linux-kernel - Re: file metadata via fs API

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Date:   Thu, 13 Aug 2020 10:52:52 +0200
From:   Karel Zak <kzak@...hat.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Steven Whitehouse <swhiteho@...hat.com>,
        David Howells <dhowells@...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

On Wed, Aug 12, 2020 at 02:43:32PM +0200, Miklos Szeredi wrote:
> On Wed, Aug 12, 2020 at 1:28 PM Karel Zak <kzak@...hat.com> wrote:
> 
> > The proposal is based on paths and open(), how do you plan to deal
> > with mount IDs? David's fsinfo() allows to ask for mount info by mount
> > ID and it works well with mount notification where you get the ID. The
> > collaboration with notification interface is critical for our use-cases.
> 
> One would use the notification to keep an up to date set of attributes
> for each watched mount, right?
> 
> 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.

The notification also reports new mount nodes, so we have no mount ID
<-> mount path mapping in userspace yet.

The another problem is that open(path) cannot be used if you have multiple
filesystems on the same mount point -- in this case (at least in theory)
you can get ID for by-path inaccessible filesystem.

> A new syscall that returns an fd pointing to the root of the mount
> might be the best solution:
> 
>    int open_mount(int root_fd, u64 mntid, int flags);

Yes, something like this is necessary. You do not want to depend
on paths if you want to read information about mountpoints.

 Karel

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