linux-kernel - Re: [PATCH net-next v23 03/23] ovpn: add basic interface creation/destruction/management routines

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Message-ID: <ab583ad2-d9b4-49ec-88a5-74b66e63839c@openvpn.net>
Date: Mon, 17 Mar 2025 11:23:50 +0100
From: Antonio Quartulli <antonio@...nvpn.net>
To: Qingfang Deng <dqfext@...il.com>
Cc: andrew+netdev@...n.ch, donald.hunter@...il.com, edumazet@...gle.com,
 horms@...nel.org, kuba@...nel.org, linux-kernel@...r.kernel.org,
 linux-kselftest@...r.kernel.org, netdev@...r.kernel.org, pabeni@...hat.com,
 ryazanov.s.a@...il.com, sd@...asysnail.net, shaw.leon@...il.com,
 shuah@...nel.org, "Jason A. Donenfeld" <Jason@...c4.com>
Subject: Re: [PATCH net-next v23 03/23] ovpn: add basic interface
 creation/destruction/management routines

On 17/03/2025 11:10, Qingfang Deng wrote:
> On Mon, Mar 17, 2025 at 6:00 PM Antonio Quartulli <antonio@...nvpn.net> wrote:
>>
>> On 17/03/2025 10:41, Qingfang Deng wrote:
>>> Hi Antonio,
>>>
>>> On Mon, Mar 17, 2025 at 5:23 PM Antonio Quartulli <antonio@...nvpn.net> wrote:
>>>>>> +static void ovpn_setup(struct net_device *dev)
>>>>>> +{
>>>>>> +    netdev_features_t feat = NETIF_F_SG | NETIF_F_HW_CSUM | NETIF_F_RXCSUM |
>>>>>
>>>>> Do not advertise NETIF_F_HW_CSUM or NETIF_F_RXCSUM, as TX/RX checksum is
>>>>> not handled in hardware.
>>>>
>>>> The idea behind these flags was that the OpenVPN protocol will take care
>>>> of authenticating packets, thus substituting what the CSUM would do here.
>>>> For this I wanted to avoid the stack to spend time computing the CSUM in
>>>> software.
>>>
>>> For the RX part (NETIF_F_RXCSUM), you might be correct, but in patch
>>> 08 you wrote:
>>>> /* we can't guarantee the packet wasn't corrupted before entering the
>>>> * VPN, therefore we give other layers a chance to check that
>>>> */
>>>> skb->ip_summed = CHECKSUM_NONE;
>>
>> Right. This was the result after a lengthy discussion with Sabrina.
>> Despite authenticating what enters the tunnel, we indeed concluded it is
>> better to let the stack verify that what entered was not corrupted.
>>
>>>
>>> So NETIF_F_RXCSUM has no effect.
>>
>> Does it mean I can drop NETIF_F_RXCSUM and also the line
>>
>> skb->ip_summed = CHECKSUM_NONE;
>>
>> at the same time?
> 
> I don't think so. skb->ip_summed might have been set to
> CHECKSUM_UNNECESSARY on the lower layer with UDP/TCP RX checksum.

Makes sense.

Ok, thanks!

Regards,


-- 
Antonio Quartulli
OpenVPN Inc.