fix dangling zombie when new parent ignores children

This fixes an arcane bug that we think was a regression introduced
by commit b2b2cbc4b2a2f389442549399a993a8306420baf.  When a parent
ignores SIGCHLD (or uses SA_NOCLDWAIT), its children would self-reap
but they don't if it's using ptrace on them.  When the parent thread
later exits and ceases to ptrace a child but leaves other live
threads in the parent's thread group, any zombie children are left
dangling.  The fix makes them self-reap then, as they would have
done earlier if ptrace had not been in use.

Signed-off-by: Roland McGrath <roland@redhat.com>

diff --git a/kernel/exit.c b/kernel/exit.c
index a2af6ca..93d2711 100644 (file)
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -702,6 +702,23 @@ static void exit_mm(struct task_struct * tsk)
        mmput(mm);
 }
 
+/*
+ * Return nonzero if @parent's children should reap themselves.
+ *
+ * Called with write_lock_irq(&tasklist_lock) held.
+ */
+static int ignoring_children(struct task_struct *parent)
+{
+       int ret;
+       struct sighand_struct *psig = parent->sighand;
+       unsigned long flags;
+       spin_lock_irqsave(&psig->siglock, flags);
+       ret = (psig->action[SIGCHLD-1].sa.sa_handler == SIG_IGN ||
+              (psig->action[SIGCHLD-1].sa.sa_flags & SA_NOCLDWAIT));
+       spin_unlock_irqrestore(&psig->siglock, flags);
+       return ret;
+}
+
 /*
  * Detach all tasks we were using ptrace on.
  * Any that need to be release_task'd are put on the @dead list.
@@ -711,6 +728,7 @@ static void exit_mm(struct task_struct * tsk)
 static void ptrace_exit(struct task_struct *parent, struct list_head *dead)
 {
        struct task_struct *p, *n;
+       int ign = -1;
 
        list_for_each_entry_safe(p, n, &parent->ptraced, ptrace_entry) {
                __ptrace_unlink(p);
@@ -726,10 +744,18 @@ static void ptrace_exit(struct task_struct *parent, struct list_head *dead)
                 * release_task() here because we already hold tasklist_lock.
                 *
                 * If it's our own child, there is no notification to do.
+                * But if our normal children self-reap, then this child
+                * was prevented by ptrace and we must reap it now.
                 */
                if (!task_detached(p) && thread_group_empty(p)) {
                        if (!same_thread_group(p->real_parent, parent))
                                do_notify_parent(p, p->exit_signal);
+                       else {
+                               if (ign < 0)
+                                       ign = ignoring_children(parent);
+                               if (ign)
+                                       p->exit_signal = -1;
+                       }
                }
 
                if (task_detached(p)) {