netdev - Re: [PATCH net-next] sockopt: Change getsockopt() of SO

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Date:	Thu, 8 Nov 2012 00:23:19 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Brian Haley <brian.haley@...com>
CC:	Pavel Emelyanov <xemul@...allels.com>,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] sockopt: Change getsockopt() of
 SO_BINDTODEVICE to return an interface name

On Mon, 2012-11-05 at 21:01 -0700, Brian Haley wrote:
> On 11/02/2012 05:34 PM, Ben Hutchings wrote:
> > On Fri, 2012-11-02 at 11:02 -0400, Brian Haley wrote:
> >> On 11/02/2012 05:36 AM, Pavel Emelyanov wrote:
> >>>> +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.
> 
> So do you think we can fix these corner cases later and get the API 
> right first?

Well you can avoid the problem here by using rtnl_{,un}lock() and
__dev_get_by_index().  But if that's too heavyweight (it depends on how
often you think this might need to be called) then I suppose it can be
left until we have a general solution for races with renaming.

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.

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