Re: [PATCH v5 2/3] fs: openat2: Extend open_how to allow userspace-selected fds

[Miklos Szeredi <miklos@szeredi.hu>]

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