linux-kernel - Re: [PATCH] net: retain NOCARRIER on protodown interfaces

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Message-ID: <a9d896a4-b279-41f6-a492-980340e125ac@redhat.com>
Date: Thu, 3 Oct 2024 10:55:46 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Volodymyr Boyko <boyko.cxx@...il.com>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>
Cc: netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: retain NOCARRIER on protodown interfaces

On 9/27/24 09:33, Volodymyr Boyko wrote:
> Make interface with enabled protodown to retain NOCARRIER state during
> transfer of operstate from its lower device.
> 
> Signed-off-by: Volodymyr Boyko <boyko.cxx@...il.com>
> ---
> Currently bringing up lower device enables carrier on upper devices
> ignoring the protodown flag.
> 
> Steps to reproduce:
> ```
> ip l a test0 up type dummy
> ip l a test0.mv0 up link test0 type macvlan mode bridge
> ip l s test0.mv0 protodown on
> sleep 1
> printf 'before flap:\n'
> ip -o l show | grep test0
> ip l set down test0 && ip l set up test0
> printf 'after flap:\n'
> ip -o l show | grep test0
> ip l del test0
> ```
> 
> output without this change:
> ```
> before flap:
> 28: test0.mv0@...t0: <NO-CARRIER,BROADCAST,MULTICAST,UP>
> 	 state LOWERLAYERDOWN protodown on
> after flap:
> 28: test0.mv0@...t0: <BROADCAST,MULTICAST,UP,LOWER_UP>
> 	 state UP protodown on
> ```
> 
> output with this change:
> ```
> before flap:
> 28: test0.mv0@...t0: <NO-CARRIER,BROADCAST,MULTICAST,UP>
> 	state DOWN protodown on
> after flap:
> 28: test0.mv0@...t0: <NO-CARRIER,BROADCAST,MULTICAST,UP>
> 	state DOWN protodown on
> ```

I'm unsure we can accept this change of behavior: existing user-space 
application may rely on the existing one. I tend to stay on the safe side.

Paolo