Re: [PATCH 1/4] pid_ns: zap_pid_ns_processes: fix the ->child_reaper changing

[sukadev@us.ibm.com]

Oleg Nesterov [oleg@tv-sign.ru] wrote:
| zap_pid_ns_processes() sets pid_ns->child_reaper = NULL, this is wrong.
| 
| Yes, we have already killed all tasks in this namespace, and sys_wait4()
| doesn't see any child. But this doesn't mean ->children list is empty,
| we may have EXIT_DEAD tasks which are not visible to do_wait(). In that
| case the subsequent forget_original_parent() will crash the kernel because
| it will try to re-parent these tasks to the NULL reaper.
| 
| Even if there are no childs, it is not good that forget_original_parent()
| uses reaper == NULL.
| 
| Change the code to set ->child_reaper = init_pid_ns.child_reaper instead.
| We could use pid_ns->parent->child_reaper as well, I think this does not
| really matter. These EXIT_DEAD tasks are not visible to the new ->parent
| after re-parenting, they will silently do release_task() eventually.
| 
| Note that we must change ->child_reaper, otherwise forget_original_parent()
| will use reaper == father, and in that case we will hit the (correct)
| BUG_ON(!list_empty(&father->children)).
| 
| Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

Acked-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>

| 
| --- 2.6.27-rc4/kernel/pid_namespace.c~1_ZAP_DONT_CLEAR_REAPER	2008-07-30 13:12:49.000000000 +0400
| +++ 2.6.27-rc4/kernel/pid_namespace.c	2008-08-24 17:22:59.000000000 +0400
| @@ -179,9 +179,12 @@ void zap_pid_ns_processes(struct pid_nam
|  		rc = sys_wait4(-1, NULL, __WALL, NULL);
|  	} while (rc != -ECHILD);
| 
| -
| -	/* Child reaper for the pid namespace is going away */
| -	pid_ns->child_reaper = NULL;
| +	/*
| +	 * We can not clear ->child_reaper or leave it alone.
| +	 * There may by stealth EXIT_DEAD tasks on ->children,
| +	 * forget_original_parent() must move them somewhere.
| +	 */
| +	pid_ns->child_reaper = init_pid_ns.child_reaper;
|  	acct_exit_ns(pid_ns);
|  	return;
|  }
| 
| --
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