Re: [Workman-devel] cgroup: status-quo and userland efforts

[Serge Hallyn]

Quoting Daniel P. Berrange (berrange@xxxxxxxxxx):
> On Fri, Jun 28, 2013 at 02:01:55PM -0400, Vivek Goyal wrote:
> > On Fri, Jun 28, 2013 at 05:05:13PM +0200, Michal Hocko wrote:
> > > On Thu 27-06-13 22:01:38, Tejun Heo wrote:
> > > > Hello, Mike.
> > > > 
> > > > On Fri, Jun 28, 2013 at 06:49:10AM +0200, Mike Galbraith wrote:
> > > > > I always thought that was a very cool feature, mkdir+echo, poof done.
> > > > > Now maybe that interface is suboptimal for serious usage, but it makes
> > > > > the things usable via dirt simple scripts, very flexible, nice.
> > > > 
> > > > Oh, that in itself is not bad.  I mean, if you're root, it's pretty
> > > > easy to play with and that part is fine.  But combined with the
> > > > hierarchical nature of cgroup and file permissions, it encourages
> > > > people to "deligate" subdirectories to less previledged domains,
> > > 
> > > OK, this really depends on what you expose to non-root users. I have
> > > seen use cases where admin prepares top-level which is root-only but
> > > it allows creating sub-groups which are under _full_ control of the
> > > subdomain. This worked nicely for memcg for example because hard limit,
> > > oom handling and other knobs are hierarchical so the subdomain cannot
> > > overwrite what admin has said.
> > > 
> > > > which
> > > > in turn leads to normal binaries to manipulate them directly, which is
> > > > where the horror begins.  We end up exposing control knobs which are
> > > > tightly coupled to kernel implementation details right into lay
> > > > binaries and scripts directly used by end users.
> > > >
> > > > I think this is the first time this happened, which is probably why
> > > > nobody really noticed the mess earlier.
> > > > 
> > > > Anyways, if you're root, you can keep doing whatever you want.
> > > 
> > > OK, so libcgroup's rules daemon will still work and place my tasks in
> > > appropriate cgroups?
> > 
> > Do you use that daemon in practice? For user session logins, I think
> > systemd has plans to put user sessions in a cgroup (kind of making
> > pam_cgroup redundant). 
> > 
> > Other functionality rulesengined was providing moving tasks automatically
> > in a cgroup based on executable name. I think that was racy and not
> > many people had liked it.
> 
> Regardless of the changes being proposed, IMHO, the cgrulesd should
> never be used. It is just outright dangerous for a daemon to be
> arbitrarily re-arranging what cgroups a process is placed in without
> the applications being aware of it. It can only be safely used in a
> scenario where cgroups are exclusively used by the administrator,
> and never used by applications for their own needs.

Even then it's not safe, since if the program quickly forks or clones a
few times, you can end up with some of the tasks being reclassified
and some not.

> > IIUC, systemd can't disable access to cgroupfs from other utilities.
> 
> The kernel can exposed a knob that would allow systemd to lock that
> down

Gah - why would you give him that idea?  :)

But yes, I'd sort of assume that was coming, eventually.

-serge
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