[PATCH v2 1/9] tty: n_gsm: fix user open not possible at responder until initiator open

[PATCH v2 1/9] tty: n_gsm: fix user open not possible at responder until initiator open

[D. Starke]

From: Daniel Starke <daniel.starke@siemens.com>

After setting up the control channel on both sides the responder side may
want to open a virtual tty to listen on until the initiator starts an
application on a user channel. The current implementation allows the
open() but no other operation, like termios. These fail with EINVAL.
The responder sided application has no means to detect an open by the
initiator sided application this way. And the initiator sided applications
usually expect the responder sided application to listen on the user
channel upon open.
Set the user channel into half-open state on responder side once a user
application opens the virtual tty to allow IO operations on it.
Furthermore, keep the user channel constipated until the initiator side
opens it to give the responder sided application the chance to detect the
new connection and to avoid data loss if the responder sided application
starts sending before the user channel is open.

Fixes: e1eaea46bb40 ("tty: n_gsm line discipline")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
---
 drivers/tty/n_gsm.c | 31 +++++++++++++++++++++++++++++--
 1 file changed, 29 insertions(+), 2 deletions(-)

This commit was not changed as there have been no comments on it in v1.

Link: https://lore.kernel.org/all/20220506144725.1946-1-daniel.starke@siemens.com/

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index fd8b86dde525..08fea3e7674d 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -1493,6 +1493,8 @@ static void gsm_dlci_close(struct gsm_dlci *dlci)
 	if (debug & 8)
 		pr_debug("DLCI %d goes closed.\n", dlci->addr);
 	dlci->state = DLCI_CLOSED;
+	/* Prevent us from sending data before the link is up again */
+	dlci->constipated = true;
 	if (dlci->addr != 0) {
 		tty_port_tty_hangup(&dlci->port, false);
 		spin_lock_irqsave(&dlci->lock, flags);
@@ -1522,6 +1524,7 @@ static void gsm_dlci_open(struct gsm_dlci *dlci)
 	del_timer(&dlci->t1);
 	/* This will let a tty open continue */
 	dlci->state = DLCI_OPEN;
+	dlci->constipated = false;
 	if (debug & 8)
 		pr_debug("DLCI %d goes open.\n", dlci->addr);
 	/* Send current modem state */
@@ -1602,6 +1605,25 @@ static void gsm_dlci_begin_open(struct gsm_dlci *dlci)
 	mod_timer(&dlci->t1, jiffies + gsm->t1 * HZ / 100);
 }
 
+/**
+ *	gsm_dlci_wait_open	-	wait for channel open procedure
+ *	@dlci: DLCI to open
+ *
+ *	Wait for a DLCI opening from the other side. Asynchronously wait until
+ *	we get a SABM and set off timers and the responses.
+ */
+static void gsm_dlci_wait_open(struct gsm_dlci *dlci)
+{
+	switch (dlci->state) {
+	case DLCI_CLOSED:
+	case DLCI_CLOSING:
+		dlci->state = DLCI_OPENING;
+		break;
+	default:
+		break;
+	}
+}
+
 /**
  *	gsm_dlci_begin_close	-	start channel open procedure
  *	@dlci: DLCI to open
@@ -1745,10 +1767,13 @@ static struct gsm_dlci *gsm_dlci_alloc(struct gsm_mux *gsm, int addr)
 	dlci->addr = addr;
 	dlci->adaption = gsm->adaption;
 	dlci->state = DLCI_CLOSED;
-	if (addr)
+	if (addr) {
 		dlci->data = gsm_dlci_data;
-	else
+		/* Prevent us from sending data before the link is up */
+		dlci->constipated = true;
+	} else {
 		dlci->data = gsm_dlci_command;
+	}
 	gsm->dlci[addr] = dlci;
 	return dlci;
 }
@@ -3163,6 +3188,8 @@ static int gsmtty_open(struct tty_struct *tty, struct file *filp)
 	/* Start sending off SABM messages */
 	if (gsm->initiator)
 		gsm_dlci_begin_open(dlci);
+	else
+		gsm_dlci_wait_open(dlci);
 	/* And wait for virtual carrier */
 	return tty_port_block_til_ready(port, tty, filp);
 }
-- 
2.34.1

[PATCH v2 2/9] tty: n_gsm: fix tty registration before control channel open

[D. Starke]

From: Daniel Starke <daniel.starke@siemens.com>

The current implementation registers/deregisters the user ttys at mux
attach/detach. That means that the user devices are available before any
control channel is open. However, user channel initialization requires an
open control channel. Furthermore, the user is not informed if the mux
restarts due to configuration changes.
Put the registration/deregistration procedure into separate function to
improve readability.
Move registration to mux activation and deregistration to mux cleanup to
keep the user devices only open as long as a control channel exists. The
user will be informed via the device driver if the mux was reconfigured in
a way that required a mux re-activation.
This makes it necessary to add T2 initialization to gsmld_open() for the
ldisc open code path (not the reconfiguration code path) to avoid deletion
of an uninitialized T2 at mux cleanup.

Fixes: d50f6dcaf22a ("tty: n_gsm: expose gsmtty device nodes at ldisc open time")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
---
 drivers/tty/n_gsm.c | 117 ++++++++++++++++++++++++++++++--------------
 1 file changed, 79 insertions(+), 38 deletions(-)

See patch 6 regarding changes since to v1.

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index 08fea3e7674d..e695956fc96c 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -235,6 +235,7 @@ struct gsm_mux {
 	struct gsm_dlci *dlci[NUM_DLCI];
 	int old_c_iflag;		/* termios c_iflag value before attach */
 	bool constipated;		/* Asked by remote to shut up */
+	bool has_devices;		/* Devices were registered */
 
 	spinlock_t tx_lock;
 	unsigned int tx_bytes;		/* TX data outstanding */
@@ -444,6 +445,68 @@ static u8 gsm_encode_modem(const struct gsm_dlci *dlci)
 	return modembits;
 }
 
+/**
+ *	gsm_register_devices	-	register all tty devices for a given mux index
+ *
+ *	@driver: the tty driver that describes the tty devices
+ *	@index:  the mux number is used to calculate the minor numbers of the
+ *	         ttys for this mux and may differ from the position in the
+ *	         mux array.
+ */
+static int gsm_register_devices(struct tty_driver *driver, unsigned int index)
+{
+	struct device *dev;
+	int i;
+	unsigned int base;
+
+	if (!driver || index >= MAX_MUX)
+		return -EINVAL;
+
+	base = index * NUM_DLCI; /* first minor for this index */
+	for (i = 1; i < NUM_DLCI; i++) {
+		/* Don't register device 0 - this is the control channel
+		 * and not a usable tty interface
+		 */
+		dev = tty_register_device(gsm_tty_driver, base + i, NULL);
+		if (IS_ERR(dev)) {
+			if (debug & 8)
+				pr_info("%s failed to register device minor %u",
+					__func__, base + i);
+			for (i--; i >= 1; i--)
+				tty_unregister_device(gsm_tty_driver, base + i);
+			return PTR_ERR(dev);
+		}
+	}
+
+	return 0;
+}
+
+/**
+ *	gsm_unregister_devices	-	unregister all tty devices for a given mux index
+ *
+ *	@driver: the tty driver that describes the tty devices
+ *	@index:  the mux number is used to calculate the minor numbers of the
+ *	         ttys for this mux and may differ from the position in the
+ *	         mux array.
+ */
+static void gsm_unregister_devices(struct tty_driver *driver,
+				   unsigned int index)
+{
+	int i;
+	unsigned int base;
+
+	if (!driver || index >= MAX_MUX)
+		return;
+
+	base = index * NUM_DLCI; /* first minor for this index */
+	for (i = 1; i < NUM_DLCI; i++) {
+		/* Don't unregister device 0 - this is the control
+		 * channel and not a usable tty interface
+		 */
+		tty_unregister_device(gsm_tty_driver, base + i);
+	}
+}
+
 /**
  *	gsm_print_packet	-	display a frame for debug
  *	@hdr: header to print before decode
@@ -2191,6 +2254,10 @@ static void gsm_cleanup_mux(struct gsm_mux *gsm, bool disc)
 	del_timer_sync(&gsm->t2_timer);
 
