netdev - [RFC PATCH 1/2] ppp: add PPPIOCBRIDGECHAN ioctl

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Date:   Fri,  6 Nov 2020 18:16:46 +0000
From:   Tom Parkin <tparkin@...alix.com>
To:     netdev@...r.kernel.org
Cc:     gnault@...hat.com, jchapman@...alix.com,
        Tom Parkin <tparkin@...alix.com>
Subject: [RFC PATCH 1/2] ppp: add PPPIOCBRIDGECHAN ioctl

This new ioctl allows two ppp channels to be bridged together: frames
arriving in one channel are transmitted in the other channel and vice
versa.

The practical use for this is primarily to support the L2TP Access
Concentrator use-case.  The end-user session is presented as a ppp
channel (typically PPPoE, although it could be e.g. PPPoA, or even PPP
over a serial link) and is switched into a PPPoL2TP session for
transmission to the LNS.  At the LNS the PPP session is terminated in
the ISP's network.

When a PPP channel is bridged to another it takes a reference on the
other's struct ppp_file.  This reference is dropped when the channel is
unregistered: if the dereference causes the bridged channel's reference
count to reach zero it is destroyed at that point.
---
 drivers/net/ppp/ppp_generic.c  | 35 +++++++++++++++++++++++++++++++++-
 include/uapi/linux/ppp-ioctl.h |  1 +
 2 files changed, 35 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index 7d005896a0f9..d893bf4470f4 100644
--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c
@@ -175,6 +175,7 @@ struct channel {
 	struct net	*chan_net;	/* the net channel belongs to */
 	struct list_head clist;		/* link in list of channels per unit */
 	rwlock_t	upl;		/* protects `ppp' */
+	struct channel *bridge;		/* "bridged" ppp channel */
 #ifdef CONFIG_PPP_MULTILINK
 	u8		avail;		/* flag used in multilink stuff */
 	u8		had_frag;	/* >= 1 fragments have been sent */
@@ -641,8 +642,9 @@ static long ppp_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 	}
 
 	if (pf->kind == CHANNEL) {
-		struct channel *pch;
+		struct channel *pch, *pchb;
 		struct ppp_channel *chan;
+		struct ppp_net *pn;
 
 		pch = PF_TO_CHANNEL(pf);
 
@@ -657,6 +659,24 @@ static long ppp_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 			err = ppp_disconnect_channel(pch);
 			break;
 
+		case PPPIOCBRIDGECHAN:
+			if (get_user(unit, p))
+				break;
+			err = -ENXIO;
+			if (pch->bridge) {
+				err = -EALREADY;
+				break;
+			}
+			pn = ppp_pernet(current->nsproxy->net_ns);
+			spin_lock_bh(&pn->all_channels_lock);
+			pchb = ppp_find_channel(pn, unit);
+			if (pchb) {
+				refcount_inc(&pchb->file.refcnt);
+				pch->bridge = pchb;
+				err = 0;
+			}
+			spin_unlock_bh(&pn->all_channels_lock);
+			break;
 		default:
 			down_read(&pch->chan_sem);
 			chan = pch->chan;
@@ -2100,6 +2120,12 @@ ppp_input(struct ppp_channel *chan, struct sk_buff *skb)
 		return;
 	}
 
+	if (pch->bridge) {
+		skb_queue_tail(&pch->bridge->file.xq, skb);
+		ppp_channel_push(pch->bridge);
+		return;
+	}
+
 	read_lock_bh(&pch->upl);
 	if (!ppp_decompress_proto(skb)) {
 		kfree_skb(skb);
@@ -2791,6 +2817,13 @@ ppp_unregister_channel(struct ppp_channel *chan)
 	up_write(&pch->chan_sem);
 	ppp_disconnect_channel(pch);
 
+	/* Drop our reference on a bridged channel, if any */
+	if (pch->bridge) {
+		if (refcount_dec_and_test(&pch->bridge->file.refcnt))
+			ppp_destroy_channel(pch->bridge);
+		pch->bridge = NULL;
+	}
+
 	pn = ppp_pernet(pch->chan_net);
 	spin_lock_bh(&pn->all_channels_lock);
 	list_del(&pch->list);
diff --git a/include/uapi/linux/ppp-ioctl.h b/include/uapi/linux/ppp-ioctl.h
index 7bd2a5a75348..4b97ab519c19 100644
--- a/include/uapi/linux/ppp-ioctl.h
+++ b/include/uapi/linux/ppp-ioctl.h
@@ -115,6 +115,7 @@ struct pppol2tp_ioc_stats {
 #define PPPIOCATTCHAN	_IOW('t', 56, int)	/* attach to ppp channel */
 #define PPPIOCGCHAN	_IOR('t', 55, int)	/* get ppp channel number */
 #define PPPIOCGL2TPSTATS _IOR('t', 54, struct pppol2tp_ioc_stats)
+#define PPPIOCBRIDGECHAN _IOW('t', 53, int)	/* bridge one channel to another */
 
 #define SIOCGPPPSTATS   (SIOCDEVPRIVATE + 0)
 #define SIOCGPPPVER     (SIOCDEVPRIVATE + 1)	/* NEVER change this!! */
-- 
2.17.1