[PATCH] EXTPROC, telnetd LINEMODE, revisited

[Howard Chu <hyc@symas.com>]

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Howard Chu wrote:
> Andi Kleen wrote:
>> Howard Chu<hyc@symas.com>   writes:
>>
>>> It's been over 10 years since I looked at this last
>>>
>>> http://lkml.indiana.edu/hypermail/linux/kernel/9911.3/0650.html
>>
>> I would suggest you repost the patch.
>
> Looks like my email got posted twice already (oops). The updated patch was
> attached each time, you didn't get it?
>
>>>  From a quick look it looks straight forward enough.
>
> The patch I posted still isn't quite right; it lets all of the input fall thru
> the regular tty input code. If ICANON is set then the tty driver will parse
> and act on any control characters in the input, but since the input was
> already fully processed on the client, any control characters remaining in the
> input should just be passed through literally. That should be an easy thing to
> fix though.

This one works on all aspects. My previous patch had a collision in the 
TIOCSIG ioctl command number, obviously other ioctls had gotten added in the 
intervening years. The ioctl definition will need to be added to the other 
arch-specific asm header files as well, but I figured that can be taken care 
of easily assuming the overall patch is OK.

The patched telnetd will generate signals for interrupt commands instead 
inserting their characters into the input buffer, so the input routines don't 
need to check for those control characters explicitly. Unfortunately there's 
no signal for the EOF command, so it still gets inserted as an EOF character. 
To preserve its functionality, I note that it can only have its command 
significance if it is the last character in the buffer. Otherwise, if it was 
input using LiteralNext, it will always be followed by some other character.

So this is now working well for telnet/telnetd. I'm moving on now to look at 
patching support into ssh and sshd. The approach will be to incorporate 
readline into the ssh client so that it can manage the command history buffer 
locally, and to patch readline to check for EXTPROC on the tty. If it's set 
then readline will just pass-thru instead of trying to set any editing modes.

For anyone interested, the telnet/telnetd patches are in debian bug#585527
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=585527

You will need to patch some headers in /usr/include to get the ioctl and 
termios bits for telnetd to compile. Those definitions are in the patch 
attached to this email.
-- 
   -- Howard Chu
   CTO, Symas Corp.           http://www.symas.com
   Director, Highland Sun     http://highlandsun.com/hyc/
   Chief Architect, OpenLDAP  http://www.openldap.org/project/

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diff --git a/drivers/char/n_tty.c b/drivers/char/n_tty.c
index bdae832..3bc496a 100644
--- a/drivers/char/n_tty.c
+++ b/drivers/char/n_tty.c
@@ -1102,6 +1102,11 @@ static inline void n_tty_receive_char(struct tty_struct *tty, unsigned char c)
 	if (I_IUCLC(tty) && L_IEXTEN(tty))
 		c = tolower(c);
 
+	if (L_EXTPROC(tty)) {
+		put_tty_queue(c, tty);
+		return;
+	}
+
 	if (tty->stopped && !tty->flow_stopped && I_IXON(tty) &&
 	    I_IXANY(tty) && c != START_CHAR(tty) && c != STOP_CHAR(tty) &&
 	    c != INTR_CHAR(tty) && c != QUIT_CHAR(tty) && c != SUSP_CHAR(tty)) {
@@ -1409,7 +1414,7 @@ static void n_tty_receive_buf(struct tty_struct *tty, const unsigned char *cp,
 
 	n_tty_set_room(tty);
 
