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Date:   Thu, 7 Feb 2019 04:05:48 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     Jann Horn <jannh@google.com>, linux-aio@kvack.org,
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Subject: Re: [PATCH 13/18] io_uring: add file set registration
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On Wed, Feb 06, 2019 at 10:56:41AM -0700, Jens Axboe wrote:
> On 2/5/19 6:01 PM, Al Viro wrote:
> > On Tue, Feb 05, 2019 at 05:27:29PM -0700, Jens Axboe wrote:
> > 
> >> This should be better, passes some basic testing, too:
> >>
> >> http://git.kernel.dk/cgit/linux-block/commit/?h=io_uring&id=01a93aa784319a02ccfa6523371b93401c9e0073
> >>
> >> Verified that we're grabbing the right refs, and don't hold any
> >> ourselves. For the file registration, forbid registration of the
> >> io_uring fd, as that is pointless and will introduce a loop regardless
> >> of fd passing.
> > 
> > *shrug*
> > 
> > So pass it to AF_UNIX socket and register _that_ - does't change the
> > underlying problem.
> 
> Maybe I'm being dense here, but it's an f_op match. Should catch a
> passed fd as well, correct?

f_op match on _what_?

> With that, how can there be a loop?

	io_uring_fd = ....
	socketpair(PF_UNIX, SOCK_STREAM, 0, sock_fds);
	register sock_fds[0] and sock_fds[1] to io_uring_fd
	send SCM_RIGHTS datagram with io_uring_fd to sock_fds[0]
	close sock_fds[0], sock_fds[1] and io_uring_fd

And there's your unreachable loop.