proc: fix ->f_pos overflows in first_tid()

1. proc_task_readdir()->first_tid() path truncates f_pos to int, this
   is wrong even on 64bit.

   We could check that f_pos < PID_MAX or even INT_MAX in
   proc_task_readdir(), but this patch simply checks the potential
   overflow in first_tid(), this check is nop on 64bit.  We do not care if
   it was negative and the new unsigned value is huge, all we need to
   ensure is that we never wrongly return !NULL.

2. Remove the 2nd "nr != 0" check before get_nr_threads(),
   nr_threads == 0 is not distinguishable from !pid_task() above.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Sameer Nanda <snanda@chromium.org>
Cc: Sergey Dyasly <dserrg@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

@@ -3097,10 +3097,14 @@ out_no_task:
  * In the case of a seek we start with the leader and walk nr
  * threads past it.
  */
-static struct task_struct *first_tid(struct pid *pid, int tid,
-					int nr, struct pid_namespace *ns)
+static struct task_struct *first_tid(struct pid *pid, int tid, loff_t f_pos,
+					struct pid_namespace *ns)
 {
 	struct task_struct *pos, *task;
+	unsigned long nr = f_pos;
+
+	if (nr != f_pos)	/* 32bit overflow? */
+		return NULL;
 
 	rcu_read_lock();
 	task = pid_task(pid, PIDTYPE_PID);
@@ -3108,14 +3112,14 @@ static struct task_struct *first_tid(struct pid *pid, int tid,
 		goto fail;
 
 	/* Attempt to start with the tid of a thread */
-	if (tid && (nr > 0)) {
+	if (tid && nr) {
 		pos = find_task_by_pid_ns(tid, ns);
 		if (pos && same_thread_group(pos, task))
 			goto found;
 	}
 
 	/* If nr exceeds the number of threads there is nothing todo */
-	if (nr && nr >= get_nr_threads(task))
+	if (nr >= get_nr_threads(task))
 		goto fail;
 
 	/* If we haven't found our starting place yet start
@@ -3123,7 +3127,7 @@ static struct task_struct *first_tid(struct pid *pid, int tid,
 	 */
 	pos = task = task->group_leader;
 	do {
-		if (nr-- <= 0)
+		if (!nr--)
 			goto found;
 	} while_each_thread(task, pos);
 fail: