linux-kernel - Re: [PATCH v3 4/4] add listmount(2) syscall

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Message-ID: <CAJfpegv78njkWdaShTskKXoGOpKAndvYYJwq7CLibiu+xmLCvg@mail.gmail.com>
Date:   Thu, 5 Oct 2023 17:47:20 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Ian Kent <raven@...maw.net>
Cc:     Paul Moore <paul@...l-moore.com>,
        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, Karel Zak <kzak@...hat.com>,
        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 v3 4/4] add listmount(2) syscall

On Thu, 5 Oct 2023 at 06:23, Ian Kent <raven@...maw.net> wrote:

> The proc interfaces essentially use <mount namespace>->list to provide
>
> the mounts that can be seen so it's filtered by mount namespace of the
>
> task that's doing the open().
>
>
> See fs/namespace.c:mnt_list_next() and just below the m_start(), m_next(),

/proc/$PID/mountinfo will list the mount namespace of $PID.  Whether
current task has permission to do so is decided at open time.

listmount() will list the children of the given mount ID.  The mount
ID is looked up in the task's mount namespace, so this cannot be used
to list mounts of other namespaces.  It's a more limited interface.

I sort of understand the reasoning behind calling into a security hook
on entry to statmount() and listmount().  And BTW I also think that if
statmount() and listmount() is limited in this way, then the same
limitation should be applied to the proc interfaces.  But that needs
to be done real carefully because it might cause regressions.  OTOH if
it's only done on the new interfaces, then what is the point, since
the old interfaces will be available indefinitely?

Also I cannot see the point in hiding some mount ID's from the list.
It seems to me that the list is just an array of numbers that in
itself doesn't carry any information.

Thanks,
Miklos