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Date:   Tue, 21 Jan 2020 15:43:35 +0000
From:   Stefan Hajnoczi <stefanha@redhat.com>
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Stefano Garzarella <sgarzare@redhat.com>,
        David Miller <davem@davemloft.net>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, Jorgen Hansen <jhansen@vmware.com>,
        Jason Wang <jasowang@redhat.com>, kvm <kvm@vger.kernel.org>,
        virtualization@lists.linux-foundation.org,
        linux-hyperv@vger.kernel.org, Dexuan Cui <decui@microsoft.com>
Subject: Re: [PATCH net-next 1/3] vsock: add network namespace support
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On Tue, Jan 21, 2020 at 06:14:48AM -0500, Michael S. Tsirkin wrote:
> On Tue, Jan 21, 2020 at 10:07:06AM +0100, Stefano Garzarella wrote:
> > On Mon, Jan 20, 2020 at 11:02 PM Michael S. Tsirkin <mst@redhat.com> wr=
ote:
> > > On Mon, Jan 20, 2020 at 05:53:39PM +0100, Stefano Garzarella wrote:
> > > > On Mon, Jan 20, 2020 at 5:04 PM Michael S. Tsirkin <mst@redhat.com>=
 wrote:
> > > > > On Mon, Jan 20, 2020 at 02:58:01PM +0100, Stefano Garzarella wrot=
e:
> > > > > > On Mon, Jan 20, 2020 at 1:03 PM Michael S. Tsirkin <mst@redhat.=
com> wrote:
> > > > > > > On Mon, Jan 20, 2020 at 11:17:35AM +0100, Stefano Garzarella =
wrote:
> > > > > > > > On Mon, Jan 20, 2020 at 10:06:10AM +0100, David Miller wrot=
e:
> > > > > > > > > From: Stefano Garzarella <sgarzare@redhat.com>
> > > > > > > > > Date: Thu, 16 Jan 2020 18:24:26 +0100
> > > > > > > > >
> > > > > > > > > > This patch adds 'netns' module param to enable this new=
 feature
> > > > > > > > > > (disabled by default), because it changes vsock's behav=
ior with
> > > > > > > > > > network namespaces and could break existing application=
s.
> > > > > > > > >
> > > > > > > > > Sorry, no.
> > > > > > > > >
> > > > > > > > > I wonder if you can even design a legitimate, reasonable,=
 use case
> > > > > > > > > where these netns changes could break things.
> > > > > > > >
> > > > > > > > I forgot to mention the use case.
> > > > > > > > I tried the RFC with Kata containers and we found that Kata=
 shim-v1
> > > > > > > > doesn't work (Kata shim-v2 works as is) because there are t=
he following
> > > > > > > > processes involved:
> > > > > > > > - kata-runtime (runs in the init_netns) opens /dev/vhost-vs=
ock and
> > > > > > > >   passes it to qemu
> > > > > > > > - kata-shim (runs in a container) wants to talk with the gu=
est but the
> > > > > > > >   vsock device is assigned to the init_netns and kata-shim =
runs in a
> > > > > > > >   different netns, so the communication is not allowed
> > > > > > > > But, as you said, this could be a wrong design, indeed they=
 already
> > > > > > > > found a fix, but I was not sure if others could have the sa=
me issue.
> > > > > > > >
> > > > > > > > In this case, do you think it is acceptable to make this ch=
ange in
> > > > > > > > the vsock's behavior with netns and ask the user to change =
the design?
> > > > > > >
> > > > > > > David's question is what would be a usecase that's broken
> > > > > > > (as opposed to fixed) by enabling this by default.
> > > > > >
> > > > > > Yes, I got that. Thanks for clarifying.
> > > > > > I just reported a broken example that can be fixed with a diffe=
rent
> > > > > > design (due to the fact that before this series, vsock devices =
were
> > > > > > accessible to all netns).
> > > > > >
> > > > > > >
> > > > > > > If it does exist, you need a way for userspace to opt-in,
> > > > > > > module parameter isn't that.
> > > > > >
> > > > > > Okay, but I honestly can't find a case that can't be solved.
> > > > > > So I don't know whether to add an option (ioctl, sysfs ?) or wa=
it for
> > > > > > a real case to come up.
> > > > > >
> > > > > > I'll try to see better if there's any particular case where we =
need
> > > > > > to disable netns in vsock.
> > > > > >
> > > > > > Thanks,
> > > > > > Stefano
> > > > >
> > > > > Me neither. so what did you have in mind when you wrote:
> > > > > "could break existing applications"?
> > > >
> > > > I had in mind:
> > > > 1. the Kata case. It is fixable (the fix is not merged on kata), bu=
t
> > > >    older versions will not work with newer Linux.
> > >
> > > meaning they will keep not working, right?
> >=20
> > Right, I mean without this series they work, with this series they work
> > only if the netns support is disabled or with a patch proposed but not
> > merged in kata.
> >=20
> > >
> > > > 2. a single process running on init_netns that wants to communicate=
 with
> > > >    VMs handled by VMMs running in different netns, but this case ca=
n be
> > > >    solved opening the /dev/vhost-vsock in the same netns of the pro=
cess
> > > >    that wants to communicate with the VMs (init_netns in this case)=
, and
> > > >    passig it to the VMM.
> > >
> > > again right now they just don't work, right?
> >=20
> > Right, as above.
> >=20
> > What do you recommend I do?
> >=20
> > Thanks,
> > Stefano
>=20
> If this breaks userspace, then we need to maintain compatibility.
> For example, have two devices, /dev/vhost-vsock and /dev/vhost-vsock-netn=
s?

/dev/vhost-vsock-netns is cleaner and simpler than my suggestion.  I
like it!

This is nice for containers (say you want to run QEMU inside a container
on the host) because you can allow only /dev/vhost-vsock-netns inside
containers.  This prevents them from opening /dev/vhost-vsock to get
access to the initial network namespace.

Stefan

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