pty: show associative slave of ptmx in fdinfo

This patch adds "tty-index" field to /proc/PID/fdinfo/N if N
specifies /dev/ptmx. The field shows the index of associative
slave pts.

Though a minor number is given for each pts instance, ptmx is not.
It means there is no way in user-space to know the association between
file descriptors for pts/n and ptmx. (n = 0, 1, ...)

This is different from pipe. About pipe such association can be solved
by inode of pipefs.

Providing the way to know the association between pts/n and ptmx helps
users understand the status of running system. lsof can utilize this field.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/pty.c

@@ -741,6 +741,11 @@ static void pty_unix98_remove(struct tty_driver *driver, struct tty_struct *tty)
 	}
 }
 
+static void pty_show_fdinfo(struct tty_struct *tty, struct seq_file *m)
+{
+	seq_printf(m, "tty-index:\t%d\n", tty->index);
+}
+
 static const struct tty_operations ptm_unix98_ops = {
 	.lookup = ptm_unix98_lookup,
 	.install = pty_unix98_install,
@@ -755,7 +760,8 @@ static const struct tty_operations ptm_unix98_ops = {
 	.ioctl = pty_unix98_ioctl,
 	.compat_ioctl = pty_unix98_compat_ioctl,
 	.resize = pty_resize,
-	.cleanup = pty_cleanup
+	.cleanup = pty_cleanup,
+	.show_fdinfo = pty_show_fdinfo,
 };
 
 static const struct tty_operations pty_unix98_ops = {

drivers/tty/tty_io.c

@@ -462,6 +462,14 @@ static int hung_up_tty_fasync(int fd, struct file *file, int on)
 	return -ENOTTY;
 }
 
+static void tty_show_fdinfo(struct seq_file *m, struct file *file)
+{
+	struct tty_struct *tty = file_tty(file);
+
+	if (tty && tty->ops && tty->ops->show_fdinfo)
+		tty->ops->show_fdinfo(tty, m);
+}
+
 static const struct file_operations tty_fops = {
 	.llseek		= no_llseek,
 	.read		= tty_read,
@@ -472,6 +480,7 @@ static const struct file_operations tty_fops = {
 	.open		= tty_open,
 	.release	= tty_release,
 	.fasync		= tty_fasync,
+	.show_fdinfo	= tty_show_fdinfo,
 };
 
 static const struct file_operations console_fops = {

include/linux/tty_driver.h

@@ -243,6 +243,7 @@
 #include <linux/list.h>
 #include <linux/cdev.h>
 #include <linux/termios.h>
+#include <linux/seq_file.h>
 
 struct tty_struct;
 struct tty_driver;
@@ -285,6 +286,7 @@ struct tty_operations {
 	int (*set_termiox)(struct tty_struct *tty, struct termiox *tnew);
 	int (*get_icount)(struct tty_struct *tty,
 				struct serial_icounter_struct *icount);
+	void (*show_fdinfo)(struct tty_struct *tty, struct seq_file *m);
 #ifdef CONFIG_CONSOLE_POLL
 	int (*poll_init)(struct tty_driver *driver, int line, char *options);
 	int (*poll_get_char)(struct tty_driver *driver, int line);