 	/* Free up any link layer users and finally the control channel */
+	if (gsm->has_devices) {
+		gsm_unregister_devices(gsm_tty_driver, gsm->num);
+		gsm->has_devices = false;
+	}
 	for (i = NUM_DLCI - 1; i >= 0; i--)
 		if (gsm->dlci[i])
 			gsm_dlci_release(gsm->dlci[i]);
@@ -2214,6 +2281,7 @@ static void gsm_cleanup_mux(struct gsm_mux *gsm, bool disc)
 static int gsm_activate_mux(struct gsm_mux *gsm)
 {
 	struct gsm_dlci *dlci;
+	int ret;
 
 	timer_setup(&gsm->t2_timer, gsm_control_retransmit, 0);
 	init_waitqueue_head(&gsm->event);
@@ -2225,9 +2293,14 @@ static int gsm_activate_mux(struct gsm_mux *gsm)
 	else
 		gsm->receive = gsm1_receive;
 
+	ret = gsm_register_devices(gsm_tty_driver, gsm->num);
+	if (ret)
+		return ret;
+
 	dlci = gsm_dlci_alloc(gsm, 0);
 	if (dlci == NULL)
 		return -ENOMEM;
+	gsm->has_devices = true;
 	gsm->dead = false;		/* Tty opens are now permissible */
 	return 0;
 }
@@ -2488,39 +2561,14 @@ static int gsmld_output(struct gsm_mux *gsm, u8 *data, int len)
  *	will need moving to an ioctl path.
  */
 
-static int gsmld_attach_gsm(struct tty_struct *tty, struct gsm_mux *gsm)
+static void gsmld_attach_gsm(struct tty_struct *tty, struct gsm_mux *gsm)
 {
-	unsigned int base;
-	int ret, i;
-
 	gsm->tty = tty_kref_get(tty);
 	/* Turn off tty XON/XOFF handling to handle it explicitly. */
 	gsm->old_c_iflag = tty->termios.c_iflag;
 	tty->termios.c_iflag &= (IXON | IXOFF);
-	ret =  gsm_activate_mux(gsm);
-	if (ret != 0)
-		tty_kref_put(gsm->tty);
-	else {
-		/* Don't register device 0 - this is the control channel and not
-		   a usable tty interface */
-		base = mux_num_to_base(gsm); /* Base for this MUX */
-		for (i = 1; i < NUM_DLCI; i++) {
-			struct device *dev;
-
-			dev = tty_register_device(gsm_tty_driver,
-							base + i, NULL);
-			if (IS_ERR(dev)) {
-				for (i--; i >= 1; i--)
-					tty_unregister_device(gsm_tty_driver,
-								base + i);
-				return PTR_ERR(dev);
-			}
-		}
-	}
-	return ret;
 }
 
-
 /**
  *	gsmld_detach_gsm	-	stop doing 0710 mux
  *	@tty: tty attached to the mux
@@ -2531,12 +2579,7 @@ static int gsmld_attach_gsm(struct tty_struct *tty, struct gsm_mux *gsm)
 
 static void gsmld_detach_gsm(struct tty_struct *tty, struct gsm_mux *gsm)
 {
-	unsigned int base = mux_num_to_base(gsm); /* Base for this MUX */
-	int i;
-
 	WARN_ON(tty != gsm->tty);
-	for (i = 1; i < NUM_DLCI; i++)
-		tty_unregister_device(gsm_tty_driver, base + i);
 	/* Restore tty XON/XOFF handling. */
 	gsm->tty->termios.c_iflag = gsm->old_c_iflag;
 	tty_kref_put(gsm->tty);
@@ -2629,7 +2672,6 @@ static void gsmld_close(struct tty_struct *tty)
 static int gsmld_open(struct tty_struct *tty)
 {
 	struct gsm_mux *gsm;
-	int ret;
 
 	if (tty->ops->write == NULL)
 		return -EINVAL;
@@ -2645,12 +2687,11 @@ static int gsmld_open(struct tty_struct *tty)
 	/* Attach the initial passive connection */
 	gsm->encoding = 1;
 
-	ret = gsmld_attach_gsm(tty, gsm);
-	if (ret != 0) {
-		gsm_cleanup_mux(gsm, false);
-		mux_put(gsm);
-	}
-	return ret;
+	gsmld_attach_gsm(tty, gsm);
+
+	timer_setup(&gsm->t2_timer, gsm_control_retransmit, 0);
+
+	return 0;
 }
 
 /**
-- 
2.34.1

[PATCH v2 3/9] tty: n_gsm: fix wrong queuing behavior in gsm_dlci_data_output()

[D. Starke]

From: Daniel Starke <daniel.starke@siemens.com>

1) The function drains the fifo for the given user tty/DLCI without
considering 'TX_THRESH_HI' and different to gsm_dlci_data_output_framed(),
which moves only one packet from the user side to the internal transmission
queue. We can only handle one packet at a time here if we want to allow
DLCI priority handling in gsm_dlci_data_sweep() to avoid link starvation.
2) Furthermore, the additional header octet from convergence layer type 2
is not counted against MTU. It is part of the UI/UIH frame message which
needs to be limited to MTU. Hence, it is wrong not to consider this octet.
3) Finally, the waiting user tty is not informed about freed space in its
send queue.

Take at most one packet worth of data out of the DLCI fifo to fix 1).
Limit the max user data size per packet to MTU - 1 in case of convergence
layer type 2 to leave space for the control signal octet which is added in
the later part of the function. This fixes 2).
Add tty_port_tty_wakeup() to wake up the user tty if new write space has
been made available to fix 3).

Fixes: 268e526b935e ("tty/n_gsm: avoid fifo overflow in gsm_dlci_data_output")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
---
 drivers/tty/n_gsm.c | 74 +++++++++++++++++++++++++--------------------
 1 file changed, 42 insertions(+), 32 deletions(-)

See patch 6 regarding changes since to v1.

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index e695956fc96c..0a9924445968 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -869,41 +869,51 @@ static int gsm_dlci_data_output(struct gsm_mux *gsm, struct gsm_dlci *dlci)
 {
 	struct gsm_msg *msg;
 	u8 *dp;
-	int len, total_size, size;
-	int h = dlci->adaption - 1;
+	int h, len, size;
 
-	total_size = 0;
-	while (1) {
-		len = kfifo_len(&dlci->fifo);
-		if (len == 0)
-			return total_size;
-
-		/* MTU/MRU count only the data bits */
-		if (len > gsm->mtu)
-			len = gsm->mtu;
-
-		size = len + h;
-
-		msg = gsm_data_alloc(gsm, dlci->addr, size, gsm->ftype);
-		/* FIXME: need a timer or something to kick this so it can't
-		   get stuck with no work outstanding and no buffer free */
-		if (msg == NULL)
-			return -ENOMEM;
-		dp = msg->data;
-		switch (dlci->adaption) {
-		case 1:	/* Unstructured */
-			break;
-		case 2:	/* Unstructed with modem bits.
-		Always one byte as we never send inline break data */
-			*dp++ = (gsm_encode_modem(dlci) << 1) | EA;
-			break;
-		}
-		WARN_ON(kfifo_out_locked(&dlci->fifo, dp , len, &dlci->lock) != len);
-		__gsm_data_queue(dlci, msg);
-		total_size += size;
+	/* for modem bits without break data */
+	h = ((dlci->adaption == 1) ? 0 : 1);
+
+	len = kfifo_len(&dlci->fifo);
+	if (len == 0)
+		return 0;
+
+	/* MTU/MRU count only the data bits but watch adaption mode */
+	if ((len + h) > gsm->mtu)
+		len = gsm->mtu - h;
+
+	size = len + h;
+
+	msg = gsm_data_alloc(gsm, dlci->addr, size, gsm->ftype);
+	/* FIXME: need a timer or something to kick this so it can't
+	 * get stuck with no work outstanding and no buffer free
+	 */
+	if (!msg)
+		return -ENOMEM;
+	dp = msg->data;
+	switch (dlci->adaption) {
+	case 1: /* Unstructured */
+		break;
+	case 2: /* Unstructured with modem bits.
+		 * Always one byte as we never send inline break data
+		 */
+		*dp++ = (gsm_encode_modem(dlci) << 1) | EA;
+		break;
+	default:
+		pr_err("%s: unsupported adaption %d\n", __func__,
+		       dlci->adaption);
+		break;
 	}
+
+	WARN_ON(len != kfifo_out_locked(&dlci->fifo, dp, len,
+		&dlci->lock));
+
+	/* Notify upper layer about available send space. */
+	tty_port_tty_wakeup(&dlci->port);
+
+	__gsm_data_queue(dlci, msg);
 	/* Bytes of data we used up */
-	return total_size;
+	return size;
 }
 
 /**
-- 
2.34.1

[PATCH v2 4/9] tty: n_gsm: fix missing timer to handle stalled links

[D. Starke]

From: Daniel Starke <daniel.starke@siemens.com>

The current implementation does not handle the situation that no data is in
the internal queue and needs to be sent out while the user tty fifo is
full.
Add a timer that moves more data from user tty down to the internal queue
which is then serialized on the ldisc. This timer is triggered if no data
was moved from a user tty to the internal queue within 10 * T1.

