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

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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>
This commit is contained in:
Qingfang Deng
2026-03-17 13:41:40 +08:00
committed by Paolo Abeni
parent 96a584db75
commit 9f4960b94f
1 changed files with 5 additions and 10 deletions
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15
net/l2tp/l2tp_ppp.c
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@@ -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;