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

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Date:   Thu, 5 May 2022 16:38:12 -0700
From:   tytso <tytso@....edu>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     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@...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>,
        Amir Goldstein <amir73il@...il.com>,
        James Bottomley <James.Bottomley@...senpartnership.com>
Subject: Re: [RFC PATCH] getting misc stats/attributes via xattr API

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:

> $ 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?

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