Re: [PATCH] inet: add RCU protection to inet->opt

[David Miller <davem@davemloft.net>]

From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Thu, 28 Apr 2011 22:49:19 +0200

> Are you sure socket is locked at this point ? ( by a priori call to
> lock_sock())

Ugh, it's not, I'll commit this fix which is safe because we're only
peeking at inet_opt to fetch the faddr value then we never reference
it again:

diff --git a/net/l2tp/l2tp_ip.c b/net/l2tp/l2tp_ip.c
index 962a607..e13c166 100644
--- a/net/l2tp/l2tp_ip.c
+++ b/net/l2tp/l2tp_ip.c
@@ -472,13 +472,15 @@ static int l2tp_ip_sendmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *m
 	if (rt == NULL) {
 		struct ip_options_rcu *inet_opt;
 
-		inet_opt = rcu_dereference_protected(inet->inet_opt,
-						     sock_owned_by_user(sk));
+		rcu_read_lock();
+		inet_opt = rcu_dereference(inet->inet_opt);
 
 		/* Use correct destination address if we have options. */
 		if (inet_opt && inet_opt->opt.srr)
 			daddr = inet_opt->opt.faddr;
 
+		rcu_read_unlock();
+
 		/* If this fails, retransmit mechanism of transport layer will
 		 * keep trying until route appears or the connection times
 		 * itself out.