linux-kernel - Re: [PATCH v5 2/3] fs: openat2: Extend open

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Message-ID: <CAJfpegt=xe-8AayW2i3AYrk3q-=Pp_A+Hctsk+=sXoMed5hFQA@mail.gmail.com>
Date:   Wed, 22 Apr 2020 09:55:56 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Michael Kerrisk <mtk.manpages@...il.com>
Cc:     Josh Triplett <josh@...htriplett.org>, io-uring@...r.kernel.org,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>, Jens Axboe <axboe@...nel.dk>,
        Aleksa Sarai <cyphar@...har.com>,
        linux-man <linux-man@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v5 2/3] fs: openat2: Extend open_how to allow
 userspace-selected fds

On Wed, Apr 22, 2020 at 8:06 AM Michael Kerrisk (man-pages)
<mtk.manpages@...il.com> wrote:
>
> [CC += linux-api]
>
> On Wed, 22 Apr 2020 at 07:20, Josh Triplett <josh@...htriplett.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?

Add this flag to the io_uring API, by all means.

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.

Thanks,
Miklos