ipv6/flowlabel: simplify pid namespace lookup

The code should be using the pid namespace from the procfs mount
instead of trying to look it up during open.

Suggested-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

net/ipv6/ip6_flowlabel.c

@@ -754,6 +754,10 @@ static struct ip6_flowlabel *ip6fl_get_idx(struct seq_file *seq, loff_t pos)
 static void *ip6fl_seq_start(struct seq_file *seq, loff_t *pos)
 	__acquires(RCU)
 {
+	struct ip6fl_iter_state *state = ip6fl_seq_private(seq);
+
+	state->pid_ns = proc_pid_ns(file_inode(seq->file));
+
 	rcu_read_lock_bh();
 	return *pos ? ip6fl_get_idx(seq, *pos - 1) : SEQ_START_TOKEN;
 }
@@ -810,36 +814,15 @@ static const struct seq_operations ip6fl_seq_ops = {
 
 static int ip6fl_seq_open(struct inode *inode, struct file *file)
 {
-	struct seq_file *seq;
-	struct ip6fl_iter_state *state;
-	int err;
-
-	err = seq_open_net(inode, file, &ip6fl_seq_ops,
+	return seq_open_net(inode, file, &ip6fl_seq_ops,
 			   sizeof(struct ip6fl_iter_state));
-
-	if (!err) {
-		seq = file->private_data;
-		state = ip6fl_seq_private(seq);
-		rcu_read_lock();
-		state->pid_ns = get_pid_ns(task_active_pid_ns(current));
-		rcu_read_unlock();
-	}
-	return err;
-}
-
-static int ip6fl_seq_release(struct inode *inode, struct file *file)
-{
-	struct seq_file *seq = file->private_data;
-	struct ip6fl_iter_state *state = ip6fl_seq_private(seq);
-	put_pid_ns(state->pid_ns);
-	return seq_release_net(inode, file);
 }
 
 static const struct file_operations ip6fl_seq_fops = {
 	.open		=	ip6fl_seq_open,
 	.read		=	seq_read,
 	.llseek		=	seq_lseek,
-	.release	=	ip6fl_seq_release,
+	.release	=	seq_release_net,
 };
 
 static int __net_init ip6_flowlabel_proc_init(struct net *net)