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Message-ID: <989bb93754a4af69c02a9f42b05549f7e72630b3.camel@themaw.net>
Subject: Re: file metadata via fs API
From:   Ian Kent <raven@themaw.net>
To:     David Howells <dhowells@redhat.com>,
        Miklos Szeredi <miklos@szeredi.hu>
Cc:     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>,
        LSM <linux-security-module@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Date:   Thu, 13 Aug 2020 09:01:31 +0800
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On Wed, 2020-08-12 at 14:06 +0100, David Howells wrote:
> 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.

It is a problem for autofs, direct mounts in particular, but also
for mount ordering at times when umounting a tree of mounts where
mounts are covered or at shutdown.

Ian