plug pre-created tap devices to libvirt guests

Tue Jun 30 11:02:01 UTC 2020

On Tue, Jun 30, 2020 at 12:59 PM Miguel Duarte de Mora Barroso
<mdbarroso at redhat.com> wrote:
>
> On Mon, Apr 6, 2020 at 4:03 PM Laine Stump <lstump at redhat.com> wrote:
> >
> > On 4/6/20 9:54 AM, Daniel P. Berrangé wrote:
> > > On Mon, Apr 06, 2020 at 03:47:01PM +0200, Miguel Duarte de Mora Barroso wrote:
> > >> Hi all,
> > >>
> > >> I'm aware that it is possible to plug pre-created macvtap devices to
> > >> libvirt guests - tracked in RFE [0].
> > >>
> > >> My interpretation of the wording in [1] and [2] is that it is also
> > >> possible to plug pre-created tap devices into libvirt guests - that
> > >> would be a requirement to allow kubevirt to run with less capabilities
> > >> in the pods that encapsulate the VMs.
> > >>
> > >> I took a look at the libvirt code ([3] & [4]), and, from my limited
> > >> understanding, I got the impression that plugging existing interfaces
> > >> via `managed='no' ` is only possible for macvtap interfaces.
> >
> >
> > No, it works for standard tap devices as well.
> >
> >
> > The reason the BZs and commit logs talk mostly about macvtap rather than
> > tap is because 1) that's what kubevirt people had asked for and 2) it
> > already *mostly* worked for tap devices, so most of the work was related
> > to macvtap (my memory is already fuzzy, but I think there were a couple
> > privileged operations we still tried to do for standard tap devices even
> > if they were precreated (standard disclaimer: I often misremember, so
> > this memory could be wrong! But definitely precreated tap devices do work).
> >
>
> It's been a while since I've started this thread, but lately I've
> understood better how tap devices work, and that new insight makes me
> wonder about a couple of things.
>
> Our ultimate goal In kubevirt is to consume a pre-created tap device
> by a kubernetes pod that doesn't have the NET_ADMIN capability.
>
> After looking at the current libvirt code, I don't think that is
> currently supported, since we'll *always* enter the
> `virNetDevTapCreate` function in [1] (I'm interested in the *tap*
> scenario).
>
> The tap device is effectively created in that function - [2] - by
> opening the clone device (/dev/net/tun), and calling `ioctl(fd,
> TUNSETIFF,...)` in it. AFAIK, both of those operations *require* the
> NET_ADMIN capability. If I'm correct, this means that the current
> libvirt implementation makes our goals impossible to achieve.
>
> I'd first like to know if I read the code correctly, and if I'm
> ultimately right - i.e. since libvirt's implementation for consuming a
> pre-existent tap device opens the clone device, where ever libvirt is
> run on will  *always* require the NET_ADMIN capability.

Adding the links that I've forgotten to add ...

[0] - https://bugzilla.redhat.com/show_bug.cgi?id=1723367
[1] - https://github.com/libvirt/libvirt/blob/master/src/qemu/qemu_interface.c#L443
[2] - https://github.com/libvirt/libvirt/blob/master/src/util/virnetdevtap.c#L243

>
>
> >
> > I think though that when someone from kubevirt actually tried using a
> > precreated macvtap device, they found that their precreated device
> > wasn't visible at all to the unprivileged libvirtd in the pod, because
> > it was in a different network namespace, or something like that. So
> > there may still be more work to do (or, again, my info might be out of
> > date and they figured out a proper solution).
> >
> >
> > >>
> > >> Would you be able to shed some light into this ? Is it possible on
> > >> libvirt-5.6.0 to plug pre-created tap devices to libvirt guests ?
> > >>
> > >> [0] - https://bugzilla.redhat.com/show_bug.cgi?id=1723367
> > > This links to the following message, which illustrates how to use pre-create
> > > tap and macvtap devices:
> > >
> > >    https://www.redhat.com/archives/libvir-list/2019-August/msg01256.html
> > >
> > > Laine: it would be useful to add something like this short guide to the
> > > knowledge base docs
> >
> >
> > You mean the wiki? Sure, I can do that.
> >
> >
> > (BTW - that was admirable reading / searching / responding - 7 minutes
> > and it wasn't even your patch! How do you do that? :-))
> >
> >
>
> On Mon, Apr 6, 2020 at 4:03 PM Laine Stump <lstump at redhat.com> wrote:
> >
> > On 4/6/20 9:54 AM, Daniel P. Berrangé wrote:
> > > On Mon, Apr 06, 2020 at 03:47:01PM +0200, Miguel Duarte de Mora Barroso wrote:
> > >> Hi all,
> > >>
> > >> I'm aware that it is possible to plug pre-created macvtap devices to
> > >> libvirt guests - tracked in RFE [0].
> > >>
> > >> My interpretation of the wording in [1] and [2] is that it is also
> > >> possible to plug pre-created tap devices into libvirt guests - that
> > >> would be a requirement to allow kubevirt to run with less capabilities
> > >> in the pods that encapsulate the VMs.
> > >>
> > >> I took a look at the libvirt code ([3] & [4]), and, from my limited
> > >> understanding, I got the impression that plugging existing interfaces
> > >> via `managed='no' ` is only possible for macvtap interfaces.
> >
> >
> > No, it works for standard tap devices as well.
> >
> >
> > The reason the BZs and commit logs talk mostly about macvtap rather than
> > tap is because 1) that's what kubevirt people had asked for and 2) it
> > already *mostly* worked for tap devices, so most of the work was related
> > to macvtap (my memory is already fuzzy, but I think there were a couple
> > privileged operations we still tried to do for standard tap devices even
> > if they were precreated (standard disclaimer: I often misremember, so
> > this memory could be wrong! But definitely precreated tap devices do work).
> >
> >
> > I think though that when someone from kubevirt actually tried using a
> > precreated macvtap device, they found that their precreated device
> > wasn't visible at all to the unprivileged libvirtd in the pod, because
> > it was in a different network namespace, or something like that. So
> > there may still be more work to do (or, again, my info might be out of
> > date and they figured out a proper solution).
> >
> >
> > >>
> > >> Would you be able to shed some light into this ? Is it possible on
> > >> libvirt-5.6.0 to plug pre-created tap devices to libvirt guests ?
> > >>
> > >> [0] - https://bugzilla.redhat.com/show_bug.cgi?id=1723367
> > > This links to the following message, which illustrates how to use pre-create
> > > tap and macvtap devices:
> > >
> > >    https://www.redhat.com/archives/libvir-list/2019-August/msg01256.html
> > >
> > > Laine: it would be useful to add something like this short guide to the
> > > knowledge base docs
> >
> >
> > You mean the wiki? Sure, I can do that.
> >
> >
> > (BTW - that was admirable reading / searching / responding - 7 minutes
> > and it wasn't even your patch! How do you do that? :-))
> >
> >