[PATCH v2] pty: show associative slave of ptmx in fdinfo

[Masatake YAMATO]

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.

Change in v2:

        Remove all #ifdef CONFIG_PROC_FS/#endif.
        Suggested by Greg KH.

Signed-off-by: Masatake YAMATO <yam...@redhat.com>
---
 drivers/tty/pty.c          | 8 +++++++-
 drivers/tty/tty_io.c       | 9 +++++++++
 include/linux/tty_driver.h | 2 ++
 3 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index d1399aa..ce27be4 100644
--- a/drivers/tty/pty.c
+++ b/drivers/tty/pty.c
@@ -742,6 +742,11 @@ 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,
@@ -756,7 +761,8 @@ 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 = {
diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 974b13d..c3c8fd0 100644
--- a/drivers/tty/tty_io.c
+++ b/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 = {
diff --git a/include/linux/tty_driver.h b/include/linux/tty_driver.h
index b742b5e..570b05b 100644
--- a/include/linux/tty_driver.h
+++ b/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);
-- 
2.9.4