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

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

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?

I reproduced this on a VM veth interface, which advertises both 
features. The real hardware I have is an Intel IGC but I have not tested 
with it.