[Devel] Re: [PATCH 03/16] net: Basic network namespace infrastructure.

[Eric W. Biederman]

"Paul E. McKenney" <paulmck at linux.vnet.ibm.com> writes:

> On Sat, Sep 08, 2007 at 03:15:34PM -0600, Eric W. Biederman wrote:
>> 
>> This is the basic infrastructure needed to support network
>> namespaces.  This infrastructure is:
>> - Registration functions to support initializing per network
>>   namespace data when a network namespaces is created or destroyed.
>> 
>> - struct net.  The network namespace data structure.
>>   This structure will grow as variables are made per network
>>   namespace but this is the minimal starting point.
>> 
>> - Functions to grab a reference to the network namespace.
>>   I provide both get/put functions that keep a network namespace
>>   from being freed.  And hold/release functions serve as weak references
>>   and will warn if their count is not zero when the data structure
>>   is freed.  Useful for dealing with more complicated data structures
>>   like the ipv4 route cache.
>> 
>> - A list of all of the network namespaces so we can iterate over them.
>> 
>> - A slab for the network namespace data structure allowing leaks
>>   to be spotted.
>
> If I understand this correctly, the only way to get to a namespace is
> via get_net_ns_by_pid(), which contains the rcu_read_lock() that matches
> the rcu_barrier() below.

Not quite.  That is the convoluted case for getting a namespace someone
else is using.  current->nsproxy->net_ns works and should require no
locking to read (only the current process may modify it) and does hold
a reference to the network namespace.  Similarly for sock->sk_net.

> So, is the get_net() in sock_copy() in this patch adding a reference to
> an element that is guaranteed to already have at least one reference?

Yes.

> If not, how are we preventing sock_copy() from running concurrently with
> cleanup_net()?  Ah, I see -- in sock_copy() we are getting a reference
> to the new struct sock that no one else can get a reference to, so OK.
> Ditto for the get_net() in sk_alloc().

> But I still don't understand what is protecting the get_net() in
> dev_seq_open().  Is there an existing reference? 

Sort of.  The directories under /proc/net are created when create
a network namespace and they are destroyed when the network namespace
is removed.  And those directories remember which network namespace
they are for and that is what dev_seq_open is referencing.

So the tricky case what happens if we open a directory under /proc/net
as we are cleaning up a network namespace.

> If so, how do we know
> that it won't be removed just as we are trying to add our reference
> (while at the same time cleanup_net() is running)?  Ditto for the other
> _open() operations in the same patch.  And for netlink_seq_open().
>
> Enlightenment?

Good spotting. It looks like you have found a legitimate race.  Grr.
I thought I had a reference to the network namespace there.  I need to
step back and think about this a bit, and see if I can come up with a
legitimate idiom.

I know the network namespace exists and I have not finished
cleanup_net because I can still get to the /proc entries.

I know I cannot use get_net for the reference in in /proc because
otherwise I could not release the network namespace unless I was to
unmount the filesystem, which is not a desirable property.

I think I can change the idiom to:

struct net *maybe_get_net(struct net *net)
{
        if (!atomic_inc_not_zero(&net->count))
        	net = NULL;
	return net;               
}

Which would make dev_seq_open be:

static int dev_seq_open(struct inode *inode, struct file *file)
{
	struct seq_file *seq;
	int res;
	res =  seq_open(file, &dev_seq_ops);
	if (!res) {
		seq = file->private_data;
		seq->private = maybe_get_net(PROC_NET(inode));
		if (!seq->private) {
			res = -ENOENT;
                        seq_release(inode, file);
		}
	}
	return res;
}

I'm still asking myself if I need any kind of locking to ensure
struct net does not go away in the mean time, if so rcu_read_lock()
should be sufficient.

I will read through the generic proc code very carefully after
I have slept and see if there is what I the code above is sufficient,
and if so update the patchset.

Eric
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