netdev - Re: [PATCH net 1/1] tipc: fix broadcast link synchronization problem

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Message-Id: <20161029.172151.2096765504289435592.davem@davemloft.net>
Date:   Sat, 29 Oct 2016 17:21:51 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     jon.maloy@...csson.com
Cc:     netdev@...r.kernel.org, paul.gortmaker@...driver.com,
        parthasarathy.bhuvaragan@...csson.com, ying.xue@...driver.com,
        maloy@...jonn.com, tipc-discussion@...ts.sourceforge.net
Subject: Re: [PATCH net 1/1] tipc: fix broadcast link synchronization
 problem

From: Jon Maloy <jon.maloy@...csson.com>
Date: Thu, 27 Oct 2016 18:51:55 -0400

> In commit 2d18ac4ba745 ("tipc: extend broadcast link initialization
> criteria") we tried to fix a problem with the initial synchronization
> of broadcast link acknowledge values. Unfortunately that solution is
> not sufficient to solve the issue.
> 
> We have seen it happen that LINK_PROTOCOL/STATE packets with a valid
> non-zero unicast acknowledge number may bypass BCAST_PROTOCOL
> initialization, NAME_DISTRIBUTOR and other STATE packets with invalid
> broadcast acknowledge numbers, leading to premature opening of the
> broadcast link. When the bypassed packets finally arrive, they are
> inadvertently accepted, and the already correctly initialized
> acknowledge number in the broadcast receive link is overwritten by
> the invalid (zero) value of the said packets. After this the broadcast
> link goes stale.
> 
> We now fix this by marking the packets where we know the acknowledge
> value is or may be invalid, and then ignoring the acks from those.
> 
> To this purpose, we claim an unused bit in the header to indicate that
> the value is invalid. We set the bit to 1 in the initial BCAST_PROTOCOL
> synchronization packet and all initial ("bulk") NAME_DISTRIBUTOR
> packets, plus those LINK_PROTOCOL packets sent out before the broadcast
> links are fully synchronized.
> 
> This minor protocol update is fully backwards compatible.
> 
> Reported-by: John Thompson <thompa.atl@...il.com>
> Tested-by: John Thompson <thompa.atl@...il.com>
> Signed-off-by: Jon Maloy <jon.maloy@...csson.com>

Applied, thanks Jon.