netdev - Re: [PATCH bpf 3/5] netkit: Fix syncing peer device mtu with primary

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Date: Fri, 24 May 2024 16:20:00 +0200
From: Daniel Borkmann <daniel@...earbox.net>
To: Nikolay Aleksandrov <razor@...ckwall.org>, martin.lau@...nel.org
Cc: bpf@...r.kernel.org, netdev@...r.kernel.org, Joe Stringer <joe@...ium.io>
Subject: Re: [PATCH bpf 3/5] netkit: Fix syncing peer device mtu with primary

On 5/24/24 4:15 PM, Nikolay Aleksandrov wrote:
> On 5/24/24 16:01, Daniel Borkmann wrote:
>> Implement the ndo_change_mtu callback in netkit in order to align the MTU
>> to the primary device. This is needed in order to sync MTUs to the latter
>> from the control plane (e.g. Cilium) which does not have access into the
>> Pod's netns.
>>
>> Fixes: 35dfaad7188c ("netkit, bpf: Add bpf programmable net device")
>> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
>> Cc: Joe Stringer <joe@...ium.io>
>> ---
>>   drivers/net/netkit.c | 20 ++++++++++++++++++++
>>   1 file changed, 20 insertions(+)
>>
> 
> This one has unexpected behaviour IMO. If the app sets the MTU and we
> silently overwrite, then it may continue working and thinking the MTU
> was changed leading to unexpected problems. I think it'd be better to
> keep the MTU synced explicitly (e.g. when set on main device, then
> set it on peer as well) and error out when trying to set it without
> the proper capabilities.

Makes sense, I'll look into this, thanks Nik!