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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     <linux-kernel@vger.kernel.org>
Cc:     Al Viro <viro@zeniv.linux.org.uk>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        Linux API <linux-api@vger.kernel.org>,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        Linux Security Module <linux-security-module@vger.kernel.org>,
        Akinobu Mita <akinobu.mita@gmail.com>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
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        Jonathan Corbet <corbet@lwn.net>,
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        Oleg Nesterov <oleg@redhat.com>,
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Date:   Fri, 28 Feb 2020 16:34:20 -0600
In-Reply-To: <878skmsbyy.fsf_-_@x220.int.ebiederm.org> (Eric W. Biederman's
        message of "Fri, 28 Feb 2020 14:17:41 -0600")
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Subject: [PATCH 4/3] pid: Improve the comment about waiting in zap_pid_ns_processes
Sender: linux-kernel-owner@vger.kernel.org
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Oleg wrote a very informative comment, but with the removal of
proc_cleanup_work it is no longer accurate.

Rewrite the comment so that it only talks about the details
that are still relevant, and hopefully is a little clearer.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
 kernel/pid_namespace.c | 31 +++++++++++++++++++------------
 1 file changed, 19 insertions(+), 12 deletions(-)

diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index 318fcc6ba301..01f8ba32cc0c 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -224,20 +224,27 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns)
 	} while (rc != -ECHILD);
 
 	/*
-	 * kernel_wait4() above can't reap the EXIT_DEAD children but we do not
-	 * really care, we could reparent them to the global init. We could
-	 * exit and reap ->child_reaper even if it is not the last thread in
-	 * this pid_ns, free_pid(pid_allocated == 0) calls proc_cleanup_work(),
-	 * pid_ns can not go away until proc_kill_sb() drops the reference.
+	 * 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().
 	 *
-	 * But this ns can also have other tasks injected by setns()+fork().
-	 * Again, ignoring the user visible semantics we do not really need
-	 * to wait until they are all reaped, but they can be reparented to
-	 * us and thus we need to ensure that pid->child_reaper stays valid
-	 * until they all go away. See free_pid()->wake_up_process().
+	 * 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.
 	 *
-	 * We rely on ignored SIGCHLD, an injected zombie must be autoreaped
-	 * if reparented.
+	 * The code relies on the 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.
+	 *
+	 * Once all of the other tasks are gone from the pid_namespace
+	 * free_pid() will awaken this task.
 	 */
 	for (;;) {
 		set_current_state(TASK_INTERRUPTIBLE);
-- 
2.20.1