Re: [PATCH net-next] sockopt: Change getsockopt() of SO_BINDTODEVICE to return an interface name

On Fri, 2012-11-02 at 11:02 -0400, Brian Haley wrote:

On 11/02/2012 05:36 AM, Pavel Emelyanov wrote:

quoted

quoted

+static int sock_getbindtodevice(struct sock *sk, char __user *optval,
+				int __user *optlen, int len)
+{
+	int ret = -ENOPROTOOPT;
+#ifdef CONFIG_NETDEVICES
+	struct net *net = sock_net(sk);
+	struct net_device *dev;
+	char devname[IFNAMSIZ];
+
+	if (sk->sk_bound_dev_if == 0) {
+		len = 0;
+		goto zero;
+	}
+
+	ret = -EINVAL;
+	if (len < IFNAMSIZ)
+		goto out;
+
+	rcu_read_lock();
+	dev = dev_get_by_index_rcu(net, sk->sk_bound_dev_if);
+	if (dev)
+		strcpy(devname, dev->name);

This still races with the device name change, potentially providing
a name which never existed in the system, doesn't it?

My only argument here is that SIOCGIFNAME has had this same code forever, and
noone has ever complained about that returning a garbled name.  Even
dev_get_by_name() only holds an rcu lock when doing a strncmp().

We'd need to audit the whole kernel to catch all the places where we potentially
look at dev->name while it could change.  Is it really worth it?

A net device name can't be changed while the device is up, or while
another task holds the RTNL lock.  I think that covers almost all uses.
I don't know whether it's worth going out to look for exceptions, but we
might as well fix the cases we know about.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.