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

[Qingfang Deng]

On 10/20/2026 11:08 PM, Eric Dumazet wrote:
> Le mar. 20 oct. 2026 à 16:30, Qingfang Deng <qingfang.deng@xxxxxxxxx> 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@xxxxxxxxx>
> > ---
> >   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.