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Date:   Mon, 6 Apr 2020 11:17:01 +0200
From:   Karel Zak <kzak@redhat.com>
To:     "J. Bruce Fields" <bfields@fieldses.org>
Cc:     Lennart Poettering <mzxreary@0pointer.de>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Ian Kent <raven@themaw.net>,
        David Howells <dhowells@redhat.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>, dray@redhat.com,
        Miklos Szeredi <mszeredi@redhat.com>,
        Steven Whitehouse <swhiteho@redhat.com>,
        Jeff Layton <jlayton@redhat.com>, andres@anarazel.de,
        keyrings@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, Aleksa Sarai <cyphar@cyphar.com>
Subject: Re: Upcoming: Notifications, FS notifications and fsinfo()
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On Fri, Apr 03, 2020 at 04:30:24PM -0400, J. Bruce Fields wrote:
> On Fri, Apr 03, 2020 at 05:12:23PM +0200, Lennart Poettering wrote:
> > BTW, while we are at it: one more thing I'd love to see exposed by
> > statx() is a simple flag whether the inode is a mount point. There's
> > plenty code that implements a test like this all over the place, and
> > it usually isn't very safe. There's one implementation in util-linux
> > for example (in the /usr/bin/mountpoint binary), and another one in
> > systemd. Would be awesome to just have a statx() return flag for that,
> > that would make things *so* much easier and more robust. because in
> > fact most code isn't very good that implements this, as much of it
> > just compares st_dev of the specified file and its parent. Better code
> > compares the mount ID, but as mentioned that's not as pretty as it
> > could be so far...
> 
> nfs-utils/support/misc/mountpoint.c:check_is_mountpoint() stats the file
> and ".." and returns true if they have different st_dev or the same
> st_ino.  Comparing mount ids sounds better.

BTW, this traditional st_dev+st_ino way is not reliable for bind mounts.
For mountpoint(1) we search the directory in /proc/self/mountinfo.

> So anyway, yes, everybody reinvents the wheel here, and this would be
> useful.

 +1

    Karel

-- 
 Karel Zak  <kzak@redhat.com>
 http://karelzak.blogspot.com