It's about the debris left behind if the admin (or someone with
delegated authority) moves the task to a wholly different cgroup.
Now we have a cgroup directory in the old cgroup, which the current
task has been removed from, for which the current user has permissions
and could then move the task back to. Is that the essence of the
problem?
That'd be one side. The other side is the one moving. Let's say the
system admin thing wants to move all processe from A proper to B. It
would do that by draining processes from A's procs file into B's and
even that is multistep and can race.
Once freezer is ported, wouldn't that allow you to stop the processes so
you can drain them? I understand your concern with draining, but surely
the same races occur if you fork? How many times would you need to scan
cgroup.procs to make sure that you didn't miss anything (and if there's
enough processes then cgroup.procs reads aren't atomic either).
--
Aleksa Sarai
Software Engineer (Containers)
SUSE Linux GmbH
https://www.cyphar.com/
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