netdev - Re: [PATCH net-next] bridge: skip fdb add if the port shouldn't learn

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Date:	Tue, 2 Jun 2015 22:57:52 -0700
From:	Scott Feldman <sfeldma@...il.com>
To:	roopa <roopa@...ulusnetworks.com>
Cc:	Nikolay Aleksandrov <nikolay@...ulusnetworks.com>,
	Stephen Hemminger <stephen@...workplumber.org>,
	Netdev <netdev@...r.kernel.org>,
	Wilson Kok <wkok@...ulusnetworks.com>,
	David Miller <davem@...emloft.net>,
	bridge@...ts.linux-foundation.org,
	Jiří Pírko <jiri@...nulli.us>
Subject: Re: [PATCH net-next] bridge: skip fdb add if the port shouldn't learn

On Tue, Jun 2, 2015 at 10:14 AM, roopa <roopa@...ulusnetworks.com> wrote:
> On 5/27/15, 9:01 AM, Scott Feldman wrote:
>>
>> On Wed, May 27, 2015 at 1:35 AM, Nikolay Aleksandrov
>> <nikolay@...ulusnetworks.com> wrote:
>>>
>>> On Wed, May 27, 2015 at 9:59 AM, Scott Feldman <sfeldma@...il.com> wrote:
>>>>
>>>> On Wed, May 27, 2015 at 12:05 AM, Nikolay Aleksandrov
>>>> <nikolay@...ulusnetworks.com> wrote:
>>>>>
>>>>> On Tue, May 26, 2015 at 7:28 PM, Stephen Hemminger
>>>>> <stephen@...workplumber.org> wrote:
>>>>>>
>>>>>> On Thu, 21 May 2015 03:42:57 -0700
>>>>>> Nikolay Aleksandrov <nikolay@...ulusnetworks.com> wrote:
>>>>>>
>>>>>>> From: Wilson Kok <wkok@...ulusnetworks.com>
>>>>>>>
>>>>>>> Check in fdb_add_entry() if the source port should learn, similar
>>>>>>> check is used in br_fdb_update.
>>>>>>> Note that new fdb entries which are added manually or
>>>>>>> as local ones are still permitted.
>>>>>>> This patch has been tested by running traffic via a bridge port and
>>>>>>> switching the port's state, also by manually adding/removing entries
>>>>>>> from the bridge's fdb.
>>>>>>>
>>>>>>> Signed-off-by: Wilson Kok <wkok@...ulusnetworks.com>
>>>>>>> Signed-off-by: Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
>>>>>>
>>>>>> What is the problem this is trying to solve?
>>>>>>
>>>>>> I think user should be allowed to manually add any entry
>>>>>> even if learning.
>>>>>
>>>>> Hi Stephen,
>>>>> I have been thinking about the use case and have discussed it
>>>>> internally with colleagues and the patch
>>>>> author, the main problem is when there's an external software that
>>>>> adds dynamic entries (learning) and
>>>>> it could experience a race condition, here's a possible situation:
>>>>> * external software learns a mac from hw, sends an add to kernel
>>>>>   * right before that, port goes blocking (or down) and kernel flushes
>>>>>     mac, sends notification about the state change and mac flush
>>>>>   * right after that, kernel gets the previous add from external
>>>>> software, it's
>>>>>     allowed to add, and then sends an add notification
>>>>>   * mean while, external software processes the link block/down and mac
>>>>> flush,
>>>>>     followed by the mac add from kernel.  At this point, external
>>>>> software can't
>>>>>     really know whether it's a user adding the mac intentionally or
>>>>> it's
>>>>>     a race.
>>>>>
>>>>> This issue can't really be avoided in user-space.
>>>>> As I've noted local and static entries are still allowed, and iproute2
>>>>> bridge utility always
>>>>> marks the entries as static (NUD_NOARP), this only affects external
>>>>> dynamic entries which
>>>>> are usually sent by something that does the learning externally.
>>>>> I'll keep digging to see if there's another way to go about this since
>>>>> I'd like to give the user
>>>>> full freedom. Personally I don't have strong feeling for this patch
>>>>> and if it's not preferred then
>>>>> I'll post a revert.
>>>>
>>>> So there is already a switchdev API to add/del an externally learned
>>>> FDB entry which holds rtnl_lock and avoids these races.  I would
>>>> suggest using that and revert this patch.
>>>>
>>>> See call_switchdev_notifiers(SWITCHDEV_FDB_ADD|SWITCHDEV_FDB_DEL) and
>>>> the handler in br.c:br_switchdev_event().
>>>>
>>>> -scott
>>>
>>> Hmm, I'm new to the switchdev API and am possibly missing something,
>>> but how do you suggest to use it here ?
>>
>> You need to call
>> call_switchdev_notifiers(SWITCHDEV_FDB_ADD|SWITCHDEV_FDB_DEL) when the
>> device learns a new mac/vlan on the port interface.
>>
>>> How can we differentiate between user-added entry and an externally
>>> learned one ?
>>
>> Externally added ones will be marked with NTF_EXT_LEARNED set in
>> ndm->ndm_flags in the netlink echo.  Manually added ones from the user
>> will have ndm->ndm_state set to NUD_NOARP in the netlink echo.
>>
>>> Do you mean to use (for example) the NTF_EXT_LEARNED flag when adding
>>> an entry from user-space so
>>> the API can get called in br_fdb_add ?
>>
>> No.  br_fdb_add is the bridge's .ndo_fdb_add handler called when user
>> manually adds an FDB entry using netlink RTM_NEWNEIGH.  For externally
>> learned entries, use the internal
>> call_switchdev_notifiers(SWITCHDEV_FDB_ADD|SWITCHDEV_FDB_DEL).
>
>
> scott, I am assuming you are ok with an external learning entity (user space
> driver or a controller) pushing entries
> with the NTF_EXT_LEARNED correct ?. Because NTF_EXT_LEARNED is in uapi (and
> analogous to RTNH_F_OFFLOAD in the fib offload world IMO)

That seems OK.  I can see how that would remove the need for this
patch, but still give you the control from user space daemon/listener
to figure out what happened.
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