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

[Eric Dumazet]

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?