Fixes: e1eaea46bb40 ("tty: n_gsm line discipline")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
---
 drivers/tty/n_gsm.c | 43 +++++++++++++++++++++++++++++++++++--------
 1 file changed, 35 insertions(+), 8 deletions(-)

See patch 6 regarding changes since to v1.

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index 0a9924445968..3a4a2394d970 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -244,6 +244,7 @@ struct gsm_mux {
 	struct list_head tx_list;	/* Pending data packets */
 
 	/* Control messages */
+	struct timer_list kick_timer;	/* Kick TX queuing on timeout */
 	struct timer_list t2_timer;	/* Retransmit timer for commands */
 	int cretries;			/* Command retry counter */
 	struct gsm_control *pending_cmd;/* Our current pending command */
@@ -833,6 +834,7 @@ static void __gsm_data_queue(struct gsm_dlci *dlci, struct gsm_msg *msg)
 	list_add_tail(&msg->list, &gsm->tx_list);
 	gsm->tx_bytes += msg->len;
 	gsm_data_kick(gsm, dlci);
+	mod_timer(&gsm->kick_timer, jiffies + 10 * gsm->t1 * HZ / 100);
 }
 
 /**
@@ -885,9 +887,6 @@ static int gsm_dlci_data_output(struct gsm_mux *gsm, struct gsm_dlci *dlci)
 	size = len + h;
 
 	msg = gsm_data_alloc(gsm, dlci->addr, size, gsm->ftype);
-	/* FIXME: need a timer or something to kick this so it can't
-	 * get stuck with no work outstanding and no buffer free
-	 */
 	if (!msg)
 		return -ENOMEM;
 	dp = msg->data;
@@ -964,9 +963,6 @@ static int gsm_dlci_data_output_framed(struct gsm_mux *gsm,
 
 	size = len + overhead;
 	msg = gsm_data_alloc(gsm, dlci->addr, size, gsm->ftype);
-
-	/* FIXME: need a timer or something to kick this so it can't
-	   get stuck with no work outstanding and no buffer free */
 	if (msg == NULL) {
 		skb_queue_tail(&dlci->skb_list, dlci->skb);
 		dlci->skb = NULL;
@@ -1062,9 +1058,9 @@ static int gsm_dlci_modem_output(struct gsm_mux *gsm, struct gsm_dlci *dlci,
  *	renegotiate DLCI priorities with optional stuff. Needs optimising.
  */
 
-static void gsm_dlci_data_sweep(struct gsm_mux *gsm)
+static int gsm_dlci_data_sweep(struct gsm_mux *gsm)
 {
-	int len;
+	int len, ret = 0;
 	/* Priority ordering: We should do priority with RR of the groups */
 	int i = 1;
 
@@ -1087,7 +1083,11 @@ static void gsm_dlci_data_sweep(struct gsm_mux *gsm)
 		/* DLCI empty - try the next */
 		if (len == 0)
 			i++;
+		else
+			ret++;
 	}
+
+	return ret;
 }
 
 /**
@@ -1806,6 +1806,30 @@ static void gsm_dlci_command(struct gsm_dlci *dlci, const u8 *data, int len)
 	}
 }
 
+/**
+ *	gsm_kick_timer	-	transmit if possible
+ *	@t: timer contained in our gsm object
+ *
+ *	Transmit data from DLCIs if the queue is empty. We can't rely on
+ *	a tty wakeup except when we filled the pipe so we need to fire off
+ *	new data ourselves in other cases.
+ */
+static void gsm_kick_timer(struct timer_list *t)
+{
+	struct gsm_mux *gsm = from_timer(gsm, t, kick_timer);
+	unsigned long flags;
+	int sent = 0;
+
+	spin_lock_irqsave(&gsm->tx_lock, flags);
+	/* If we have nothing running then we need to fire up */
+	if (gsm->tx_bytes < TX_THRESH_LO)
+		sent = gsm_dlci_data_sweep(gsm);
+	spin_unlock_irqrestore(&gsm->tx_lock, flags);
+
+	if (sent && debug & 4)
+		pr_info("%s TX queue stalled\n", __func__);
+}
+
 /*
  *	Allocate/Free DLCI channels
  */
@@ -2261,6 +2285,7 @@ static void gsm_cleanup_mux(struct gsm_mux *gsm, bool disc)
 	}
 
 	/* Finish outstanding timers, making sure they are done */
+	del_timer_sync(&gsm->kick_timer);
 	del_timer_sync(&gsm->t2_timer);
 
 	/* Free up any link layer users and finally the control channel */
@@ -2293,6 +2318,7 @@ static int gsm_activate_mux(struct gsm_mux *gsm)
 	struct gsm_dlci *dlci;
 	int ret;
 
+	timer_setup(&gsm->kick_timer, gsm_kick_timer, 0);
 	timer_setup(&gsm->t2_timer, gsm_control_retransmit, 0);
 	init_waitqueue_head(&gsm->event);
 	spin_lock_init(&gsm->control_lock);
@@ -2699,6 +2725,7 @@ static int gsmld_open(struct tty_struct *tty)
 
 	gsmld_attach_gsm(tty, gsm);
 
+	timer_setup(&gsm->kick_timer, gsm_kick_timer, 0);
 	timer_setup(&gsm->t2_timer, gsm_control_retransmit, 0);
 
 	return 0;
-- 
2.34.1

[PATCH v2 5/9] tty: n_gsm: fix non flow control frames during mux flow off

[D. Starke]

From: Daniel Starke <daniel.starke@siemens.com>

n_gsm is based on the 3GPP 07.010 and its newer version is the 3GPP 27.010.
See https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=1516
The changes from 07.010 to 27.010 are non-functional. Therefore, I refer to
the newer 27.010 here. Chapter 5.4.6.3.6 states that FCoff stops the
transmission on all channels except the control channel. This is already
implemented in gsm_data_kick(). However, chapter 5.4.8.1 explains that this
shall result in the same behavior as software flow control on the ldisc in
advanced option mode. That means only flow control frames shall be sent
during flow off. The current implementation does not consider this case.

Change gsm_data_kick() to send only flow control frames if constipated to
abide the standard. gsm_read_ea_val() and gsm_is_flow_ctrl_msg() are
introduced as helper functions for this.
It is planned to use gsm_read_ea_val() in later code cleanups for other
functions, too.

Fixes: c01af4fec2c8 ("n_gsm : Flow control handling in Mux driver")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
---
 drivers/tty/n_gsm.c | 54 ++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 53 insertions(+), 1 deletion(-)

See patch 6 regarding changes since to v1.

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index 3a4a2394d970..5287416a917f 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -421,6 +421,27 @@ static int gsm_read_ea(unsigned int *val, u8 c)
 	return c & EA;
 }
 
+/**
+ *	gsm_read_ea_val	-	read a value until EA
+ *	@val: variable holding value
+ *	@data: buffer of data
+ *	@dlen: length of data
+ *
+ *	Processes an EA value. Updates the passed variable and
+ *	returns the processed data length.
+ */
+static unsigned int gsm_read_ea_val(unsigned int *val, const u8 *data, int dlen)
+{
+	unsigned int len = 0;
+
+	for (; dlen > 0; dlen--) {
+		len++;
+		if (gsm_read_ea(val, *data++))
+			break;
+	}
+	return len;
+}
+
 /**
  *	gsm_encode_modem	-	encode modem data bits
  *	@dlci: DLCI to encode from
@@ -727,6 +748,37 @@ static struct gsm_msg *gsm_data_alloc(struct gsm_mux *gsm, u8 addr, int len,
 	return m;
 }
 
