Re: [RFC] How to handle the rules engine for cgroups

["Paul Menage" <menage@google.com>]

Hi Vivek,

On Tue, Jul 1, 2008 at 12:11 PM, Vivek Goyal <vgoyal@redhat.com> wrote:
>
> - netlink is not a reliable protocol.
>        - Messages can be dropped and one can loose message. That means a
>          newly forked process might never go into right group as meant.

One way that you could avoid the unreliability would be to not use
netlink, but instead use cgroups itself.

What we're looking for is a way to easily distinguish between
processes that are in the right cgroups, and processes that might be
in the wrong cgroups. Additionally, we want the children of such
processes to inherit the same status until we've dealt with them, and
not be able to change their status themselves.

That sounds a bit like a cgroup. How about the following?

- create a cgroup subsystem called "setuid".

- have a uid_changed() hook called by sys_setuid() and friends; this
hook would simply attach current to the root cgroup in the "setuid"
hierarchy if it wasn't already in that cgroup (which can be determined
with a couple of dereferences from current and no locking, so not
slowing down the normal case).

- userspace uses this by:

mount the setuid hierarchy, e.g. at /mnt/setuid
create a child cgroup /mnt/setuid/processed
while true:
  wait for /mnt/setuid/tasks to be non-empty
  read a pid from /mnt/setuid/tasks
  move that pid to the appropriate cgroups in memory/cpu/etc
hierarchies if necessary
  move that pid to /mnt/setuid/processed/tasks

i.e. any pid in the root cgroup of the setuid hierarchy is one that
needs attention and may need to be moved to different cgroups

A couple of enhancements to make this more usable might include:

- adding an API (via a new syscall or an eventfd?) to wait for a
cgroup to be non-empty, to avoid having to poll /mnt/setuid/tasks more
than necessary

- allow the user to designate certain processes and their children as
uninteresting so that their setuid calls don't trigger them being
moved back to the root (perhaps indicated via membership of an
"ignored" cgroup in the setuid hierarchy?)

This should be more reliable than netlink since it doesn't involve
userspace having to keep up with a stream of events - we're not
queuing up events, we're just shifting process group memberships.

Similar approaches could be used for a "setgid" hierarchy and an
"execve" hierarchy.

Paul