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Subject[PATCH 3/4] reparent_thread: fix a zombie leak if /sbin/init ignores SIGCHLD
If /sbin/init ignores SIGCHLD and we re-parent a zombie, it is leaked.
reparent_thread() does do_notify_parent() which sets ->exit_signal = -1
in this case. This means that nobody except us can reap it, the detached
task is not visible to do_wait().

Change reparent_thread() to return a boolean (like __pthread_detach) to
indicate that the thread is dead and must be released. Also change
forget_original_parent() to add the child to ptrace_dead list in this
case.

The naming becomes insane, the next patch does the cleanup.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>

--- 6.29-rc3/kernel/exit.c~7_FIX_INIT_IGN_CHLD 2009-01-29 06:57:09.000000000 +0100
+++ 6.29-rc3/kernel/exit.c 2009-01-29 08:03:18.000000000 +0100
@@ -804,8 +804,11 @@ static void ptrace_exit_finish(struct ta
}
}

-static void reparent_thread(struct task_struct *p, struct task_struct *father)
+/* Returns nonzero if the child should be released. */
+static int reparent_thread(struct task_struct *p, struct task_struct *father)
{
+ int dead;
+
if (p->pdeath_signal)
/* We already hold the tasklist_lock here. */
group_send_sig_info(p->pdeath_signal, SEND_SIG_NOINFO, p);
@@ -813,12 +816,12 @@ static void reparent_thread(struct task_
list_move_tail(&p->sibling, &p->real_parent->children);

if (task_detached(p))
- return;
+ return 0;
/* If this is a threaded reparent there is no need to
* notify anyone anything has happened.
*/
if (same_thread_group(p->real_parent, father))
- return;
+ return 0;

/* We don't want people slaying init. */
p->exit_signal = SIGCHLD;
@@ -826,11 +829,19 @@ static void reparent_thread(struct task_
/* If we'd notified the old parent about this child's death,
* also notify the new parent.
*/
+ dead = 0;
if (!p->ptrace &&
- p->exit_state == EXIT_ZOMBIE && thread_group_empty(p))
+ p->exit_state == EXIT_ZOMBIE && thread_group_empty(p)) {
do_notify_parent(p, p->exit_signal);
+ if (task_detached(p)) {
+ p->exit_state = EXIT_DEAD;
+ dead = 1;
+ }
+ }

kill_orphaned_pgrp(p, father);
+
+ return dead;
}

/*
@@ -890,7 +901,8 @@ static void forget_original_parent(struc
BUG_ON(p->ptrace);
p->parent = p->real_parent;
}
- reparent_thread(p, father);
+ if (reparent_thread(p, father))
+ list_add(&p->ptrace_entry, &ptrace_dead);;
}

write_unlock_irq(&tasklist_lock);


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