+/**
+ *	gsm_is_flow_ctrl_msg	-	checks if flow control message
+ *	@msg: message to check
+ *
+ *	Returns true if the given message is a flow control command of the
+ *	control channel. False is returned in any other case.
+ */
+static bool gsm_is_flow_ctrl_msg(struct gsm_msg *msg)
+{
+	unsigned int cmd;
+
+	if (msg->addr > 0)
+		return false;
+
+	switch (msg->ctrl & ~PF) {
+	case UI:
+	case UIH:
+		cmd = 0;
+		if (gsm_read_ea_val(&cmd, msg->data + 2, msg->len - 2) < 1)
+			break;
+		switch (cmd & ~PF) {
+		case CMD_FCOFF:
+		case CMD_FCON:
+			return true;
+		}
+		break;
+	}
+
+	return false;
+}
+
 /**
  *	gsm_data_kick		-	poke the queue
  *	@gsm: GSM Mux
@@ -746,7 +798,7 @@ static void gsm_data_kick(struct gsm_mux *gsm, struct gsm_dlci *dlci)
 	int len;
 
 	list_for_each_entry_safe(msg, nmsg, &gsm->tx_list, list) {
-		if (gsm->constipated && msg->addr)
+		if (gsm->constipated && !gsm_is_flow_ctrl_msg(msg))
 			continue;
 		if (gsm->encoding != 0) {
 			gsm->txframe[0] = GSM1_SOF;
-- 
2.34.1

[PATCH v2 6/9] tty: n_gsm: fix deadlock and link starvation in outgoing data path

[D. Starke]

From: Daniel Starke <daniel.starke@siemens.com>

The current implementation queues up new control and user packets as needed
and processes this queue down to the ldisc in the same code path.
That means that the upper and the lower layer are hard coupled in the code.
Due to this deadlocks can happen as seen below while transmitting data,
especially during ldisc congestion. Furthermore, the data channels starve
the control channel on high transmission load on the ldisc.

Introduce an additional control channel data queue to prevent timeouts and
link hangups during ldisc congestion. This is being processed before the
user channel data queue in gsm_data_kick(), i.e. with the highest priority.
Put the queue to ldisc data path into a workqueue and trigger it whenever
new data has been put into the transmission queue. Change
gsm_dlci_data_sweep() accordingly to fill up the transmission queue until
TX_THRESH_HI. This solves the locking issue, keeps latency low and provides
good performance on high data load.
Note that now all packets from a DLCI are removed from the internal queue
if the associated DLCI was closed. This ensures that no data is sent by the
introduced write task to an already closed DLCI.

BUG: spinlock recursion on CPU#0, test_v24_loop/124
 lock: serial8250_ports+0x3a8/0x7500, .magic: dead4ead, .owner: test_v24_loop/124, .owner_cpu: 0
CPU: 0 PID: 124 Comm: test_v24_loop Tainted: G           O      5.18.0-rc2 #3
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
Call Trace:
 <IRQ>
 dump_stack_lvl+0x34/0x44
 do_raw_spin_lock+0x76/0xa0
 _raw_spin_lock_irqsave+0x72/0x80
 uart_write_room+0x3b/0xc0
 gsm_data_kick+0x14b/0x240 [n_gsm]
 gsmld_write_wakeup+0x35/0x70 [n_gsm]
 tty_wakeup+0x53/0x60
 tty_port_default_wakeup+0x1b/0x30
 serial8250_tx_chars+0x12f/0x220
 serial8250_handle_irq.part.0+0xfe/0x150
 serial8250_default_handle_irq+0x48/0x80
 serial8250_interrupt+0x56/0xa0
 __handle_irq_event_percpu+0x78/0x1f0
 handle_irq_event+0x34/0x70
 handle_fasteoi_irq+0x90/0x1e0
 __common_interrupt+0x69/0x100
 common_interrupt+0x48/0xc0
 asm_common_interrupt+0x1e/0x40
RIP: 0010:__do_softirq+0x83/0x34e
Code: 2a 0a ff 0f b7 ed c7 44 24 10 0a 00 00 00 48 c7 c7 51 2a 64 82 e8 2d
e2 d5 ff 65 66 c7 05 83 af 1e 7e 00 00 fb b8 ff ff ff ff <49> c7 c2 40 61
80 82 0f bc c5 41 89 c4 41 83 c4 01 0f 84 e6 00 00
RSP: 0018:ffffc90000003f98 EFLAGS: 00000286
RAX: 00000000ffffffff RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffff82642a51 RDI: ffffffff825bb5e7
RBP: 0000000000000200 R08: 00000008de3271a8 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000000030 R14: 0000000000000000 R15: 0000000000000000
 ? __do_softirq+0x73/0x34e
 irq_exit_rcu+0xb5/0x100
 common_interrupt+0xa4/0xc0
 </IRQ>
 <TASK>
 asm_common_interrupt+0x1e/0x40
RIP: 0010:_raw_spin_unlock_irqrestore+0x2e/0x50
Code: 00 55 48 89 fd 48 83 c7 18 53 48 89 f3 48 8b 74 24 10 e8 85 28 36 ff
48 89 ef e8 cd 58 36 ff 80 e7 02 74 01 fb bf 01 00 00 00 <e8> 3d 97 33 ff
65 8b 05 96 23 2b 7e 85 c0 74 03 5b 5d c3 0f 1f 44
RSP: 0018:ffffc9000020fd08 EFLAGS: 00000202
RAX: 0000000000000000 RBX: 0000000000000246 RCX: 0000000000000000
RDX: 0000000000000004 RSI: ffffffff8257fd74 RDI: 0000000000000001
RBP: ffff8880057de3a0 R08: 00000008de233000 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000000100 R14: 0000000000000202 R15: ffff8880057df0b8
 ? _raw_spin_unlock_irqrestore+0x23/0x50
 gsmtty_write+0x65/0x80 [n_gsm]
 n_tty_write+0x33f/0x530
 ? swake_up_all+0xe0/0xe0
 file_tty_write.constprop.0+0x1b1/0x320
 ? n_tty_flush_buffer+0xb0/0xb0
 new_sync_write+0x10c/0x190
 vfs_write+0x282/0x310
 ksys_write+0x68/0xe0
 do_syscall_64+0x3b/0x90
 entry_SYSCALL_64_after_hwframe+0x44/0xae
RIP: 0033:0x7f3e5e35c15c
Code: 8b 7c 24 08 89 c5 e8 c5 ff ff ff 89 ef 89 44 24 08 e8 58 bc 02 00 8b
44 24 08 48 83 c4 10 5d c3 48 63 ff b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff
ff 76 10 48 8b 15 fd fc 05 00 f7 d8 64 89 02 48 83
RSP: 002b:00007ffcee77cd18 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007ffcee77cd70 RCX: 00007f3e5e35c15c
RDX: 0000000000000100 RSI: 00007ffcee77cd90 RDI: 0000000000000003
RBP: 0000000000000100 R08: 0000000000000000 R09: 7efefefefefefeff
R10: 00007f3e5e3bddeb R11: 0000000000000246 R12: 00007ffcee77ce8f
R13: 0000000000000001 R14: 000056214404e010 R15: 00007ffcee77cd90
 </TASK>

Fixes: e1eaea46bb40 ("tty: n_gsm line discipline")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
---
 drivers/tty/n_gsm.c | 410 ++++++++++++++++++++++++++++++--------------
 1 file changed, 280 insertions(+), 130 deletions(-)

The previous commit was split into patch 2 to 9. The remarks regarding
gsm_read_ea_val() have been incorporated. Furthermore, the use of tasklet
was replaced by workqueue. This came with a 10-20% performance penalty at
full transmission load.
Also the wait while atomic issue has been resolved in gsmld_close(). This
was reported by the kernel test robot for the v1 patch.
Functions have not been re-grouped by their purpose to make the review
easier.

