[PATCH net-next v2] l2tp: ppp: use max L2TP header size for PPP channel hdrlen

[Qingfang Deng <dqfext@gmail.com>]

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 <dqfext@gmail.com>
---
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