[PATCH 1/1] namespaces: introduce sys_hijack (v11)

Serge E. Hallyn serue at us.ibm.com
Fri Aug 1 07:11:53 PDT 2008

Quoting Bastian Blank (bastian at waldi.eu.org):
> On Thu, Jul 31, 2008 at 01:32:13PM -0500, Serge E. Hallyn wrote:
> > Introduce sys_hijack (for i386 and s390 only so far).  An open
> > fd for a cgroup 'tasks' file is specified.  The main purpose
> > is to allow entering an empty cgroup without having to keep a
> > task alive in the target cgroup.
> What is the problem if no task is alive in the target?

Oh, that comment dates back to when I first introduced the
attach-by-ns_cgroup feature.  Before that one had to specify a process
id of an existing task, resulting in hijacking that task.

Eventually, we dropped the hijack_by_pid entirely.

> > The effect is a sort of namespace enter.  The following program
> > uses sys_hijack to 'enter' all namespaces of the specified
> > cgroup.
> I currently fail to see what the differences to a normal cgroup attach
> is.

A normal cgroup attach doesn't switch a task's root and nsproxies.

> >         For instance in one terminal, do
> > 
> > 	mount -t cgroup -ons cgroup /cgroup
> > 	hostname
> > 	  qemu
> > 	ns_exec -u /bin/sh
> > 	  hostname serge
> >           echo $$
> >             2996
> > 	  cat /proc/$$/cgroup
> > 	    ns:/node_2996
> > 
> > In another terminal then do
> > 
> > 	hostname
> > 	  qemu
> > 	cat /proc/$$/cgroup
> > 	  ns:/
> > 	hijack /cgroup/node_2996/tasks
> Why can't this be done by a echo $$ >> /cgroup/node_2996/attach?

Do you mean "why does that current functionality not suffice", or "why
didn't you implement the feature with those semantics"?

Current functionality doesn't suffice because namespaces and
fs_struct are not switched with cgroup attach.  Cgroup attach is
just about tracking tasks, and keeping stats and enforcing limits or
guarantees on the groups.

The problem with implementing this feature using the attach
semantics is that it would move an existing task into the new
cgroup.  That would get much more complicated, especially when
you consider pid namespaces, where we explicitly refuse to
unshare for the same reason.

That is why, with hijack, we clone a new task which is started
afresh in the new namespaces.


> > 	  hostname
> > 	    serge
> > 	  cat /proc/$$/cgroup
> > 	    ns:/node_2996
> Bastian
> -- 
> Star Trek Lives!

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