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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 6 Apr 2020 09:34:08 -0700
Message-ID: <CAHk-=wjW735UE+byK1xsM9UvpF2ubh7bCMaAOwz575U7hRCKyA@mail.gmail.com>
Subject: Re: Upcoming: Notifications, FS notifications and fsinfo()
To:     Karel Zak <kzak@redhat.com>
Cc:     "J. Bruce Fields" <bfields@fieldses.org>,
        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>,
        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 <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Aleksa Sarai <cyphar@cyphar.com>
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On Mon, Apr 6, 2020 at 2:17 AM Karel Zak <kzak@redhat.com> wrote:
>
> On Fri, Apr 03, 2020 at 04:30:24PM -0400, J. Bruce Fields wrote:
> >
> > 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.

These days you should probably use openat2() with RESOLVE_NO_XDEV.

No need for any mountinfo or anything like that. Just look up the
pathname and say "don't cross mount-points", and you'll get an error
if it's a mount crossing lookup.

So this kind of thing is _not_ an argument for another kernel querying
interface.  We got a new (and better) model for a lot of this.

              Linus