linux-kernel - Re: [PATCH v6 9/9] devcg: propagate local changes down the hierarchy

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Message-ID: <20130211143241.GF30962@redhat.com>
Date:	Mon, 11 Feb 2013 09:32:42 -0500
From:	Aristeu Rozanski <aris@...hat.com>
To:	"Serge E. Hallyn" <serge@...lyn.com>
Cc:	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
	Tejun Heo <tj@...nel.org>,
	Serge Hallyn <serge.hallyn@...onical.com>
Subject: Re: [PATCH v6 9/9] devcg: propagate local changes down the hierarchy

On Sat, Feb 09, 2013 at 04:04:02AM +0000, Serge E. Hallyn wrote:
> Quoting Aristeu Rozanski (aris@...hat.com):
> > devcg: propagate local changes down the hierarchy
> > 
> > This patch makes all changes propagate down in hierarchy respecting when
> > possible local configurations.
> > 
> > Behavior changes will clean up exceptions in all the children except when the
> > parent changes the behavior from allow to deny and the child's behavior was
> > already deny, in which case the local exceptions will be reused. The inverse
> > is not possible: you can't have a parent with behavior deny and a child with
> > behavior accept.
> > 
> > New exceptions allowing additional access to devices won't be propagated, but
> > it'll be possible to add an exception to access all of part of the newly
> > allowed device(s).
> > 
> > New exceptions disallowing access to devices will be propagated down and the
> > local group's exceptions will be revalidated for the new situation.
> > Example:
> >       A
> >      / \
> >         B
> > 
> >     group        behavior          exceptions
> >     A            allow             "b 8:* rwm", "c 116:1 rw"
> >     B            deny              "c 1:3 rwm", "c 116:2 rwm", "b 3:* rwm"
> > 
> > If a new exception is added to group A:
> > 	# echo "c 116:* r" > A/devices.deny
> > it'll propagate down and after revalidating B's local exceptions, the exception
> > "c 116:2 rwm" will be removed.
> > 
> > In case parent behavior or exceptions change and local settings are not
> > allowed anymore, they'll be deleted.
> 
> Do you have a use case which would be broken if we simply refuse to
> allow behavior changes for any cgroup with children?
> 
> It seems like that would drastically simplify much of this.  We would
> no longer need local.exceptions at all, right?  Your comment says
> 
>          * local set rules, saved so when a parent propagates new rules, the
>          * local preferences can be preserved
> 
> but if there were no parent behavior changes, then any exception change
> in a parent could be enforced by simply removing violating exceptions
> in the child, and subsequently refusing the addition of new rules in the
> child which are not allowed in the parent.  Both of which you already do.
> 
> Or am I thinking wrongly?

That would be an option even simpler than not keeping local settings. In
production I doubt the sysadmin will keep playing with permissions,
although until one gets right, it'll be annoying as hell to have to
remove the whole hierarchy because you forgot to add a certain device to
the list.

-- 
Aristeu

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