ptrace: Fix ptrace when task is in task_is_stopped() state

This patch fixes a regression in ptrace, introduced by commit 9e74eb39
(backport of 9899d11f) which makes assumptions about ptrace behavior
which are not true in the 2.6.32 kernel.

BugLink: http://bugs.launchpad.net/bugs/1145234

9899d11f makes the assumption that task_is_stopped() is not a valid state
in ptrace because it is built on top of a series of patches which change
how the TASK_STOPPED state is tracked (321fb561 which requires d79fdd6d
and several other patches).

Because Lucid does not have the set of patches that make task_is_stopped()
an invalid state in ptrace_check_attach, partially revert 9e74eb39 so
that ptrace_check_attach() correctly handles task_is_stopped(). However
we must replace the assignment of TASK_TRACED with __TASK_TRACED to
ensure TASK_WAKEKILL is cleared.
Signed-off-by: John Johansen <john.johansen@canonical.com>
Acked-by: Colin King <colin.king@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Luis Henriques <luis.henriques@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>

kernel/ptrace.c

@@ -81,14 +81,18 @@ void __ptrace_unlink(struct task_struct *child)
 }
 
 /* Ensure that nothing can wake it up, even SIGKILL */
-static bool ptrace_freeze_traced(struct task_struct *task)
+static bool ptrace_freeze_traced(struct task_struct *task, int kill)
 {
-	bool ret = false;
+	bool ret = true;
 
 	spin_lock_irq(&task->sighand->siglock);
-	if (task_is_traced(task) && !__fatal_signal_pending(task)) {
+	if (task_is_stopped(task) && !__fatal_signal_pending(task))
 		task->state = __TASK_TRACED;
-		ret = true;
+	else if (!kill) {
+		if (task_is_traced(task) && !__fatal_signal_pending(task))
+			task->state = __TASK_TRACED;
+		else
+			ret = false;
 	}
 	spin_unlock_irq(&task->sighand->siglock);
 
@@ -131,7 +135,7 @@ int ptrace_check_attach(struct task_struct *child, int kill)
 		 * child->sighand can't be NULL, release_task()
 		 * does ptrace_unlink() before __exit_signal().
 		 */
-		if (kill || ptrace_freeze_traced(child))
+		if (ptrace_freeze_traced(child, kill))
 			ret = 0;
 	}
 	read_unlock(&tasklist_lock);