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

[Josh Triplett]

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@xxxxxxxxx> wrote:
> >
> > [CC += linux-api]
> >
> > On Wed, 22 Apr 2020 at 07:20, Josh Triplett <josh@xxxxxxxxxxxxxxxx> 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.)

> 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.