kernel: rename is_single_threaded(task) to current_is_single_threaded(void)

- is_single_threaded(task) is not safe unless task == current,
  we can't use task->signal or task->mm.

- it doesn't make sense unless task == current, the task can
  fork right after the check.

Rename it to current_is_single_threaded() and kill the argument.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>

include/linux/sched.h

@@ -2075,7 +2075,7 @@ static inline unsigned long wait_task_inactive(struct task_struct *p,
 #define for_each_process(p) \
 	for (p = &init_task ; (p = next_task(p)) != &init_task ; )
 
-extern bool is_single_threaded(struct task_struct *);
+extern bool current_is_single_threaded(void);
 
 /*
  * Careful: do_each_thread/while_each_thread is a double loop so

lib/is_single_threaded.c

@@ -15,8 +15,9 @@
 /*
  * Returns true if the task does not share ->mm with another thread/process.
  */
-bool is_single_threaded(struct task_struct *task)
+bool current_is_single_threaded(void)
 {
+	struct task_struct *task = current;
 	struct mm_struct *mm = task->mm;
 	struct task_struct *p, *t;
 	bool ret;

security/keys/process_keys.c

@@ -702,7 +702,7 @@ long join_session_keyring(const char *name)
 	/* only permit this if there's a single thread in the thread group -
 	 * this avoids us having to adjust the creds on all threads and risking
 	 * ENOMEM */
-	if (!is_single_threaded(current))
+	if (!current_is_single_threaded())
 		return -EMLINK;
 
 	new = prepare_creds();

security/selinux/hooks.c

@@ -5187,7 +5187,7 @@ static int selinux_setprocattr(struct task_struct *p,
 
 		/* Only allow single threaded processes to change context */
 		error = -EPERM;
-		if (!is_single_threaded(p)) {
+		if (!current_is_single_threaded()) {
 			error = security_bounded_transition(tsec->sid, sid);
 			if (error)
 				goto abort_change;