l2tp: ppp: use max L2TP header size for PPP channel hdrlen

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>
Link: https://patch.msgid.link/20260317054141.524879-1-dqfext@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

diff --git a/net/l2tp/l2tp_ppp.c b/net/l2tp/l2tp_ppp.c
index ae4543d5597b6677c758b4919db71a28909cf3ff..99d6582f41de2171557053d72bb7621c825822db 100644 (file)
--- 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;