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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Wed, 30 Sep 2020 17:44:54 +0200
Message-ID: <CAJfpegvB7XJH7sPni7Vj7R4ZwSrDfevfeRRBgvESSgGg=C5tdQ@mail.gmail.com>
Subject: Re: [PATCH V9 1/4] fuse: Definitions and ioctl() for passthrough
To:     Alessio Balsini <balsini@android.com>
Cc:     Akilesh Kailash <akailash@google.com>,
        Amir Goldstein <amir73il@gmail.com>,
        Antonio SJ Musumeci <trapexit@spawn.link>,
        David Anderson <dvander@google.com>,
        Giuseppe Scrivano <gscrivan@redhat.com>,
        Jann Horn <jannh@google.com>, Jens Axboe <axboe@kernel.dk>,
        Martijn Coenen <maco@android.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Paul Lawrence <paullawrence@google.com>,
        Stefano Duo <stefanoduo@google.com>,
        Zimuzo Ezeozue <zezeozue@google.com>,
        fuse-devel <fuse-devel@lists.sourceforge.net>,
        kernel-team <kernel-team@android.com>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
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On Thu, Sep 24, 2020 at 3:13 PM Alessio Balsini <balsini@android.com> wrote:
>
> Introduce the new FUSE passthrough ioctl(), which allows userspace to
> specify a direct connection between a FUSE file and a lower file system
> file.
> Such ioctl() requires userspace to specify:
> - the file descriptor of one of its opened files,
> - the unique identifier of the FUSE request associated with a pending
>   open/create operation,
> both encapsulated into a fuse_passthrough_out data structure.
> The ioctl() will search for the pending FUSE request matching the unique
> identifier, and update the passthrough file pointer of the request with the
> file pointer referenced by the passed file descriptor.
> When that pending FUSE request is handled, the passthrough file pointer
> is copied to the fuse_file data structure, so that the link between FUSE
> and lower file system is consolidated.

How about returning an ID from the ioctl (like the fuse2 porototype)
and returning that in fuse_open_out.passthrough_fh?

Seems a more straightforward interface to me.

Thanks,
Miklos