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

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Date:   Thu, 23 Apr 2020 01:06:44 -0700
From:   Josh Triplett <josh@...htriplett.org>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Michael Kerrisk <mtk.manpages@...il.com>, 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 Thu, Apr 23, 2020 at 09:45:45AM +0200, Miklos Szeredi wrote:
> On Thu, Apr 23, 2020 at 9:33 AM Josh Triplett <josh@...htriplett.org> wrote:
> > > What are the plans for those syscalls that don't easily lend
> > > themselves to this modification (such as accept(2))?
> >
> > accept4 has a flags argument with more flags available, so it'd be
> > entirely possible to cleanly extend it further without introducing a new
> > version.
>
> Variable argument syscalls, you are thinking?

That or repurposing an existing pointer-sized argument as an
open_how-style struct, yes. But in any case, I'm not proposing that; I'm
proposing changes to the existing highly extensible openat2 syscall.

> > > I mean, you could open the file descriptor outside of io_uring in such
> > > cases, no?
> >
> > I would prefer to not introduce that limitation in the first place, and
> > instead open normal file descriptors.
> >
> > > The point of O_SPECIFIC_FD is to be able to perform short
> > > sequences of open/dosomething/close without having to block and having
> > > to issue separate syscalls.
> >
> > "close" is not a required component. It's entirely possible to use
> > io_uring to open a file descriptor, do various things with it, and then
> > leave it open for subsequent usage via either other io_uring chains or
> > standalone syscalls.
> 
> If this use case arraises,

This wasn't a hypothetical "someone might want this". I'm stating that
this is a requirement I'm seeking to meet with this patch series, and
one I intend to use. The primary use case is interoperability with
other code using file descriptors and not using io_uring.