linux-kernel - Re: file metadata via fs API

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAJfpegv4sC2zm+N5tvEmYaEFvvWJRHfdGqXUoBzbeKj81uNCvQ@mail.gmail.com>
Date:   Wed, 12 Aug 2020 14:43:32 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Karel Zak <kzak@...hat.com>
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 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.

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