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>
Cc: Roland McGrath <roland@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 17 insertions, 5 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index 5be0a406fae..3e09b7cb3b2 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -810,8 +810,11 @@ static void ptrace_exit_finish(struct task_struct *parent,
         }
 }
 
-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);
@@ -819,12 +822,12 @@ static void reparent_thread(struct task_struct *p, struct task_struct *father)
         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;
@@ -832,11 +835,19 @@ static void reparent_thread(struct task_struct *p, struct task_struct *father)
         /* 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;
 }
 
 /*
@@ -896,7 +907,8 @@ static void forget_original_parent(struct task_struct *father)
                         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);