ppp: use IFF_NO_QUEUE in virtual interfaces

[ Upstream commit 95d0d09 ]

For PPPoE, PPTP, and PPPoL2TP, the start_xmit() function directly
forwards packets to the underlying network stack and never returns
anything other than 1. So these interfaces do not require a qdisc,
and the IFF_NO_QUEUE flag should be set.

Introduces a direct_xmit flag in struct ppp_channel to indicate when
IFF_NO_QUEUE should be applied. The flag is set in ppp_connect_channel()
for relevant protocols.

While at it, remove the usused latency member from struct ppp_channel.

Signed-off-by: Qingfang Deng <dqfext@gmail.com>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://patch.msgid.link/20250301135517.695809-1-dqfext@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Stable-dep-of: 543adf0 ("ppp: annotate data races in ppp_generic")
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: LGA1150

drivers/net/ppp/ppp_generic.c

@@ -3508,6 +3508,10 @@ ppp_connect_channel(struct channel *pch, int unit)
 		ret = -ENOTCONN;
 		goto outl;
 	}
+	if (pch->chan->direct_xmit)
+		ppp->dev->priv_flags |= IFF_NO_QUEUE;
+	else
+		ppp->dev->priv_flags &= ~IFF_NO_QUEUE;
 	spin_unlock_bh(&pch->downl);
 	if (pch->file.hdrlen > ppp->file.hdrlen)
 		ppp->file.hdrlen = pch->file.hdrlen;

drivers/net/ppp/pppoe.c

@@ -699,6 +699,7 @@ static int pppoe_connect(struct socket *sock, struct sockaddr *uservaddr,
 		po->chan.mtu = dev->mtu - sizeof(struct pppoe_hdr) - 2;
 		po->chan.private = sk;
 		po->chan.ops = &pppoe_chan_ops;
+		po->chan.direct_xmit = true;
 
 		error = ppp_register_net_channel(dev_net(dev), &po->chan);
 		if (error) {

drivers/net/ppp/pptp.c

@@ -469,6 +469,7 @@ static int pptp_connect(struct socket *sock, struct sockaddr *uservaddr,
 	po->chan.mtu -= PPTP_HEADER_OVERHEAD;
 
 	po->chan.hdrlen = 2 + sizeof(struct pptp_gre_header);
+	po->chan.direct_xmit = true;
 	error = ppp_register_channel(&po->chan);
 	if (error) {
 		pr_err("PPTP: failed to register PPP channel (%d)\n", error);

include/linux/ppp_channel.h

@@ -42,8 +42,7 @@ struct ppp_channel {
 	int		hdrlen;		/* amount of headroom channel needs */
 	void		*ppp;		/* opaque to channel */
 	int		speed;		/* transfer rate (bytes/second) */
-	/* the following is not used at present */
-	int		latency;	/* overhead time in milliseconds */
+	bool		direct_xmit;	/* no qdisc, xmit directly */
 };
 
 #ifdef __KERNEL__

net/l2tp/l2tp_ppp.c

@@ -810,6 +810,7 @@ static int pppol2tp_connect(struct socket *sock, struct sockaddr *uservaddr,
 	po->chan.private = sk;
 	po->chan.ops	 = &pppol2tp_chan_ops;
 	po->chan.mtu	 = pppol2tp_tunnel_mtu(tunnel);
+	po->chan.direct_xmit	= true;
 
 	error = ppp_register_net_channel(sock_net(sk), &po->chan);
 	if (error) {