Link: https://lore.kernel.org/all/20220506144725.1946-2-daniel.starke@siemens.com/
Reported-by: kernel test robot <oliver.sang@intel.com>

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index 5287416a917f..6d68aaf1dae8 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -5,6 +5,14 @@
  *
  *	* THIS IS A DEVELOPMENT SNAPSHOT IT IS NOT A FINAL RELEASE *
  *
+ * Outgoing path:
+ * tty -> DLCI fifo -> scheduler -> GSM MUX data queue    ---o-> ldisc
+ * control message               -> GSM MUX control queue --´
+ *
+ * Incoming path:
+ * ldisc -> gsm_queue() -o--> tty
+ *                        `-> gsm_control_response()
+ *
  * TO DO:
  *	Mostly done:	ioctls for setting modes/timing
  *	Partly done:	hooks so you can pull off frames to non tty devs
@@ -210,6 +218,9 @@ struct gsm_mux {
 	/* Events on the GSM channel */
 	wait_queue_head_t event;
 
+	/* ldisc send work */
+	struct work_struct tx_work;
+
 	/* Bits for GSM mode decoding */
 
 	/* Framing Layer */
@@ -241,7 +252,8 @@ struct gsm_mux {
 	unsigned int tx_bytes;		/* TX data outstanding */
 #define TX_THRESH_HI		8192
 #define TX_THRESH_LO		2048
-	struct list_head tx_list;	/* Pending data packets */
+	struct list_head tx_ctrl_list;	/* Pending control packets */
+	struct list_head tx_data_list;	/* Pending data packets */
 
 	/* Control messages */
 	struct timer_list kick_timer;	/* Kick TX queuing on timeout */
@@ -371,6 +383,11 @@ static const u8 gsm_fcs8[256] = {
 
 static int gsmld_output(struct gsm_mux *gsm, u8 *data, int len);
 static int gsm_modem_update(struct gsm_dlci *dlci, u8 brk);
+static struct gsm_msg *gsm_data_alloc(struct gsm_mux *gsm, u8 addr, int len,
+								u8 ctrl);
+static int gsm_send_packet(struct gsm_mux *gsm, struct gsm_msg *msg);
+static void gsmld_write_trigger(struct gsm_mux *gsm);
+static void gsmld_write_task(struct work_struct *work);
 
 /**
  *	gsm_fcs_add	-	update FCS
@@ -636,57 +653,73 @@ static int gsm_stuff_frame(const u8 *input, u8 *output, int len)
  *	@cr: command/response bit seen as initiator
  *	@control:  control byte including PF bit
  *
- *	Format up and transmit a control frame. These do not go via the
- *	queueing logic as they should be transmitted ahead of data when
- *	they are needed.
- *
- *	FIXME: Lock versus data TX path
+ *	Format up and transmit a control frame. These should be transmitted
+ *	ahead of data when they are needed.
  */
-
-static void gsm_send(struct gsm_mux *gsm, int addr, int cr, int control)
+static int gsm_send(struct gsm_mux *gsm, int addr, int cr, int control)
 {
-	int len;
-	u8 cbuf[10];
-	u8 ibuf[3];
+	struct gsm_msg *msg;
+	u8 *dp;
 	int ocr;
+	unsigned long flags;
+
+	msg = gsm_data_alloc(gsm, addr, 0, control);
+	if (!msg)
+		return -ENOMEM;
 
 	/* toggle C/R coding if not initiator */
 	ocr = cr ^ (gsm->initiator ? 0 : 1);
 
-	switch (gsm->encoding) {
-	case 0:
-		cbuf[0] = GSM0_SOF;
-		cbuf[1] = (addr << 2) | (ocr << 1) | EA;
-		cbuf[2] = control;
-		cbuf[3] = EA;	/* Length of data = 0 */
-		cbuf[4] = 0xFF - gsm_fcs_add_block(INIT_FCS, cbuf + 1, 3);
-		cbuf[5] = GSM0_SOF;
-		len = 6;
-		break;
-	case 1:
-	case 2:
-		/* Control frame + packing (but not frame stuffing) in mode 1 */
-		ibuf[0] = (addr << 2) | (ocr << 1) | EA;
-		ibuf[1] = control;
-		ibuf[2] = 0xFF - gsm_fcs_add_block(INIT_FCS, ibuf, 2);
-		/* Stuffing may double the size worst case */
-		len = gsm_stuff_frame(ibuf, cbuf + 1, 3);
-		/* Now add the SOF markers */
-		cbuf[0] = GSM1_SOF;
-		cbuf[len + 1] = GSM1_SOF;
-		/* FIXME: we can omit the lead one in many cases */
-		len += 2;
-		break;
-	default:
-		WARN_ON(1);
-		return;
-	}
-	gsmld_output(gsm, cbuf, len);
-	if (!gsm->initiator) {
-		cr = cr & gsm->initiator;
-		control = control & ~PF;
+	msg->data -= 3;
+	dp = msg->data;
+	*dp++ = (addr << 2) | (ocr << 1) | EA;
+	*dp++ = control;
+
+	if (gsm->encoding == 0)
+		*dp++ = EA; /* Length of data = 0 */
+
+	*dp = 0xFF - gsm_fcs_add_block(INIT_FCS, msg->data, dp - msg->data);
+	msg->len = (dp - msg->data) + 1;
+
+	gsm_print_packet("Q->", addr, cr, control, NULL, 0);
+
+	spin_lock_irqsave(&gsm->tx_lock, flags);
+	list_add_tail(&msg->list, &gsm->tx_ctrl_list);
+	gsm->tx_bytes += msg->len;
+	spin_unlock_irqrestore(&gsm->tx_lock, flags);
+	gsmld_write_trigger(gsm);
+
+	return 0;
+}
+
+/**
+ *	gsm_dlci_clear_queues	-	remove outstanding data for a DLCI
+ *	@gsm: mux
+ *	@dlci: clear for this DLCI
+ *
+ *	Clears the data queues for a given DLCI.
+ */
+static void gsm_dlci_clear_queues(struct gsm_mux *gsm, struct gsm_dlci *dlci)
+{
+	struct gsm_msg *msg, *nmsg;
+	int addr = dlci->addr;
+	unsigned long flags;
+
+	/* Clear DLCI write fifo first */
+	spin_lock_irqsave(&dlci->lock, flags);
+	kfifo_reset(&dlci->fifo);
+	spin_unlock_irqrestore(&dlci->lock, flags);
+
+	/* Clear data packets in MUX write queue */
+	spin_lock_irqsave(&gsm->tx_lock, flags);
+	list_for_each_entry_safe(msg, nmsg, &gsm->tx_data_list, list) {
+		if (msg->addr != addr)
+			continue;
+		gsm->tx_bytes -= msg->len;
+		list_del(&msg->list);
+		kfree(msg);
 	}
-	gsm_print_packet("-->", addr, cr, control, NULL, 0);
+	spin_unlock_irqrestore(&gsm->tx_lock, flags);
 }
 
 /**
@@ -748,6 +781,46 @@ static struct gsm_msg *gsm_data_alloc(struct gsm_mux *gsm, u8 addr, int len,
 	return m;
 }
 
+/**
+ *	gsm_send_packet	-	sends a single packet
+ *	@gsm: GSM Mux
+ *	@msg: packet to send
+ *
+ *	The given packet is encoded and sent out. No memory is freed.
+ *	The caller must hold the gsm tx lock.
+ */
+static int gsm_send_packet(struct gsm_mux *gsm, struct gsm_msg *msg)
+{
+	int len, ret;
+
+
+	if (gsm->encoding == 0) {
+		gsm->txframe[0] = GSM0_SOF;
+		memcpy(gsm->txframe + 1, msg->data, msg->len);
+		gsm->txframe[msg->len + 1] = GSM0_SOF;
+		len = msg->len + 2;
+	} else {
+		gsm->txframe[0] = GSM1_SOF;
+		len = gsm_stuff_frame(msg->data, gsm->txframe + 1, msg->len);
+		gsm->txframe[len + 1] = GSM1_SOF;
+		len += 2;
+	}
+
+	if (debug & 4)
+		print_hex_dump_bytes("gsm_send_packet: ", DUMP_PREFIX_OFFSET,
+				     gsm->txframe, len);
+	gsm_print_packet("-->", msg->addr, gsm->initiator, msg->ctrl, msg->data,
+			 msg->len);
+
+	ret = gsmld_output(gsm, gsm->txframe, len);
+	if (ret <= 0)
+		return ret;
+	/* FIXME: Can eliminate one SOF in many more cases */
+	gsm->tx_bytes -= msg->len;
+
+	return 0;
+}
+
 /**
  *	gsm_is_flow_ctrl_msg	-	checks if flow control message
  *	@msg: message to check
@@ -780,61 +853,81 @@ static bool gsm_is_flow_ctrl_msg(struct gsm_msg *msg)
 }
 
