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