From ac77f0077c3265197d378158c85a55eee6d21508 Mon Sep 17 00:00:00 2001
From: Daniel Starke <daniel.starke@siemens.com>
Date: Fri, 1 Jul 2022 08:16:44 +0200
Subject: [PATCH] tty: n_gsm: fix user open not possible at responder until initiator open
Git-commit: ac77f0077c3265197d378158c85a55eee6d21508
Patch-mainline: v6.0-rc1
References: git-fixes

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")
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20220701061652.39604-1-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Takashi Iwai <tiwai@suse.de>

---
 drivers/tty/n_gsm.c |   31 +++++++++++++++++++++++++++++--
 1 file changed, 29 insertions(+), 2 deletions(-)

--- a/drivers/tty/n_gsm.c
+++ b/drivers/tty/n_gsm.c
@@ -1510,6 +1510,8 @@ static void gsm_dlci_close(struct gsm_dl
 	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);
@@ -1539,6 +1541,7 @@ static void gsm_dlci_open(struct gsm_dlc
 	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 */
@@ -1620,6 +1623,25 @@ static void gsm_dlci_begin_open(struct g
 }
 
 /**
+ *	gsm_dlci_set_opening	-	change state to opening
+ *	@dlci: DLCI to open
+ *
+ *	Change internal state to wait for DLCI open from initiator side.
+ *	We set off timers and responses upon reception of an SABM.
+ */
+static void gsm_dlci_set_opening(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
  *
@@ -1762,10 +1784,13 @@ static struct gsm_dlci *gsm_dlci_alloc(s
 	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;
 }
@@ -3174,6 +3199,8 @@ static int gsmtty_open(struct tty_struct
 	/* Start sending off SABM messages */
 	if (gsm->initiator)
 		gsm_dlci_begin_open(dlci);
+	else
+		gsm_dlci_set_opening(dlci);
 	/* And wait for virtual carrier */
 	return tty_port_block_til_ready(port, tty, filp);
 }