From: Oleg Nesterov <oleg@tv-sign.ru>

It is not possible to see the PT_PTRACED task without ->signal/sighand under
tasklist_lock, release_task() does ptrace_unlink() first.  If the task was
already released before, ptrace_attach() can't succeed and set PT_PTRACED. 
Remove this check.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 kernel/ptrace.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff -puN kernel/ptrace.c~ptrace_check_attach-remove-unneeded-signal-=-null-check kernel/ptrace.c
--- a/kernel/ptrace.c~ptrace_check_attach-remove-unneeded-signal-=-null-check
+++ a/kernel/ptrace.c
@@ -99,9 +99,12 @@ int ptrace_check_attach(struct task_stru
 	 * be changed by us so it's not changing right after this.
 	 */
 	read_lock(&tasklist_lock);
-	if ((child->ptrace & PT_PTRACED) && child->parent == current &&
-	     child->signal != NULL) {
+	if ((child->ptrace & PT_PTRACED) && child->parent == current) {
 		ret = 0;
+		/*
+		 * child->sighand can't be NULL, release_task()
+		 * does ptrace_unlink() before __exit_signal().
+		 */
 		spin_lock_irq(&child->sighand->siglock);
 		if (is_task_stopped(child))
 			__set_task_state(child, TASK_TRACED);
_