linux-kernel - Re: [PATCH net v2] bridge: mcast: fix disabled snooping after long uptime

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Message-ID: <43a266e9-7cde-472a-9846-c16756be8c09@blackwall.org>
Date: Sat, 27 Jan 2024 21:32:25 +0200
From: Nikolay Aleksandrov <razor@...ckwall.org>
To: Linus Lüssing <linus.luessing@...3.blue>,
 netdev@...r.kernel.org
Cc: bridge@...ts.linux.dev, b.a.t.m.a.n@...ts.open-mesh.org,
 linux-kernel@...r.kernel.org, Roopa Prabhu <roopa@...dia.com>,
 "David S . Miller" <davem@...emloft.net>, Eric Dumazet
 <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net v2] bridge: mcast: fix disabled snooping after long
 uptime

On 27/01/2024 19:50, Linus Lüssing wrote:
> The original idea of the delay_time check was to not apply multicast
> snooping too early when an MLD querier appears. And to instead wait at
> least for MLD reports to arrive before switching from flooding to group
> based, MLD snooped forwarding, to avoid temporary packet loss.
> 
> However in a batman-adv mesh network it was noticed that after 248 days of
> uptime 32bit MIPS based devices would start to signal that they had
> stopped applying multicast snooping due to missing queriers - even though
> they were the elected querier and still sending MLD queries themselves.
> 
> While time_is_before_jiffies() generally is safe against jiffies
> wrap-arounds, like the code comments in jiffies.h explain, it won't
> be able to track a difference larger than ULONG_MAX/2. With a 32bit
> large jiffies and one jiffies tick every 10ms (CONFIG_HZ=100) on these MIPS
> devices running OpenWrt this would result in a difference larger than
> ULONG_MAX/2 after 248 (= 2^32/100/60/60/24/2) days and
> time_is_before_jiffies() would then start to return false instead of
> true. Leading to multicast snooping not being applied to multicast
> packets anymore.
> 
> Fix this issue by using a proper timer_list object which won't have this
> ULONG_MAX/2 difference limitation.
> 
> Fixes: b00589af3b04 ("bridge: disable snooping if there is no querier")
> Signed-off-by: Linus Lüssing <linus.luessing@...3.blue>
> ---
> Changelog v2:
> * removed "inline" from br_multicast_query_delay_expired()
> 
>  net/bridge/br_multicast.c | 20 +++++++++++++++-----
>  net/bridge/br_private.h   |  4 ++--
>  2 files changed, 17 insertions(+), 7 deletions(-)
> 

Acked-by: Nikolay Aleksandrov <razor@...ckwall.org>