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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Wed, 12 Aug 2020 14:43:32 +0200
Message-ID: <CAJfpegv4sC2zm+N5tvEmYaEFvvWJRHfdGqXUoBzbeKj81uNCvQ@mail.gmail.com>
Subject: Re: file metadata via fs API
To:     Karel Zak <kzak@redhat.com>
Cc:     Steven Whitehouse <swhiteho@redhat.com>,
        David Howells <dhowells@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>
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On Wed, Aug 12, 2020 at 1:28 PM Karel Zak <kzak@redhat.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.

If that assumption is not true, we could add a new interface for
opening the root of the mount by ID.  Fsinfo uses the dfd as a root
for checking connectivity and the filename as the mount ID + a special
flag indicating that it's not "dfd + path" we are dealing with but
"rootfd + mntid".  That sort of semantic multiplexing is quite ugly
and I wouldn't suggest doing that with openat(2).

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);

Yeah, yeah this is adding just another syscall interface, but notice how:

 a) it does one simple thing, no multiplexing at all

 b) is general purpose, and could be used for example in conjunction
with open_by_handle_at(2), that also requires an fd pointing to a
mount.

Thanks,
Miklos