udev in containers

Eric W. Biederman ebiederm at xmission.com
Fri Jan 28 12:18:47 PST 2011

"Serge E. Hallyn" <serge.hallyn at canonical.com> writes:

> Hi,
> Now that we are allowing udev to run in containers, Daniel has
> noticed that updates to sysfs uevent files will trigger a flurry
> of activity in all containers on the host.  While not a problem
> with just a few containers, this can severaly impact performance
> with hundreds or more containers.
> (Daniel, would it be possible for you to get some measurements
> on host and in a container versus # of active containers, with
> and without udev?  Do you have a otehrwise unused machien you
> could try that on?)
> Is there anything we can/should do about this?
> Two approaches, neither sufficiently thought out yet, would be
> to generalize the directory tagging currently used for
> /sys/class/net, and full-fledged implementation of a device
> namespace.
> The directory tagging would probably only work if we can assign
> multiple tags to a device, but we could for instance make
> /sys/block tagged, and really no container probably needs to see
> /sys/block/sda.
> The device namespace would be similar, except I suspect it
> would not only hide certain devices from certain namespaces,
> but it would actually virtualize the device major:minor
> mapping, for checkpoint/restart, so that /dev/sda could be
> redirected to another device more completely than simply
> fudging the nodes under /dev.
> Comments?  Designs?  Plans?

To answer you earlier question: What did I expect the device namespace
to look like.

- Only purely virtual devices like  /dev/pts, /dev/null, /dev/nbd and /dev/loop0 present.
- Fully virtualized major/minor look up preventing us from even talking
  about devices in other namespaces.
- Support from the user/security namespace so that mknod and mount are safe.

I get a certain uncomfortable feeling about mknod and mount running free
in a container without restrictions that make container without restrictions...


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