-	if (!tty->icanon && (tty->read_cnt >= tty->minimum_to_wake)) {
+	if ((!tty->icanon && (tty->read_cnt >= tty->minimum_to_wake)) || L_EXTPROC(tty)) {
 		kill_fasync(&tty->fasync, SIGIO, POLL_IN);
 		if (waitqueue_active(&tty->read_wait))
 			wake_up_interruptible(&tty->read_wait);
@@ -1585,7 +1590,7 @@ static int n_tty_open(struct tty_struct *tty)
 static inline int input_available_p(struct tty_struct *tty, int amt)
 {
 	tty_flush_to_ldisc(tty);
-	if (tty->icanon) {
+	if (tty->icanon && !L_EXTPROC(tty)) {
 		if (tty->canon_data)
 			return 1;
 	} else if (tty->read_cnt >= (amt ? amt : 1))
@@ -1632,6 +1637,11 @@ static int copy_from_read_buf(struct tty_struct *tty,
 		spin_lock_irqsave(&tty->read_lock, flags);
 		tty->read_tail = (tty->read_tail + n) & (N_TTY_BUF_SIZE-1);
 		tty->read_cnt -= n;
+		/* Check if last character is EOF */
+		if (L_EXTPROC(tty) && tty->icanon) {
+			if (!tty->read_cnt && *b[n-1] == EOF_CHAR(tty))
+				n--;
+		}
 		spin_unlock_irqrestore(&tty->read_lock, flags);
 		*b += n;
 		*nr -= n;
@@ -1767,6 +1777,13 @@ do_it_again:
 				break;
 			}
 			nr--;
+			if (cs & TIOCPKT_IOCTL) {
+				c = sizeof(struct termios);
+				if (c > nr) c = nr;
+				copy_to_user(b, tty->link->termios, c);
+				nr -= c;
+				b += c;
+			}
 			break;
 		}
 		/* This statement must be first before checking for input
@@ -1812,7 +1829,7 @@ do_it_again:
 			nr--;
 		}
 
-		if (tty->icanon) {
+		if (tty->icanon && !L_EXTPROC(tty)) {
 			/* N.B. avoid overrun if nr == 0 */
 			while (nr && tty->read_cnt) {
 				int eol;
diff --git a/drivers/char/pty.c b/drivers/char/pty.c
index d83a431..c429e9f 100644
--- a/drivers/char/pty.c
+++ b/drivers/char/pty.c
@@ -171,6 +171,15 @@ static int pty_set_lock(struct tty_struct *tty, int __user *arg)
 	return 0;
 }
 
+/* Send a signal to the slave */
+static int pty_signal(struct tty_struct *tty, int sig)
+{
+	if (tty->link) {
+		kill_pgrp(tty->link->pgrp, sig, 1);
+	}
+	return 0;
+}
+
 static void pty_flush_buffer(struct tty_struct *tty)
 {
 	struct tty_struct *to = tty->link;
@@ -321,6 +330,8 @@ static int pty_bsd_ioctl(struct tty_struct *tty, struct file *file,
 	switch (cmd) {
 	case TIOCSPTLCK: /* Set PT Lock (disallow slave open) */
 		return pty_set_lock(tty, (int __user *) arg);
+	case TIOCSIG:    /* Send signal to other side of pty */
+		return pty_signal(tty, (int) arg);
 	}
 	return -ENOIOCTLCMD;
 }
@@ -476,6 +487,8 @@ static int pty_unix98_ioctl(struct tty_struct *tty, struct file *file,
 		return pty_set_lock(tty, (int __user *)arg);
 	case TIOCGPTN: /* Get PT Number */
 		return put_user(tty->index, (unsigned int __user *)arg);
+	case TIOCSIG:    /* Send signal to other side of pty */
+		return pty_signal(tty, (int) arg);
 	}
 
 	return -ENOIOCTLCMD;
diff --git a/drivers/char/tty_ioctl.c b/drivers/char/tty_ioctl.c
index 6bd5f88..0c18899 100644
--- a/drivers/char/tty_ioctl.c
+++ b/drivers/char/tty_ioctl.c
@@ -517,19 +517,25 @@ static void change_termios(struct tty_struct *tty, struct ktermios *new_termios)
 
