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

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Date:	Wed, 27 May 2015 00:59:15 -0700
From:	Scott Feldman <sfeldma@...il.com>
To:	Nikolay Aleksandrov <nikolay@...ulusnetworks.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 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
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