Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Tue, 25 Sep 2018 18:46:13 -0400
From:   "Theodore Y. Ts'o" <tytso@mit.edu>
To:     Adam Borowski <kilobyte@angband.pl>
Cc:     Jeff Layton <jlayton@redhat.com>,
        Alan Cox <gnomes@lxorguk.ukuu.org.uk>,
        =?utf-8?B?54Sm5pmT5Yas?= <milestonejxd@gmail.com>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        Rogier Wolff <R.E.Wolff@BitWizard.nl>
Subject: Re: POSIX violation by writeback error
Message-ID: <20180925224613.GI2933@thunk.org>
Mail-Followup-To: "Theodore Y. Ts'o" <tytso@mit.edu>,
        Adam Borowski <kilobyte@angband.pl>,
        Jeff Layton <jlayton@redhat.com>,
        Alan Cox <gnomes@lxorguk.ukuu.org.uk>,
        =?utf-8?B?54Sm5pmT5Yas?= <milestonejxd@gmail.com>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        Rogier Wolff <R.E.Wolff@BitWizard.nl>
References: <CAJDTihx2yaR-_-9Ks1PoFcrKNZgUOoLdN-wRTTMV76Jg_dCLrw@mail.gmail.com>
 <486f6105fd4076c1af67dae7fdfe6826019f7ff4.camel@redhat.com>
 <20180925003044.239531c7@alans-desktop>
 <0662a4c5d2e164d651a6a116d06da380f317100f.camel@redhat.com>
 <20180925154627.GC2933@thunk.org>
 <20180925173510.273hshp4iapd6dcd@angband.pl>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20180925173510.273hshp4iapd6dcd@angband.pl>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Tue, Sep 25, 2018 at 07:35:11PM +0200, Adam Borowski wrote:
> Isn't this what the snippet for O_TMPFILE in "man 2 open" does?:
> 
>                   char path[PATH_MAX];
>                   fd = open("/path/to/dir", O_TMPFILE | O_RDWR,
>                                           S_IRUSR | S_IWUSR);
> 
>                   /* File I/O on 'fd'... */
> 
>                   snprintf(path, PATH_MAX,  "/proc/self/fd/%d", fd);
>                   linkat(AT_FDCWD, path, AT_FDCWD, "/path/for/file",
>                                           AT_SYMLINK_FOLLOW);

Huh.  I stand corrected.  I had assumed O_TMPFILE worked like any
other file where the link count was zero, and linkat(2) wouldn't allow
this.  But obviously, this does work.  In fact, from the linkat(2) man
page, using:

	linkat(fd, NULL, AT_FDCWD, "/path/for/file", AT_EMPTY_PATH);

is an even simpler way that doesn't /proc being mounted.

TIL...

					- Ted