Re: [B.A.T.M.A.N.] [RFC PATCH] batman-adv: mitigate issue when empty vlan is received

Sven Eckelmann
2018-05-08T08:45:51Z

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n Montag, 7. Mai 2018 12:23:57 CEST Sven Eckelmann wrote:
> This would sound to me like the vlan should just be
> created instead of adding this workaround. But maybe you should discuss the
> details here with Antonio because at the end it is just about the different
> ways of handling "!vlan" case from your patch.

Just had a look at the cleanup code for vlans and it seems like each vlan
in vlan_list must contain at least one entry (according to
vlan->tt.num_entries) and otherwise batadv_tt_global_size_mod will drop it.
So it isn't as easy as just adding a simple vlan for each received vlan
entry via TT response.

So your patch (or a variant of it) will most likely be the way to go for
receivers.

Now to your commit message:

On Sonntag, 6. Mai 2018 21:55:59 CEST Leonardo Mörlein wrote:

> batman-adv: mitigate issue when empty vlan is received

Something more specific would be nice. Maybe somebody else has a better
idea but what about:

batman-adv: Mitigate TT loop when receiving empty VLAN

> This patch is not a fix for the issue itself, but a fix for the
> other nodes, which are also influenced by the issue.

I personally don't like to read "this patch" in commit messages. This part
should maybe be reformulated and put at the end (before the Fixes line).
Something like:

While the problem is caused by the sender, the receiver can mitigate the
problem by ignoring (potentially) empty VLANs during the CRC check.

[...]
> - This behaviour is a bug! Batman-adv nodes must not send TT
> announcements for empty vlans.

Where is this defined as incorrect message? But yes, the current
implementation cannot handle it.

> - The receiving batman-adv can not handle incoming TT announcements
> with empty vlans. (The crc check routine batadv_tt_global_check_crc()
> fails.)

This is to vague as discussed earlier. Maybe something more like:

- The receiving batman-adv can not handle incoming TT announcements with
empty vlans. No new struct batadv_orig_node_vlan entry will be added
to batadv_orig_node->vlan_list. This will cause the crc check routine
batadv_tt_global_check_crc() to fail because it cannot find the local
representation of this empty vlan.

[...]
> - This causes a lot of unneeded TT traffic. In Freifunk Hannover we
> decreased the amount of from ~20kpp/s to ~4kpp/s on our supernodes.
> We have ~800 nodes, which are connected via vpn to one of six
> supernodes. Those supernodes are connected with each other in a fully
> connected mesh.

This part doesn't seem to fit in the list and might just be a new paragraph
at the end to summarize the impact of the problem.

A "Fixes: " line is missing here. Please check whether this sounds about
right:

Fixes: 7ea7b4a14275 ("batman-adv: make the TT CRC logic VLAN specific")

Kind regards,
Sven