
> Do you have a plan to modify the kernel fuse_passthrough_out data structure
> the same way as you mentioned?
> 
> Thanks!
> qixiaoyu