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Date:   Wed, 9 Jun 2021 09:21:08 +0200
From:   Christian Brauner <christian.brauner@ubuntu.com>
To:     Hannes Reinecke <hare@suse.de>
Cc:     "Eric W. Biederman" <ebiederm@xmission.com>,
        gregkh@linuxfoundation.org, containers@lists.linux.dev,
        linux-kernel@vger.kernel.org, lkml@metux.net
Subject: Re: device namespaces
Message-ID: <20210609072108.ldhsxfnfql4pacqx@wittgenstein>
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On Wed, Jun 09, 2021 at 09:02:36AM +0200, Hannes Reinecke wrote:
> On 6/9/21 8:38 AM, Christian Brauner wrote:
> > On Tue, Jun 08, 2021 at 12:16:43PM -0500, Eric W. Biederman wrote:
> > > Hannes Reinecke <hare@suse.de> writes:
> > > 
> > > > On 6/8/21 4:29 PM, Christian Brauner wrote:
> > > > > On Tue, Jun 08, 2021 at 04:10:08PM +0200, Hannes Reinecke wrote:
> [ .. ]
> > > > Granted, modifying sysfs layout is not something for the faint-hearted,
> > > > and one really has to look closely to ensure you end up with a
> > > > consistent layout afterwards.
> > > > 
> > > > But let's see how things go; might well be that it turns out to be too
> > > > complex to consider. Can't tell yet.
> > > 
> > > I would suggest aiming for something like devptsfs without the
> > > complication of /dev/ptmx.
> > > 
> > > That is a pseudo filesystem that has a control node and virtual block
> > > devices that were created using that control node.
> > 
> > Also see android/binder/binderfs.c
> > 
> Ah. Will have a look.

I implemented this a few years back and I think it should've made it
onto Android by default now. So that approach does indeed work well, it
seems:
https://chromium.googlesource.com/aosp/platform/system/core/+/master/rootdir/init.rc#257

This should be easier to follow than the devpts case because you don't
need to wade through the {t,p}ty layer.

> 
> > > 
> > > That is the cleanest solution I know and is not strictly limited to use
> > > with containers so it can also gain greater traction.  The interaction
> > > with devtmpfs should be simply having devtmpfs create a mount point for
> > > that filesystem.
> > > 
> > > This could be a new cleaner api for things like loopback devices.
> > 
> > I sent a patchset that implemented this last year.
> > 
> Do you have a pointer/commit hash for this?

Yes, sure:
https://lore.kernel.org/linux-block/20200424162052.441452-1-christian.brauner@ubuntu.com/

You can also just pull my branch. I think it's still based on v5.7 or sm:
https://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git/log/?h=loopfs

I'm happy to collaborate on this too.

Christian