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

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Message-ID: <CABQ03vEbgtHLyFmRs7ffpXGkTmjX9G=RbAhWwDrj-5rKysys5g@mail.gmail.com>
Date:	Wed, 27 May 2015 10:35:54 +0200
From:	Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
To:	Scott Feldman <sfeldma@...il.com>
Cc:	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 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 ?
How can we differentiate between user-added entry and an externally
learned one ?
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 ?
Minor note: br_fdb_add (ndo_fdb_add) is already called with rtnl held,
so the API will have to be used directly.

Thanks,
 Nik
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