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

[Josh Triplett]

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@xxxxxxxxxxxxxxxx> 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.