[PATCH] simplify/fix first_tid()

first_tid:

	/* If nr exceeds the number of threads there is nothing todo */
	if (nr) {
		if (nr >= get_nr_threads(leader))
			goto done;
	}

This is not reliable: sub-threads can exit after this check, so the
'for' loop below can overlap and proc_task_readdir() can return an
already filldir'ed dirents.

	for (; pos && pid_alive(pos); pos = next_thread(pos)) {
		if (--nr > 0)
			continue;

Off-by-one error, will return 'leader' when nr == 1.

This patch tries to fix these problems and simplify the code.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/proc/base.c

@@ -2227,38 +2227,34 @@ int proc_pid_readdir(struct file * filp, void * dirent, filldir_t filldir)
 static struct task_struct *first_tid(struct task_struct *leader,
 					int tid, int nr)
 {
-	struct task_struct *pos = NULL;
+	struct task_struct *pos;
 
 	rcu_read_lock();
 	/* Attempt to start with the pid of a thread */
 	if (tid && (nr > 0)) {
 		pos = find_task_by_pid(tid);
-		if (pos && (pos->group_leader != leader))
-			pos = NULL;
-		if (pos)
-			nr = 0;
+		if (pos && (pos->group_leader == leader))
+			goto found;
 	}
 
 	/* If nr exceeds the number of threads there is nothing todo */
-	if (nr) {
-		if (nr >= get_nr_threads(leader))
-			goto done;
-	}
+	pos = NULL;
+	if (nr && nr >= get_nr_threads(leader))
+		goto out;
 
-	/* If we haven't found our starting place yet start with the
-	 * leader and walk nr threads forward.
+	/* If we haven't found our starting place yet start
+	 * with the leader and walk nr threads forward.
 	 */
-	if (!pos && (nr >= 0))
-		pos = leader;
-
-	for (; pos && pid_alive(pos); pos = next_thread(pos)) {
-		if (--nr > 0)
-			continue;
-		get_task_struct(pos);
-		goto done;
-	}
+	for (pos = leader; nr > 0; --nr) {
+		pos = next_thread(pos);
+		if (pos == leader) {
 			pos = NULL;
-done:
+			goto out;
+		}
+	}
+found:
+	get_task_struct(pos);
+out:
 	rcu_read_unlock();
 	return pos;
 }