 /**
- *	gsm_data_kick		-	poke the queue
+ *	gsm_data_kick	-	poke the queue
  *	@gsm: GSM Mux
- *	@dlci: DLCI sending the data
  *
  *	The tty device has called us to indicate that room has appeared in
- *	the transmit queue. Ram more data into the pipe if we have any
+ *	the transmit queue. Ram more data into the pipe if we have any.
  *	If we have been flow-stopped by a CMD_FCOFF, then we can only
- *	send messages on DLCI0 until CMD_FCON
- *
- *	FIXME: lock against link layer control transmissions
+ *	send messages on DLCI0 until CMD_FCON. The caller must hold
+ *	the gsm tx lock.
  */
-
-static void gsm_data_kick(struct gsm_mux *gsm, struct gsm_dlci *dlci)
+static int gsm_data_kick(struct gsm_mux *gsm)
 {
 	struct gsm_msg *msg, *nmsg;
-	int len;
+	struct gsm_dlci *dlci;
+	int ret;
 
-	list_for_each_entry_safe(msg, nmsg, &gsm->tx_list, list) {
+	clear_bit(TTY_DO_WRITE_WAKEUP, &gsm->tty->flags);
+
+	/* Serialize control messages and control channel messages first */
+	list_for_each_entry_safe(msg, nmsg, &gsm->tx_ctrl_list, list) {
 		if (gsm->constipated && !gsm_is_flow_ctrl_msg(msg))
+			return -EAGAIN;
+		ret = gsm_send_packet(gsm, msg);
+		switch (ret) {
+		case -ENOSPC:
+			return -ENOSPC;
+		case -ENODEV:
+			/* ldisc not open */
+			gsm->tx_bytes -= msg->len;
+			list_del(&msg->list);
+			kfree(msg);
 			continue;
-		if (gsm->encoding != 0) {
-			gsm->txframe[0] = GSM1_SOF;
-			len = gsm_stuff_frame(msg->data,
-						gsm->txframe + 1, msg->len);
-			gsm->txframe[len + 1] = GSM1_SOF;
-			len += 2;
-		} else {
-			gsm->txframe[0] = GSM0_SOF;
-			memcpy(gsm->txframe + 1 , msg->data, msg->len);
-			gsm->txframe[msg->len + 1] = GSM0_SOF;
-			len = msg->len + 2;
-		}
-
-		if (debug & 4)
-			print_hex_dump_bytes("gsm_data_kick: ",
-					     DUMP_PREFIX_OFFSET,
-					     gsm->txframe, len);
-		if (gsmld_output(gsm, gsm->txframe, len) <= 0)
+		default:
+			if (ret >= 0) {
+				list_del(&msg->list);
+				kfree(msg);
+			}
 			break;
-		/* FIXME: Can eliminate one SOF in many more cases */
-		gsm->tx_bytes -= msg->len;
-
-		list_del(&msg->list);
-		kfree(msg);
+		}
+	}
 
-		if (dlci) {
-			tty_port_tty_wakeup(&dlci->port);
-		} else {
-			int i = 0;
+	if (gsm->constipated)
+		return -EAGAIN;
 
-			for (i = 0; i < NUM_DLCI; i++)
-				if (gsm->dlci[i])
-					tty_port_tty_wakeup(&gsm->dlci[i]->port);
+	/* Serialize other channels */
+	if (list_empty(&gsm->tx_data_list))
+		return 0;
+	list_for_each_entry_safe(msg, nmsg, &gsm->tx_data_list, list) {
+		dlci = gsm->dlci[msg->addr];
+		/* Send only messages for DLCIs with valid state */
+		if (dlci->state != DLCI_OPEN) {
+			gsm->tx_bytes -= msg->len;
+			list_del(&msg->list);
+			kfree(msg);
+			continue;
+		}
+		ret = gsm_send_packet(gsm, msg);
+		switch (ret) {
+		case -ENOSPC:
+			return -ENOSPC;
+		case -ENODEV:
+			/* ldisc not open */
+			gsm->tx_bytes -= msg->len;
+			list_del(&msg->list);
+			kfree(msg);
+			continue;
+		default:
+			if (ret >= 0) {
+				list_del(&msg->list);
+				kfree(msg);
+			}
+			break;
 		}
 	}
+
+	return 1;
 }
 
 /**
@@ -883,9 +976,21 @@ static void __gsm_data_queue(struct gsm_dlci *dlci, struct gsm_msg *msg)
 	msg->data = dp;
 
 	/* Add to the actual output queue */
-	list_add_tail(&msg->list, &gsm->tx_list);
+	switch (msg->ctrl & ~PF) {
+	case UI:
+	case UIH:
+		if (msg->addr > 0) {
+			list_add_tail(&msg->list, &gsm->tx_data_list);
+			break;
+		}
+		fallthrough;
+	default:
+		list_add_tail(&msg->list, &gsm->tx_ctrl_list);
+		break;
+	}
 	gsm->tx_bytes += msg->len;
-	gsm_data_kick(gsm, dlci);
+
+	gsmld_write_trigger(gsm);
 	mod_timer(&gsm->kick_timer, jiffies + 10 * gsm->t1 * HZ / 100);
 }
 
@@ -1112,32 +1217,39 @@ static int gsm_dlci_modem_output(struct gsm_mux *gsm, struct gsm_dlci *dlci,
 
 static int gsm_dlci_data_sweep(struct gsm_mux *gsm)
 {
-	int len, ret = 0;
 	/* Priority ordering: We should do priority with RR of the groups */
-	int i = 1;
-
-	while (i < NUM_DLCI) {
-		struct gsm_dlci *dlci;
+	int i, len, ret = 0;
+	bool sent;
+	struct gsm_dlci *dlci;
 
-		if (gsm->tx_bytes > TX_THRESH_HI)
-			break;
-		dlci = gsm->dlci[i];
-		if (dlci == NULL || dlci->constipated) {
-			i++;
-			continue;
+	while (gsm->tx_bytes < TX_THRESH_HI) {
+		for (sent = false, i = 1; i < NUM_DLCI; i++) {
+			dlci = gsm->dlci[i];
+			/* skip unused or blocked channel */
+			if (!dlci || dlci->constipated)
+				continue;
+			/* skip channels with invalid state */
+			if (dlci->state != DLCI_OPEN)
+				continue;
+			/* count the sent data per adaption */
+			if (dlci->adaption < 3 && !dlci->net)
+				len = gsm_dlci_data_output(gsm, dlci);
+			else
+				len = gsm_dlci_data_output_framed(gsm, dlci);
+			/* on error exit */
+			if (len < 0)
+				return ret;
+			if (len > 0) {
+				ret++;
+				sent = true;
+				/* The lower DLCs can starve the higher DLCs! */
+				break;
+			}
+			/* try next */
 		}
-		if (dlci->adaption < 3 && !dlci->net)
-			len = gsm_dlci_data_output(gsm, dlci);
-		else
-			len = gsm_dlci_data_output_framed(gsm, dlci);
-		if (len < 0)
+		if (!sent)
 			break;
-		/* DLCI empty - try the next */
-		if (len == 0)
-			i++;
-		else
-			ret++;
-	}
+	};
 
 	return ret;
 }
@@ -1385,7 +1497,6 @@ static void gsm_control_message(struct gsm_mux *gsm, unsigned int command,
 						const u8 *data, int clen)
 {
 	u8 buf[1];
-	unsigned long flags;
 
 	switch (command) {
 	case CMD_CLD: {
@@ -1407,9 +1518,7 @@ static void gsm_control_message(struct gsm_mux *gsm, unsigned int command,
 		gsm->constipated = false;
 		gsm_control_reply(gsm, CMD_FCON, NULL, 0);
 		/* Kick the link in case it is idling */
-		spin_lock_irqsave(&gsm->tx_lock, flags);
-		gsm_data_kick(gsm, NULL);
-		spin_unlock_irqrestore(&gsm->tx_lock, flags);
+		gsmld_write_trigger(gsm);
 		break;
 	case CMD_FCOFF:
 		/* Modem wants us to STFU */
@@ -1612,8 +1721,6 @@ static int gsm_control_wait(struct gsm_mux *gsm, struct gsm_control *control)
 
 static void gsm_dlci_close(struct gsm_dlci *dlci)
 {
-	unsigned long flags;
-
 	del_timer(&dlci->t1);
 	if (debug & 8)
 		pr_debug("DLCI %d goes closed.\n", dlci->addr);
@@ -1622,17 +1729,16 @@ static void gsm_dlci_close(struct gsm_dlci *dlci)
 	dlci->constipated = true;
 	if (dlci->addr != 0) {
 		tty_port_tty_hangup(&dlci->port, false);
-		spin_lock_irqsave(&dlci->lock, flags);
-		kfifo_reset(&dlci->fifo);
-		spin_unlock_irqrestore(&dlci->lock, flags);
+		gsm_dlci_clear_queues(dlci->gsm, dlci);
 		/* Ensure that gsmtty_open() can return. */
 		tty_port_set_initialized(&dlci->port, 0);
 		wake_up_interruptible(&dlci->port.open_wait);
 	} else
 		dlci->gsm->dead = true;
-	wake_up(&dlci->gsm->event);
 	/* A DLCI 0 close is a MUX termination so we need to kick that
 	   back to userspace somehow */
+	gsm_dlci_data_kick(dlci);
+	wake_up(&dlci->gsm->event);
 }
 
