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SubjectRe: [PATCH 2/4] pid_ns: (BUG 11391) change ->child_reaper when init->group_leader exits
Quoting Oleg Nesterov (oleg@tv-sign.ru):
> We don't change pid_ns->child_reaper when the main thread of the
> subnamespace init exits. As Robert Rex <robert.rex@exasol.com>
> pointed out this is wrong.

I think there was another way that could go wrong as well - if the main
thread exited but sighand had another user, then the child_reaper would
not be changed, zap_pid_ns_processes() would not be called, and children
would continue to run and at death try to signal the dead main thread.

> Yes, the re-parenting itself works correctly, but if the reparented
> task exits it needs ->parent->nsproxy->pid_ns in do_notify_parent(),
> and if the main thread is zombie its ->nsproxy was already cleared
> by exit_task_namespaces().
>
> Introduce the new function, find_new_reaper(), which finds the new
> ->parent for the re-parenting and changes ->child_reaper if needed.
> Kill the now unneeded exit_child_reaper().
>
> Also move the changing of ->child_reaper from zap_pid_ns_processes()
> to find_new_reaper(), this consolidates the games with ->child_reaper
> and makes it stable under tasklist_lock.
>
> Reported-by: Robert Rex <robert.rex@exasol.com>
> Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

Sat on this for a bit - the only thing about this that worried me was
that zap_pid_ns_processes() is moved much further down in do_exit().
After looking I see no reason why that would be a problem, though.

Acked-by: Serge Hallyn <serue@us.ibm.com>

Thanks, Oleg.

>
> kernel/pid_namespace.c | 6 ---
> kernel/exit.c | 78 +++++++++++++++++++++----------------------------
> 2 files changed, 34 insertions(+), 50 deletions(-)
>
> --- 2.6.27-rc4/kernel/pid_namespace.c~2_SWITCH_REAPER 2008-08-24 17:22:59.000000000 +0400
> +++ 2.6.27-rc4/kernel/pid_namespace.c 2008-08-24 18:22:44.000000000 +0400
> @@ -179,12 +179,6 @@ void zap_pid_ns_processes(struct pid_nam
> rc = sys_wait4(-1, NULL, __WALL, NULL);
> } while (rc != -ECHILD);
>
> - /*
> - * 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;
> }
> --- 2.6.27-rc4/kernel/exit.c~2_SWITCH_REAPER 2008-08-24 16:46:51.000000000 +0400
> +++ 2.6.27-rc4/kernel/exit.c 2008-08-24 18:22:37.000000000 +0400
> @@ -831,26 +831,50 @@ static void reparent_thread(struct task_
> * the child reaper process (ie "init") in our pid
> * space.
> */
> +static struct task_struct *find_new_reaper(struct task_struct *father)
> +{
> + struct pid_namespace *pid_ns = task_active_pid_ns(father);
> + struct task_struct *thread;
> +
> + thread = father;
> + while_each_thread(father, thread) {
> + if (thread->flags & PF_EXITING)
> + continue;
> + if (unlikely(pid_ns->child_reaper == father))
> + pid_ns->child_reaper = thread;
> + return thread;
> + }
> +
> + if (unlikely(pid_ns->child_reaper == father)) {
> + write_unlock_irq(&tasklist_lock);
> + if (unlikely(pid_ns == &init_pid_ns))
> + panic("Attempted to kill init!");
> +
> + zap_pid_ns_processes(pid_ns);
> + write_lock_irq(&tasklist_lock);
> + /*
> + * 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;
> + }
> +
> + return pid_ns->child_reaper;
> +}
> +
> static void forget_original_parent(struct task_struct *father)
> {
> - struct task_struct *p, *n, *reaper = father;
> + struct task_struct *p, *n, *reaper;
> LIST_HEAD(ptrace_dead);
>
> write_lock_irq(&tasklist_lock);
> -
> + reaper = find_new_reaper(father);
> /*
> * First clean up ptrace if we were using it.
> */
> ptrace_exit(father, &ptrace_dead);
>
> - do {
> - reaper = next_thread(reaper);
> - if (reaper == father) {
> - reaper = task_child_reaper(father);
> - break;
> - }
> - } while (reaper->flags & PF_EXITING);
> -
> list_for_each_entry_safe(p, n, &father->children, sibling) {
> p->real_parent = reaper;
> if (p->parent == father) {
> @@ -959,39 +983,6 @@ static void check_stack_usage(void)
> static inline void check_stack_usage(void) {}
> #endif
>
> -static inline void exit_child_reaper(struct task_struct *tsk)
> -{
> - if (likely(tsk->group_leader != task_child_reaper(tsk)))
> - return;
> -
> - if (tsk->nsproxy->pid_ns == &init_pid_ns)
> - panic("Attempted to kill init!");
> -
> - /*
> - * @tsk is the last thread in the 'cgroup-init' and is exiting.
> - * Terminate all remaining processes in the namespace and reap them
> - * before exiting @tsk.
> - *
> - * Note that @tsk (last thread of cgroup-init) may not necessarily
> - * be the child-reaper (i.e main thread of cgroup-init) of the
> - * namespace i.e the child_reaper may have already exited.
> - *
> - * Even after a child_reaper exits, we let it inherit orphaned children,
> - * because, pid_ns->child_reaper remains valid as long as there is
> - * at least one living sub-thread in the cgroup init.
> -
> - * This living sub-thread of the cgroup-init will be notified when
> - * a child inherited by the 'child-reaper' exits (do_notify_parent()
> - * uses __group_send_sig_info()). Further, when reaping child processes,
> - * do_wait() iterates over children of all living sub threads.
> -
> - * i.e even though 'child_reaper' thread is listed as the parent of the
> - * orphaned children, any living sub-thread in the cgroup-init can
> - * perform the role of the child_reaper.
> - */
> - zap_pid_ns_processes(tsk->nsproxy->pid_ns);
> -}
> -
> NORET_TYPE void do_exit(long code)
> {
> struct task_struct *tsk = current;
> @@ -1051,7 +1042,6 @@ NORET_TYPE void do_exit(long code)
> }
> group_dead = atomic_dec_and_test(&tsk->signal->live);
> if (group_dead) {
> - exit_child_reaper(tsk);
> hrtimer_cancel(&tsk->signal->real_timer);
> exit_itimers(tsk->signal);
> }


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