linux-kernel - Re: file metadata via fs API

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Message-ID: <131358.1597237585@warthog.procyon.org.uk>
Date:   Wed, 12 Aug 2020 14:06:25 +0100
From:   David Howells <dhowells@...hat.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     dhowells@...hat.com, Karel Zak <kzak@...hat.com>,
        Steven Whitehouse <swhiteho@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Jeff Layton <jlayton@...hat.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Nicolas Dichtel <nicolas.dichtel@...nd.com>,
        Christian Brauner <christian@...uner.io>,
        Lennart Poettering <lennart@...ttering.net>,
        Linux API <linux-api@...r.kernel.org>,
        Ian Kent <raven@...maw.net>,
        LSM <linux-security-module@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: file metadata via fs API

Miklos Szeredi <miklos@...redi.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