CLONE_PARENT after setns(CLONE_NEWPID)

[Serge Hallyn]

Hi Oleg,

commit 40a0d32d1eaffe6aac7324ca92604b6b3977eb0e :
"fork: unify and tighten up CLONE_NEWUSER/CLONE_NEWPID checks"
breaks lxc-attach in 3.12.  That code forks a child which does
setns() and then does a clone(CLONE_PARENT).  That way the
grandchild can be in the right namespaces (which the child was
not) and be a child of the original task, which is the monitor.

lxc-attach in 3.11 was working fine with no side effects that I
could see.  Is there a real danger in allowing CLONE_PARENT
when current->nsproxy->pidns_for_children is not our pidns,
or was this done out of an "over-abundance of caution"?  Can we
safely revert that new extra check?

thanks,
-serge
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