linux-kernel - Re: [PATCH] fs: add an ioctl to get an owning userns for a superblock

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Date:   Wed, 10 May 2017 20:24:00 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Andrei Vagin <avagin@...tuozzo.com>
Cc:     Andrei Vagin <avagin@...nvz.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        <linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-api@...r.kernel.org>, <criu@...nvz.org>,
        Linux Containers <containers@...ts.linux-foundation.org>
Subject: Re: [PATCH] fs: add an ioctl to get an owning userns for a superblock

Andrei Vagin <avagin@...tuozzo.com> writes:

> On Tue, May 09, 2017 at 07:34:00PM -0500, Eric W. Biederman wrote:
>> Andrei Vagin <avagin@...nvz.org> writes:
>> 
>> > The introduced ioctl returns a file descriptor that refers to a owning
>> > user namespace for a superblock which is associated with a target file
>> > descriptor.
>> >
>> > EPERM is returned if the current process doesn't have CAP_SYS_ADMIN in
>> > the returned user namespace.
>> >
>> > This information is required to dump and restore mount namespaces. We
>> > need to know to which user namespace a superblock is belonged to.
>> >
>> > We already have the SIOCGSKNS ioctl for sockets to get a network
>> > namespace, so it looks reasonable to use the same interface for
>> > superblocks too.
>> >
>> > This functionality can be useful for users in order to understand
>> > a running system.
>> 
>> This will probably work.  And the capability check eases any concerns
>> I might have that this would be a trivial information leak.
>> 
>> That said can we hold off just a little bit.  If open_fs work actually
>> turns into a real interface that would seem to be the perfect place
>> to stick this functionality.
>
> Sure, we can. Do you know any place where to read more information about
> open_fs? I think I have heared a few times about this idea, but it would be
> good to get more details.


Look for David Howells <dhowells@...hat.com> recent patches on lkml he
has implemented an initial rfc for it.

Eric