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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Tue, 10 May 2022 15:15:05 +0200
Message-ID: <CAJfpegtVgyumJiFM_ujjuRTjg07vwOd4h9AT+mbh+n1Qn-LqqA@mail.gmail.com>
Subject: Re: [RFC PATCH] getting misc stats/attributes via xattr API
To:     Christian Brauner <brauner@kernel.org>
Cc:     linux-fsdevel@vger.kernel.org, Dave Chinner <david@fromorbit.com>,
        "Theodore Ts'o" <tytso@mit.edu>, Karel Zak <kzak@redhat.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org,
        Linux API <linux-api@vger.kernel.org>,
        linux-man <linux-man@vger.kernel.org>,
        LSM <linux-security-module@vger.kernel.org>,
        Ian Kent <raven@themaw.net>,
        David Howells <dhowells@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <christian@brauner.io>,
        Amir Goldstein <amir73il@gmail.com>,
        James Bottomley <James.Bottomley@hansenpartnership.com>
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On Tue, 10 May 2022 at 13:53, Christian Brauner <brauner@kernel.org> wrote:

> > What exactly are the attributes that systemd requires?
>
> We keep a repo with ideas for (kernel) extensions - we should probably
> publish that somewhere - but the list we used for a prototype roughly
> contains:
>
> * mount flags MOUNT_ATTR_RDONLY etc.
> * time flags MOUNT_ATTR_RELATIME etc. (could probably be combined with
>   mount flags. We missed the opportunity to make them proper enums
>   separate from other mount flags imho.)
> * propagation "flags" (MS_SHARED)
> * peer group
> * mnt_id of the mount
> * mnt_id of the mount's parent
> * owning userns

Sounds good thus far.   And hey, we don't even need a new syscall:
statx(2) could handle these fine.

> There's a bit more advanced stuff systemd would really want but which I
> think is misplaced in a mountinfo system call including:
> * list of primary and auxiliary block device major/minor

It's when you need to return variable size arrays or list of strings
that the statx kind of interface falls down.

For that a hierarchical namespace is a much better choice, as it can
represent arbitrary levels of arrays, while doing that with a
specialized syscall is going to be cumbersome.

> I just have a really hard time understanding how this belongs into the
> (f)getxattr() system call family and why it would be a big deal to just
> make this a separate system call.

Fragmenting syntactically equivalent interfaces is bad, unifying them
is good.  Dave's example of adding a new syscall for retrieving
multiple xattrs  is a prime example.

Thanks,
Miklos