ptrace_check_attach: remove unneeded ->signal != NULL check

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>
Acked-by: Roland McGrath <roland@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 2 deletions

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 2dc00298e678..fdb34e86f923 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -99,9 +99,12 @@ int ptrace_check_attach(struct task_struct *child, int kill)
          * 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 (task_is_stopped(child))
                         child->state = TASK_TRACED;