 /**
@@ -1655,6 +1761,7 @@ static void gsm_dlci_open(struct gsm_dlci *dlci)
 	/* Send current modem state */
 	if (dlci->addr)
 		gsm_modem_update(dlci, 0);
+	gsm_dlci_data_kick(dlci);
 	wake_up(&dlci->gsm->event);
 }
 
@@ -2209,7 +2316,7 @@ static void gsm1_receive(struct gsm_mux *gsm, unsigned char c)
 	} else if ((c & ISO_IEC_646_MASK) == XOFF) {
 		gsm->constipated = false;
 		/* Kick the link in case it is idling */
-		gsm_data_kick(gsm, NULL);
+		gsmld_write_trigger(gsm);
 		return;
 	}
 	if (c == GSM1_SOF) {
@@ -2340,6 +2447,9 @@ static void gsm_cleanup_mux(struct gsm_mux *gsm, bool disc)
 	del_timer_sync(&gsm->kick_timer);
 	del_timer_sync(&gsm->t2_timer);
 
+	/* Finish writing to ldisc */
+	flush_work(&gsm->tx_work);
+
 	/* Free up any link layer users and finally the control channel */
 	if (gsm->has_devices) {
 		gsm_unregister_devices(gsm_tty_driver, gsm->num);
@@ -2351,9 +2461,12 @@ static void gsm_cleanup_mux(struct gsm_mux *gsm, bool disc)
 	mutex_unlock(&gsm->mutex);
 	/* Now wipe the queues */
 	tty_ldisc_flush(gsm->tty);
-	list_for_each_entry_safe(txq, ntxq, &gsm->tx_list, list)
+	list_for_each_entry_safe(txq, ntxq, &gsm->tx_ctrl_list, list)
+		kfree(txq);
+	INIT_LIST_HEAD(&gsm->tx_ctrl_list);
+	list_for_each_entry_safe(txq, ntxq, &gsm->tx_data_list, list)
 		kfree(txq);
-	INIT_LIST_HEAD(&gsm->tx_list);
+	INIT_LIST_HEAD(&gsm->tx_data_list);
 }
 
 /**
@@ -2372,6 +2485,7 @@ static int gsm_activate_mux(struct gsm_mux *gsm)
 
 	timer_setup(&gsm->kick_timer, gsm_kick_timer, 0);
 	timer_setup(&gsm->t2_timer, gsm_control_retransmit, 0);
+	INIT_WORK(&gsm->tx_work, gsmld_write_task);
 	init_waitqueue_head(&gsm->event);
 	spin_lock_init(&gsm->control_lock);
 	spin_lock_init(&gsm->tx_lock);
@@ -2481,7 +2595,8 @@ static struct gsm_mux *gsm_alloc_mux(void)
 	spin_lock_init(&gsm->lock);
 	mutex_init(&gsm->mutex);
 	kref_init(&gsm->ref);
-	INIT_LIST_HEAD(&gsm->tx_list);
+	INIT_LIST_HEAD(&gsm->tx_ctrl_list);
+	INIT_LIST_HEAD(&gsm->tx_data_list);
 
 	gsm->t1 = T1;
 	gsm->t2 = T2;
@@ -2639,6 +2754,47 @@ static int gsmld_output(struct gsm_mux *gsm, u8 *data, int len)
 	return gsm->tty->ops->write(gsm->tty, data, len);
 }
 
+
+/**
+ *	gsmld_write_trigger	-	schedule ldisc write task
+ *	@gsm: our mux
+ */
+static void gsmld_write_trigger(struct gsm_mux *gsm)
+{
+	if (!gsm || !gsm->dlci[0] || gsm->dlci[0]->dead)
+		return;
+	schedule_work(&gsm->tx_work);
+}
+
+
+/**
+ *	gsmld_write_task	-	ldisc write task
+ *	@work: our tx write work
+ *
+ *	Writes out data to the ldisc if possible. We are doing this here to
+ *	avoid dead-locking. This returns if no space or data is left for output.
+ */
+static void gsmld_write_task(struct work_struct *work)
+{
+	struct gsm_mux *gsm = container_of(work, struct gsm_mux, tx_work);
+	unsigned long flags;
+	int i, ret;
+
+	/* All outstanding control channel and control messages and one data
+	 * frame is sent.
+	 */
+	ret = -ENODEV;
+	spin_lock_irqsave(&gsm->tx_lock, flags);
+	if (gsm->tty)
+		ret = gsm_data_kick(gsm);
+	spin_unlock_irqrestore(&gsm->tx_lock, flags);
+
+	if (ret >= 0)
+		for (i = 0; i < NUM_DLCI; i++)
+			if (gsm->dlci[i])
+				tty_port_tty_wakeup(&gsm->dlci[i]->port);
+}
+
 /**
  *	gsmld_attach_gsm	-	mode set up
  *	@tty: our tty structure
@@ -2779,6 +2935,7 @@ static int gsmld_open(struct tty_struct *tty)
 
 	timer_setup(&gsm->kick_timer, gsm_kick_timer, 0);
 	timer_setup(&gsm->t2_timer, gsm_control_retransmit, 0);
+	INIT_WORK(&gsm->tx_work, gsmld_write_task);
 
 	return 0;
 }
@@ -2795,16 +2952,9 @@ static int gsmld_open(struct tty_struct *tty)
 static void gsmld_write_wakeup(struct tty_struct *tty)
 {
 	struct gsm_mux *gsm = tty->disc_data;
-	unsigned long flags;
 
 	/* Queue poll */
-	clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
-	spin_lock_irqsave(&gsm->tx_lock, flags);
-	gsm_data_kick(gsm, NULL);
-	if (gsm->tx_bytes < TX_THRESH_LO) {
-		gsm_dlci_data_sweep(gsm);
-	}
-	spin_unlock_irqrestore(&gsm->tx_lock, flags);
+	gsmld_write_trigger(gsm);
 }
 
 /**
-- 
2.34.1

[PATCH v2 7/9] tty: n_gsm: fix packet re-transmission without open control channel

[D. Starke]

From: Daniel Starke <daniel.starke@siemens.com>

In the current implementation control packets are re-transmitted even if
the control channel closed down during T2. This is wrong.
Check whether the control channel is open before re-transmitting any
packets. Note that control channel open/close is handled by T1 and not T2
and remains unaffected by this.

Fixes: e1eaea46bb40 ("tty: n_gsm line discipline")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
---
 drivers/tty/n_gsm.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

See patch 6 regarding changes since to v1.

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index 6d68aaf1dae8..dfc537eb2b49 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -1624,7 +1624,7 @@ static void gsm_control_retransmit(struct timer_list *t)
 	spin_lock_irqsave(&gsm->control_lock, flags);
 	ctrl = gsm->pending_cmd;
 	if (ctrl) {
-		if (gsm->cretries == 0) {
+		if (gsm->cretries == 0 || !gsm->dlci[0] || gsm->dlci[0]->dead) {
 			gsm->pending_cmd = NULL;
 			ctrl->error = -ETIMEDOUT;
 			ctrl->done = 1;
-- 
2.34.1

[PATCH v2 8/9] tty: n_gsm: fix resource allocation order in gsm_activate_mux()

[D. Starke]

From: Daniel Starke <daniel.starke@siemens.com>

Within gsm_activate_mux() all timers and locks are initiated before the
actual resource for the control channel is allocated. This can lead to race
conditions.

Allocate the control channel DLCI object first to avoid race conditions.

Fixes: e1eaea46bb40 ("tty: n_gsm line discipline")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
---
 drivers/tty/n_gsm.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

See patch 6 regarding changes since to v1.

