1 files changed, 39 insertions, 20 deletions

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index e609843..4348586 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -41,7 +41,26 @@ void __ptrace_link(struct task_struct *child, struct task_struct *new_parent)
  * __ptrace_unlink - unlink ptracee and restore its execution state
  * @child: ptracee to be unlinked
  *
- * Remove @child from the ptrace list, move it back to the original parent.
+ * Remove @child from the ptrace list, move it back to the original parent,
+ * and restore the execution state so that it conforms to the group stop
+ * state.
+ *
+ * Unlinking can happen via two paths - explicit PTRACE_DETACH or ptracer
+ * exiting.  For PTRACE_DETACH, unless the ptracee has been killed between
+ * ptrace_check_attach() and here, it's guaranteed to be in TASK_TRACED.
+ * If the ptracer is exiting, the ptracee can be in any state.
+ *
+ * After detach, the ptracee should be in a state which conforms to the
+ * group stop.  If the group is stopped or in the process of stopping, the
+ * ptracee should be put into TASK_STOPPED; otherwise, it should be woken
+ * up from TASK_TRACED.
+ *
+ * If the ptracee is in TASK_TRACED and needs to be moved to TASK_STOPPED,
+ * it goes through TRACED -> RUNNING -> STOPPED transition which is similar
+ * to but in the opposite direction of what happens while attaching to a
+ * stopped task.  However, in this direction, the intermediate RUNNING
+ * state is not hidden even from the current ptracer and if it immediately
+ * re-attaches and performs a WNOHANG wait(2), it may fail.
  *
  * CONTEXT:
  * write_lock_irq(tasklist_lock)
@@ -55,25 +74,25 @@ void __ptrace_unlink(struct task_struct *child)
 	list_del_init(&child->ptrace_entry);
 
 	spin_lock(&child->sighand->siglock);
-	if (task_is_traced(child)) {
-		/*
-		 * If group stop is completed or in progress, it should
-		 * participate in the group stop.  Set GROUP_STOP_PENDING
-		 * before kicking it.
-		 *
-		 * This involves TRACED -> RUNNING -> STOPPED transition
-		 * which is similar to but in the opposite direction of
-		 * what happens while attaching to a stopped task.
-		 * However, in this direction, the intermediate RUNNING
-		 * state is not hidden even from the current ptracer and if
-		 * it immediately re-attaches and performs a WNOHANG
-		 * wait(2), it may fail.
-		 */
-		if (child->signal->flags & SIGNAL_STOP_STOPPED ||
-		    child->signal->group_stop_count)
-			child->group_stop |= GROUP_STOP_PENDING;
-		signal_wake_up(child, 1);
-	}
+
+	/*
+	 * Reinstate GROUP_STOP_PENDING if group stop is in effect and
+	 * @child isn't dead.
+	 */
+	if (!(child->flags & PF_EXITING) &&
+	    (child->signal->flags & SIGNAL_STOP_STOPPED ||
+	     child->signal->group_stop_count))
+		child->group_stop |= GROUP_STOP_PENDING;
+
+	/*
+	 * If transition to TASK_STOPPED is pending or in TASK_TRACED, kick
+	 * @child in the butt.  Note that @resume should be used iff @child
+	 * is in TASK_TRACED; otherwise, we might unduly disrupt
+	 * TASK_KILLABLE sleeps.
+	 */
+	if (child->group_stop & GROUP_STOP_PENDING || task_is_traced(child))
+		signal_wake_up(child, task_is_traced(child));
+
 	spin_unlock(&child->sighand->siglock);
 }