Re: [PATCH 3/3] rcu-tasks: Fix synchronize_rcu_tasks() VS zap_pid_ns_processes()

[Frederic Weisbecker]

On Tue, Dec 06, 2022 at 05:49:28PM +0100, Oleg Nesterov wrote:
> On 11/30, Eric W. Biederman wrote:
> >
> > 2) I keep thinking zap_pid_ns_processes() should be changed so that
> >    after it sends SIGKILL to all of the relevant processes to not wait,
> 
> At least I think it should not wait for the tasks injected into this ns.
> 
> Because this looks like a kernel bug even if we forget about this deadlock.
> 
> Say we create a task P using clone(CLONE_NEWPID), then inject a task T into
> P's pid-namespace via setns/fork. This make the process P "unkillable", it
> will hang in zap_pid_ns_processes() "forever" until T->parent reaps a zombie
> task T killed by P.

I think this was made that way on purpose, see the comment in
zap_pid_ns_processes():

	/*
	 * kernel_wait4() misses EXIT_DEAD children, and EXIT_ZOMBIE
	 * process whose parents processes are outside of the pid
	 * namespace.  Such processes are created with setns()+fork().
	 *
	 * If those EXIT_ZOMBIE processes are not reaped by their
	 * parents before their parents exit, they will be reparented
	 * to pid_ns->child_reaper.  Thus pidns->child_reaper needs to
	 * stay valid until they all go away.
	 *
	 * The code relies on the pid_ns->child_reaper ignoring
	 * SIGCHILD to cause those EXIT_ZOMBIE processes to be
	 * autoreaped if reparented.
	 *
	 * Semantically it is also desirable to wait for EXIT_ZOMBIE
	 * processes before allowing the child_reaper to be reaped, as
	 * that gives the invariant that when the init process of a
	 * pid namespace is reaped all of the processes in the pid
	 * namespace are gone.

I can't say I like the fact that a parent not belonging to a new namespace
can create more than one child within that namespace but anyway this all look
like an ABI that can't be reverted now.

Thanks.