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index dfc537eb2b49..cc349f5a37fb 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -2483,6 +2483,10 @@ static int gsm_activate_mux(struct gsm_mux *gsm)
 	struct gsm_dlci *dlci;
 	int ret;
 
+	dlci = gsm_dlci_alloc(gsm, 0);
+	if (dlci == NULL)
+		return -ENOMEM;
+
 	timer_setup(&gsm->kick_timer, gsm_kick_timer, 0);
 	timer_setup(&gsm->t2_timer, gsm_control_retransmit, 0);
 	INIT_WORK(&gsm->tx_work, gsmld_write_task);
@@ -2499,9 +2503,6 @@ static int gsm_activate_mux(struct gsm_mux *gsm)
 	if (ret)
 		return ret;
 
-	dlci = gsm_dlci_alloc(gsm, 0);
-	if (dlci == NULL)
-		return -ENOMEM;
 	gsm->has_devices = true;
 	gsm->dead = false;		/* Tty opens are now permissible */
 	return 0;
-- 
2.34.1

[PATCH v2 9/9] tty: n_gsm: fix race condition in gsmld_write()

[D. Starke]

From: Daniel Starke <daniel.starke@siemens.com>

The function may be used by the user directly and also by the n_gsm
internal functions. They can lead into a race condition which results in
interleaved frames if both are writing at the same time. The receiving side
is not able to decode those interleaved frames correctly.

Add a lock around the low side tty write to avoid race conditions and frame
interleaving between user originated writes and n_gsm writes.

Fixes: e1eaea46bb40 ("tty: n_gsm line discipline")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
---
 drivers/tty/n_gsm.c | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

See patch 6 regarding changes since to v1.

diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
index cc349f5a37fb..640c0f0aa7b1 100644
--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -2999,11 +2999,24 @@ static ssize_t gsmld_read(struct tty_struct *tty, struct file *file,
 static ssize_t gsmld_write(struct tty_struct *tty, struct file *file,
 			   const unsigned char *buf, size_t nr)
 {
-	int space = tty_write_room(tty);
+	struct gsm_mux *gsm = tty->disc_data;
+	unsigned long flags;
+	int space;
+	int ret;
+
+	if (!gsm)
+		return -ENODEV;
+
+	ret = -ENOBUFS;
+	spin_lock_irqsave(&gsm->tx_lock, flags);
+	space = tty_write_room(tty);
 	if (space >= nr)
-		return tty->ops->write(tty, buf, nr);
-	set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
-	return -ENOBUFS;
+		ret = tty->ops->write(tty, buf, nr);
+	else
+		set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
+	spin_unlock_irqrestore(&gsm->tx_lock, flags);
+
+	return ret;
 }
 
 /**
-- 
2.34.1

Re: [PATCH v2 1/9] tty: n_gsm: fix user open not possible at responder until initiator open

[Greg KH]

On Thu, May 19, 2022 at 09:07:49AM +0200, D. Starke wrote:
> From: Daniel Starke <daniel.starke@siemens.com>
> 
> After setting up the control channel on both sides the responder side may
> want to open a virtual tty to listen on until the initiator starts an
> application on a user channel. The current implementation allows the
> open() but no other operation, like termios. These fail with EINVAL.
> The responder sided application has no means to detect an open by the
> initiator sided application this way. And the initiator sided applications
> usually expect the responder sided application to listen on the user
> channel upon open.
> Set the user channel into half-open state on responder side once a user
> application opens the virtual tty to allow IO operations on it.
> Furthermore, keep the user channel constipated until the initiator side
> opens it to give the responder sided application the chance to detect the
> new connection and to avoid data loss if the responder sided application
> starts sending before the user channel is open.
> 
> Fixes: e1eaea46bb40 ("tty: n_gsm line discipline")
> Cc: stable@vger.kernel.org
> Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
> ---
>  drivers/tty/n_gsm.c | 31 +++++++++++++++++++++++++++++--
>  1 file changed, 29 insertions(+), 2 deletions(-)
> 
> This commit was not changed as there have been no comments on it in v1.
> 
> Link: https://lore.kernel.org/all/20220506144725.1946-1-daniel.starke@siemens.com/
> 
> diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
> index fd8b86dde525..08fea3e7674d 100644
> --- a/drivers/tty/n_gsm.c
> +++ b/drivers/tty/n_gsm.c
> @@ -1493,6 +1493,8 @@ static void gsm_dlci_close(struct gsm_dlci *dlci)
>  	if (debug & 8)
>  		pr_debug("DLCI %d goes closed.\n", dlci->addr);
>  	dlci->state = DLCI_CLOSED;
> +	/* Prevent us from sending data before the link is up again */
> +	dlci->constipated = true;
>  	if (dlci->addr != 0) {
>  		tty_port_tty_hangup(&dlci->port, false);
>  		spin_lock_irqsave(&dlci->lock, flags);
> @@ -1522,6 +1524,7 @@ static void gsm_dlci_open(struct gsm_dlci *dlci)
>  	del_timer(&dlci->t1);
>  	/* This will let a tty open continue */
>  	dlci->state = DLCI_OPEN;
> +	dlci->constipated = false;
>  	if (debug & 8)
>  		pr_debug("DLCI %d goes open.\n", dlci->addr);
>  	/* Send current modem state */
> @@ -1602,6 +1605,25 @@ static void gsm_dlci_begin_open(struct gsm_dlci *dlci)
>  	mod_timer(&dlci->t1, jiffies + gsm->t1 * HZ / 100);
>  }
>  
> +/**
> + *	gsm_dlci_wait_open	-	wait for channel open procedure
> + *	@dlci: DLCI to open
> + *
> + *	Wait for a DLCI opening from the other side. Asynchronously wait until
> + *	we get a SABM and set off timers and the responses.
> + */
> +static void gsm_dlci_wait_open(struct gsm_dlci *dlci)
> +{
> +	switch (dlci->state) {
> +	case DLCI_CLOSED:
> +	case DLCI_CLOSING:
> +		dlci->state = DLCI_OPENING;
> +		break;
> +	default:
> +		break;
> +	}
> +}

The documentation for this function is odd, you are not waiting for
anything.  You are just changing the state.  This makes no sense as-is,
sorry.

greg k-h

Re: [PATCH v2 4/9] tty: n_gsm: fix missing timer to handle stalled links

[Jiri Slaby]

On 19. 05. 22, 9:07, D. Starke wrote:
> From: Daniel Starke <daniel.starke@siemens.com>
> 
> The current implementation does not handle the situation that no data is in
> the internal queue and needs to be sent out while the user tty fifo is
> full.
> Add a timer that moves more data from user tty down to the internal queue
> which is then serialized on the ldisc. This timer is triggered if no data
> was moved from a user tty to the internal queue within 10 * T1.
> 
> Fixes: e1eaea46bb40 ("tty: n_gsm line discipline")
> Cc: stable@vger.kernel.org
> Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
> ---
>   drivers/tty/n_gsm.c | 43 +++++++++++++++++++++++++++++++++++--------
>   1 file changed, 35 insertions(+), 8 deletions(-)
> 
> See patch 6 regarding changes since to v1.
> 
> diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
> index 0a9924445968..3a4a2394d970 100644
> --- a/drivers/tty/n_gsm.c
> +++ b/drivers/tty/n_gsm.c
...
> @@ -833,6 +834,7 @@ static void __gsm_data_queue(struct gsm_dlci *dlci, struct gsm_msg *msg)
>   	list_add_tail(&msg->list, &gsm->tx_list);
>   	gsm->tx_bytes += msg->len;
>   	gsm_data_kick(gsm, dlci);
> +	mod_timer(&gsm->kick_timer, jiffies + 10 * gsm->t1 * HZ / 100);

The formula deserves an explanation. And why 10 * X / 100, and not X / 10?

> @@ -1062,9 +1058,9 @@ static int gsm_dlci_modem_output(struct gsm_mux *gsm, struct gsm_dlci *dlci,
>    *	renegotiate DLCI priorities with optional stuff. Needs optimising.
>    */
>   
> -static void gsm_dlci_data_sweep(struct gsm_mux *gsm)
> +static int gsm_dlci_data_sweep(struct gsm_mux *gsm)
>   {
> -	int len;
> +	int len, ret = 0;

Why is ret signed?

>   	/* Priority ordering: We should do priority with RR of the groups */
>   	int i = 1;
>   
> @@ -1087,7 +1083,11 @@ static void gsm_dlci_data_sweep(struct gsm_mux *gsm)
>   		/* DLCI empty - try the next */
>   		if (len == 0)
>   			i++;
> +		else
> +			ret++;
>   	}
> +
> +	return ret;
>   }
>   
>   /**

thanks,
-- 
js
suse labs