linux-kernel - Re: [RFC PATCH] getting misc stats/attributes via xattr API

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Date:   Fri, 6 May 2022 03:06:59 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     tytso <tytso@....edu>
Cc:     Miklos Szeredi <miklos@...redi.hu>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Dave Chinner <david@...morbit.com>,
        Karel Zak <kzak@...hat.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Christian Brauner <brauner@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        linux-man <linux-man@...r.kernel.org>,
        LSM <linux-security-module@...r.kernel.org>,
        Ian Kent <raven@...maw.net>,
        David Howells <dhowells@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <christian@...uner.io>,
        James Bottomley <James.Bottomley@...senpartnership.com>
Subject: Re: [RFC PATCH] getting misc stats/attributes via xattr API

On Fri, May 6, 2022 at 2:38 AM tytso <tytso@....edu> wrote:
>
> On Tue, May 03, 2022 at 02:23:23PM +0200, Miklos Szeredi wrote:
> >
> > : - root
> > bar - an attribute
> > foo: - a folder (can contain attributes and/or folders)
> >
> > The contents of a folder is represented by a null separated list of names.
> >
> > Examples:
> >
> > $ getfattr -etext -n ":" .
> > # file: .
> > :="mnt:\000mntns:"
>
> In your example, does it matter what "." is?  It looks like in some
> cases, it makes no difference at all, and in other cases, like this,
> '.' *does* matter:

It does. If "." was a directory in /proc/ or in ext4 it might have had
more entries.

>
> > $ getfattr -etext -n ":mnt:info" .
> > # file: .
> > :mnt:info="21 1 254:0 / / rw,relatime - ext4 /dev/root rw\012"
>
> Is that right?
>
> > $ getfattr -etext -n ":mntns:" .
> > # file: .
> > :mntns:="21:\00022:\00024:\00025:\00023:\00026:\00027:\00028:\00029:\00030:\00031:"
>
> What is this returning?  All possible mount name spaces?  Or all of
> the mount spaces where '.' happens to exist?

This confused me too.
It is not returning the mount namespaces, it is returning all the mount ids
in the mount namespace of ".".
":mntns:mounts:" might have been a better choice of key.

Thanks,
Amir.

>
> Also, using the null character means that we can't really use shell
> scripts calling getfattr.  I understand that the problem is that in
> some cases, you might want to return a pathname, and NULL is the only
> character which is guaranteed not to show up in a pathname.  However,
> it makes parsing the returned value in a shell script exciting.
>
>                                          - Ted