 	/* See if packet mode change of state. */
 	if (tty->link && tty->link->packet) {
+		int extproc = (old_termios.c_lflag & EXTPROC) |
+				(tty->termios->c_lflag & EXTPROC);
 		int old_flow = ((old_termios.c_iflag & IXON) &&
 				(old_termios.c_cc[VSTOP] == '\023') &&
 				(old_termios.c_cc[VSTART] == '\021'));
 		int new_flow = (I_IXON(tty) &&
 				STOP_CHAR(tty) == '\023' &&
 				START_CHAR(tty) == '\021');
-		if (old_flow != new_flow) {
+		if ((old_flow != new_flow) || extproc) {
 			spin_lock_irqsave(&tty->ctrl_lock, flags);
-			tty->ctrl_status &= ~(TIOCPKT_DOSTOP | TIOCPKT_NOSTOP);
-			if (new_flow)
-				tty->ctrl_status |= TIOCPKT_DOSTOP;
-			else
-				tty->ctrl_status |= TIOCPKT_NOSTOP;
+			if (old_flow != new_flow) {
+				tty->ctrl_status &= ~(TIOCPKT_DOSTOP | TIOCPKT_NOSTOP);
+				if (new_flow)
+					tty->ctrl_status |= TIOCPKT_DOSTOP;
+				else
+					tty->ctrl_status |= TIOCPKT_NOSTOP;
+			}
+			if (extproc)
+				tty->ctrl_status |= TIOCPKT_IOCTL;
 			spin_unlock_irqrestore(&tty->ctrl_lock, flags);
 			wake_up_interruptible(&tty->link->read_wait);
 		}
diff --git a/fs/compat_ioctl.c b/fs/compat_ioctl.c
index 641640d..766636d 100644
--- a/fs/compat_ioctl.c
+++ b/fs/compat_ioctl.c
@@ -966,6 +966,7 @@ COMPATIBLE_IOCTL(TIOCGPGRP)
 COMPATIBLE_IOCTL(TIOCGPTN)
 COMPATIBLE_IOCTL(TIOCSPTLCK)
 COMPATIBLE_IOCTL(TIOCSERGETLSR)
+COMPATIBLE_IOCTL(TIOCSIG)
 #ifdef TCGETS2
 COMPATIBLE_IOCTL(TCGETS2)
 COMPATIBLE_IOCTL(TCSETS2)
diff --git a/include/asm-generic/ioctls.h b/include/asm-generic/ioctls.h
index a799e20..87661c8 100644
--- a/include/asm-generic/ioctls.h
+++ b/include/asm-generic/ioctls.h
@@ -69,6 +69,7 @@
 #define TCSETX		0x5433
 #define TCSETXF		0x5434
 #define TCSETXW		0x5435
+#define TIOCSIG		_IOW('T', 0x36, int)  /* pty: generate signal */
 
 #define FIONCLEX	0x5450
 #define FIOCLEX		0x5451
@@ -104,6 +105,7 @@
 #define TIOCPKT_START		 8
 #define TIOCPKT_NOSTOP		16
 #define TIOCPKT_DOSTOP		32
+#define TIOCPKT_IOCTL		64
 
 #define TIOCSER_TEMT	0x01	/* Transmitter physically empty */
 
diff --git a/include/asm-generic/termbits.h b/include/asm-generic/termbits.h
index 1c9773d..232b478 100644
--- a/include/asm-generic/termbits.h
+++ b/include/asm-generic/termbits.h
@@ -178,6 +178,7 @@ struct ktermios {
 #define FLUSHO	0010000
 #define PENDIN	0040000
 #define IEXTEN	0100000
+#define EXTPROC	0200000
 
 /* tcflow() and TCXONC use these */
 #define	TCOOFF		0
diff --git a/include/linux/tty.h b/include/linux/tty.h
index 4409967..3cfe448 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -178,6 +178,7 @@ struct tty_bufhead {
 #define L_FLUSHO(tty)	_L_FLAG((tty), FLUSHO)
 #define L_PENDIN(tty)	_L_FLAG((tty), PENDIN)
 #define L_IEXTEN(tty)	_L_FLAG((tty), IEXTEN)
+#define L_EXTPROC(tty)	_L_FLAG((tty), EXTPROC)
 
 struct device;
 struct signal_struct;