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From:   David Howells <dhowells@redhat.com>
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To:     Miklos Szeredi <miklos@szeredi.hu>
Cc:     dhowells@redhat.com, Karel Zak <kzak@redhat.com>,
        Steven Whitehouse <swhiteho@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Jeff Layton <jlayton@redhat.com>,
        Miklos Szeredi <mszeredi@redhat.com>,
        Nicolas Dichtel <nicolas.dichtel@6wind.com>,
        Christian Brauner <christian@brauner.io>,
        Lennart Poettering <lennart@poettering.net>,
        Linux API <linux-api@vger.kernel.org>,
        Ian Kent <raven@themaw.net>,
        LSM <linux-security-module@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: file metadata via fs API
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Date:   Wed, 12 Aug 2020 14:06:25 +0100
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Miklos Szeredi <miklos@szeredi.hu> wrote:

> That presumably means the mount ID <-> mount path mapping already
> exists, which means it's just possible to use the open(mount_path,
> O_PATH) to obtain the base fd.

No, you can't.  A path more correspond to multiple mounts stacked on top of
each other, e.g.:

	mount -t tmpfs none /mnt
	mount -t tmpfs none /mnt
	mount -t tmpfs none /mnt

Now you have three co-located mounts and you can't use the path to
differentiate them.  I think this might be an issue in autofs, but Ian would
need to comment on that.

David