Re: [RFC net] net: force software segmentation of PPPoE packets
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From: Eric Dumazet <edumazet@kernel.org>
Date: 2026-10-20 15:08:56
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Le mar. 20 oct. 2026 à 16:30, Qingfang Deng [off-list ref] a écrit :
quoted hunk ↗ jump to hunk
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 <redacted> --- 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?