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    SubjectRe: [PATCH v5 2/3] fs: openat2: Extend open_how to allow userspace-selected fds
    On Thu, Apr 23, 2020 at 2:48 AM Josh Triplett <josh@joshtriplett.org> wrote:
    >
    > On Wed, Apr 22, 2020 at 09:55:56AM +0200, Miklos Szeredi wrote:
    > > On Wed, Apr 22, 2020 at 8:06 AM Michael Kerrisk (man-pages)
    > > <mtk.manpages@gmail.com> wrote:
    > > >
    > > > [CC += linux-api]
    > > >
    > > > On Wed, 22 Apr 2020 at 07:20, Josh Triplett <josh@joshtriplett.org> wrote:
    > > > >
    > > > > Inspired by the X protocol's handling of XIDs, allow userspace to select
    > > > > the file descriptor opened by openat2, so that it can use the resulting
    > > > > file descriptor in subsequent system calls without waiting for the
    > > > > response to openat2.
    > > > >
    > > > > In io_uring, this allows sequences like openat2/read/close without
    > > > > waiting for the openat2 to complete. Multiple such sequences can
    > > > > overlap, as long as each uses a distinct file descriptor.
    > >
    > > If this is primarily an io_uring feature, then why burden the normal
    > > openat2 API with this?
    >
    > This feature was inspired by io_uring; it isn't exclusively of value
    > with io_uring. (And io_uring doesn't normally change the semantics of
    > syscalls.)

    What's the use case of O_SPECIFIC_FD beyond io_uring?

    >
    > > This would also allow Implementing a private fd table for io_uring.
    > > I.e. add a flag interpreted by file ops (IORING_PRIVATE_FD), including
    > > openat2 and freely use the private fd space without having to worry
    > > about interactions with other parts of the system.
    >
    > I definitely don't want to add a special kind of file descriptor that
    > doesn't work in normal syscalls taking file descriptors. A file
    > descriptor allocated via O_SPECIFIC_FD is an entirely normal file
    > descriptor, and works anywhere a file descriptor normally works.

    What's the use case of allocating a file descriptor within io_uring
    and using it outside of io_uring?

    Thanks,
    Miklos

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