Subject: Re: [PATCH v2 net] bonding: Fix stacked device detection in arp monitoring

From: Veaceslav Falico <[email-masked]>
Date: Wed, 7 May 2014 20:59:37 +0200

On Wed, May 07, 2014 at 02:47:36PM -0400, Vlad Yasevich wrote:
>On 05/07/2014 02:11 PM, Veaceslav Falico wrote:
>> On Wed, May 07, 2014 at 07:49:10PM +0200, Veaceslav Falico wrote:
>>> On Wed, May 07, 2014 at 01:08:09PM -0400, Vlad Yasevich wrote:
>>> ...snip...
>>>> Yes.  I verified that it works.  The reason is that we are traversing
>>>> the all_adj_list.upper list which contains all of the upper devices at
>>>> each level.  So, at vlan100 level, we will see vlan200 and all will be
>>>> well.
>>>
>>> Hrm, two scenarios, with the following config:
>>>
>>> bond0 -> whatever1 -> vlan1 -> whatever2 -> vlan2 -> whatever3_IP
>>>
>>> end == whatever3_IP
>>>
>> ...snip...
>>>
>>> So, the end patch (not compiled, not tested...) would look something like
>>> (only the bond_check_path() is changed and copied here, everything else
>>> remains the same):
>>>
>>> +       bool upper_found = false;
>>> +
>>> +       netdev_for_each_all_upper_dev_rcu(start, upper, iter) {
>>> +               if (upper == end)
>>> +                       upper_found = true;
>>> +
>>> +               if (idx < 2 && is_vlan_dev(upper) &&
>>> +                   bond_check_path(upper, end, tag, idx+1)) {
>>> +                       tag[idx].vlan_proto = vlan_dev_vlan_proto(upper);
>>> +                       tag[idx].vlan_id = vlan_dev_vlan_id(upper);
>>> +                       return true;
>>
>> Actually, screw that, we might find here the vlan2 first and end up with
>> only 1 vlan (vlan2, skipping vlan1).
>>
>
>hmm..  I am not sure that's actually possible...
>
>__netdev_adjacent_dev_insert() will always insert at the tail.  If you
>have a stack of vlans:
>   vlan1 (vid 10, 802.1Q)
>     |
>     v
>   vlan2 (vid 20, 802.1AD)
>     |
>     v
>   bond0
>
>then vlan1 will always be at the end of the list, and after vlan2.
>Even if we remove things, the higher the device, the later it will be in
>the list.
>
>So, in the event of the configuration:
>bond0 -> whatever1 -> vlan1 -> whatever2 -> vlan2 -> whatever3_IP
>
>waterver_IP will be last in the list due to list_add_tail_rcu()
>usage.

Yeah, you're right, sorry for misunderstanding (I've been tricked by the
master thing, but it doesn't actually add anything to all_upper).

Anyway, so the only concern is:

bond0 -> whatever1 -> vlan1 -> whatever2 -> vlan2 -> whatever3_IP
					\-> vlan3
bond_check_path start==bond0 idx=0
finds vlan1, tag[0] set, recursion start==vlan1 idx=1
\->
	bond_check_path start==vlan1 idx=1
	finds vlan2, tag[1] set, recursion start==vlan2 idx=2
	\->	returns right away with false as idx >= 2.

	finds vlan3 (!!!) that isn't related with whatever_IP, tag[1] set with the
	wrong vlan, recursion start==vlan3 idx=2
	\->	return right away with false as idx >= 2.

	finds whatever3_IP, returns true.
returns true

and this way we end up with vlan1 -> vlan3, instead of vlan1 -> vlan2.

Can be fixed by that "don't go deeper/populate tag[] if idx == 2" trick.

>
>Did I misunderstand something in the code? I can't seem to make it
>the above config fail in my tests.
>
>-vlad
>> The way to fix this might be to get the most "lengthy" path of vlans, as
>> in:
>>
>> + * Return the maximum length of stacked vlans + device found, 0 if the end
>> + * device is not found.
>> + */
>> +static int bond_check_path(struct net_device *start, struct net_device
>> *end,
>> +                          struct bond_vlan_tag *tag, int idx)
>> +{
>> +       struct net_device *upper;
>> +       struct list_head  *iter;
>> +       int length, max_length = 0;
>> +
>> +       netdev_for_each_all_upper_dev_rcu(start, upper, iter) {
>> +               if (upper == end && !max_length)
>> +                       max_length = 1;
>> +
>> +               if (idx < 2 && is_vlan_dev(upper)) {
>> +                       length = bond_check_path(upper, end, tag, idx + 1);
>> +
>> +                       if (max_length < length + 1) {
>> +                               tag[idx].vlan_proto =
>> vlan_dev_vlan_proto(upper);
>> +                               tag[idx].vlan_id = vlan_dev_vlan_id(upper);
>> +                               max_length = length + 1;
>> +                       }
>> +               }
>> +       }
>> +       return max_length;
>> +}
>>
>> Hope that helps.
>>
>>> +               }
>>> +       }
>>> +       return upper_found;
>> ...snip...
>