linux-kernel - Re: [PATCH v4 0/6] querying mount attributes

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Date:   Mon, 6 Nov 2023 13:10:53 +0100
From:   Karel Zak <kzak@...hat.com>
To:     Ian Kent <raven@...maw.net>
Cc:     Miklos Szeredi <mszeredi@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, linux-man@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        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>,
        Matthew House <mattlloydhouse@...il.com>,
        Florian Weimer <fweimer@...hat.com>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v4 0/6] querying mount attributes

On Wed, Nov 01, 2023 at 07:52:45PM +0800, Ian Kent wrote:
> On 25/10/23 22:01, Miklos Szeredi wrote:
> Looks ok to me,covers the primary cases I needed when I worked
> on using fsinfo() in systemd.

Our work on systemd was about two areas: get mount info (stat/listmount()  
now) from the kernel, and get the mount ID from notification.                 

There was watch_queue.h with WATCH_TYPE_MOUNT_NOTIFY and struct       
mount_notification->auxiliary_mount (aka mount ID) and event subtype  
to get the change status (new mount, umount, etc.) 

For example David's:
 https://patchwork.kernel.org/project/linux-security-module/patch/155991711016.15579.4449417925184028666.stgit@warthog.procyon.org.uk/

Do we have any replacement for this?

> Karel, is there anything missing you would need for adding
> libmount support?

Miklos's statmount() and listmount() API is excellent from my point of
view. It looks pretty straightforward to use, and with the unique
mount ID, it's safe too. It will be ideal for things like umount(8)
(and recursive umount, etc.).

For complex scenarios (systemd), we need to get from the kernel the
unique ID's after any change in the mount table to save resources and
call statmount() only for the affected mount node. Parse mountinfo
sucks, call for(listmount(-1)) { statmount() } sucks too :-)

    Karel

-- 
 Karel Zak  <kzak@...hat.com>
 http://karelzak.blogspot.com