Re: [PATCH v6 9/9] devcg: propagate local changes down the hierarchy
From: Aristeu Rozanski
Date: Mon Feb 11 2013 - 09:32:51 EST
On Sat, Feb 09, 2013 at 04:04:02AM +0000, Serge E. Hallyn wrote:
> Quoting Aristeu Rozanski (aris@xxxxxxxxxx):
> > 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
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