Re: [RFC net] net: force software segmentation of PPPoE packets

[Qingfang Deng <qingfang.deng@linux.dev>]

Hi,

On 10/20/2026 11:08 PM, Eric Dumazet wrote:
> Le mar. 20 oct. 2026 à 16:30, Qingfang Deng <qingfang.deng@linux.dev> a écrit :
>>
>> Ordinary TSO does not update the PPPoE payload length. A PPPoE GRO packet
>> forwarded by a bridge can retain hardware GSO features and reach the
>> driver without software segmentation, if the egress Ethernet interface
>> advertises TSO.
>>
>> To force software segmentation, clear the GSO feature mask for
>> ETH_P_PPP_SES in harmonize_features().
>>
>> Fixes: 55a5d8fca836 ("net: pppoe: implement GRO/GSO support")
>> Signed-off-by: Qingfang Deng <qingfang.deng@linux.dev>
>> ---
>>   net/core/dev.c | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/net/core/dev.c b/net/core/dev.c
>> index a6951ea08961..27483a2ad747 100644
>> --- a/net/core/dev.c
>> +++ b/net/core/dev.c
>> @@ -3831,6 +3831,10 @@ static netdev_features_t harmonize_features(struct sk_buff *skb,
>>          type = skb_network_protocol(skb, NULL);
>>          features = net_mpls_features(skb, features, type);
>>
>> +       /* Ordinary TSO does not update the PPPoE payload length. */
>> +       if (IS_ENABLED(CONFIG_PPPOE) && type == htons(ETH_P_PPP_SES))
>> +               features &= ~NETIF_F_GSO_MASK;
>> +
>>          if (skb->ip_summed != CHECKSUM_NONE &&
>>              !can_checksum_protocol(features, type)) {
>>                  features &= ~(NETIF_F_CSUM_MASK | NETIF_F_GSO_MASK);
>> --
>> 2.34.1
>>
> 
> Ouch.
> 
> harmonize_features() is called for every packet, GSO or not.
> 
> This is a GSO-only problem, please do not add more protocol-specific
> tests in this path. gso_features_check() is only called for GSO
> packets, and the cost there is amortized over all segments.
> 
> Devices without NETIF_F_HW_CSUM already lose GSO for ETH_P_PPP_SES
> because of the can_checksum_protocol() check in harmonize_features(),
> and fraglist GRO packets (the main use case for PPPoE GRO) are
> segmented in software anyway.
> 
> This leaves bridged, non fraglist GRO PPPoE packets sent to a
> NETIF_F_HW_CSUM + TSO device. Have you hit this on real hardware?

+Cc: Alexander Duyck

I ran the real hardware test on Intel igc.

igc_tso() requires skb_network_header() to point to the IP header, which 
is not the case for PPPoE packets, so it won't work on an unmodified 
driver. I therefore used a kernel module to inject PPPoE GSO skbs 
directly to the driver's TX path, deliberately pointing 
skb_network_header() to IP.

Test results show that the hardware can still perform TCP segmentation, 
but it cannot update the length field in the PPPoE header. So 
theoretically, we might be able to make GSO_PARTIAL work in PPPoE.

Kind regards,
Qingfang