signals: move cred_guard_mutex from task_struct to signal_struct

Oleg Nesterov pointed out we have to prevent multiple-threads-inside-exec
itself and we can reuse ->cred_guard_mutex for it.  Yes, concurrent
execve() has no worth.

Let's move ->cred_guard_mutex from task_struct to signal_struct.  It
naturally prevent multiple-threads-inside-exec.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Roland McGrath <roland@redhat.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/cred.c

@@ -325,7 +325,7 @@ EXPORT_SYMBOL(prepare_creds);
 
 /*
  * Prepare credentials for current to perform an execve()
- * - The caller must hold current->cred_guard_mutex
+ * - The caller must hold ->cred_guard_mutex
  */
 struct cred *prepare_exec_creds(void)
 {
@@ -384,8 +384,6 @@ int copy_creds(struct task_struct *p, unsigned long clone_flags)
 	struct cred *new;
 	int ret;
 
-	mutex_init(&p->cred_guard_mutex);
-
 	if (
 #ifdef CONFIG_KEYS
 		!p->cred->thread_keyring &&

kernel/fork.c

@@ -908,6 +908,8 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
 	sig->oom_adj = current->signal->oom_adj;
 	sig->oom_score_adj = current->signal->oom_score_adj;
 
+	mutex_init(&sig->cred_guard_mutex);
+
 	return 0;
 }
 

kernel/ptrace.c

@@ -181,7 +181,7 @@ int ptrace_attach(struct task_struct *task)
 	 * under ptrace.
 	 */
 	retval = -ERESTARTNOINTR;
-	if (mutex_lock_interruptible(&task->cred_guard_mutex))
+	if (mutex_lock_interruptible(&task->signal->cred_guard_mutex))
 		goto out;
 
 	task_lock(task);
@@ -208,7 +208,7 @@ int ptrace_attach(struct task_struct *task)
 unlock_tasklist:
 	write_unlock_irq(&tasklist_lock);
 unlock_creds:
-	mutex_unlock(&task->cred_guard_mutex);
+	mutex_unlock(&task->signal->cred_guard_mutex);
 out:
 	return retval;
 }