chan.hdrlen is read once at channel registration by ppp_register_net_channel(), and used to set the PPP net device's hard_header_len. It was set to PPPOL2TP_L2TP_HDR_SIZE_NOSEQ (6), which is 4 bytes too small if sequence numbers are later enabled via setsockopt(PPPOL2TP_SO_SENDSEQ), causing unnecessary skb reallocations on the TX path. The setsockopt handler attempted to change netdev's hard_header_len by updating chan.hdrlen, but the PPP layer never re-reads it after the registration, so the update had no effect. To avoid the unnecessary reallocations, set chan.hdrlen to PPPOL2TP_L2TP_HDR_SIZE_SEQ (10) unconditionally at registration and remove the ineffective update in the setsockopt callback. Signed-off-by: Qingfang Deng --- v2: rebase to net-next and drop fixes tag - https://lore.kernel.org/netdev/20260313034732.209792-1-dqfext@gmail.com/ net/l2tp/l2tp_ppp.c | 15 +++++---------- 1 file changed, 5 insertions(+), 10 deletions(-) diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c index ae4543d5597b..99d6582f41de 100644 --- a/net/l2tp/l2tp_ppp.c +++ b/net/l2tp/l2tp_ppp.c @@ -787,11 +787,12 @@ static int pppol2tp_connect(struct socket *sock, struct sockaddr_unsized *userva goto out_no_ppp; } - /* The only header we need to worry about is the L2TP - * header. This size is different depending on whether - * sequence numbers are enabled for the data channel. + /* Reserve enough headroom for the L2TP header with sequence numbers, + * which is the largest possible. This is used by the PPP layer to set + * the net device's hard_header_len at registration, which must be + * sufficient regardless of whether sequence numbers are enabled later. */ - po->chan.hdrlen = PPPOL2TP_L2TP_HDR_SIZE_NOSEQ; + po->chan.hdrlen = PPPOL2TP_L2TP_HDR_SIZE_SEQ; po->chan.private = sk; po->chan.ops = &pppol2tp_chan_ops; @@ -1176,12 +1177,6 @@ static int pppol2tp_session_setsockopt(struct sock *sk, break; } session->send_seq = !!val; - { - struct pppox_sock *po = pppox_sk(sk); - - po->chan.hdrlen = val ? PPPOL2TP_L2TP_HDR_SIZE_SEQ : - PPPOL2TP_L2TP_HDR_SIZE_NOSEQ; - } l2tp_session_set_header_len(session, session->tunnel->version, session->tunnel->encap); break; -- 2.43.0