[PATCH] tty: Add a new file /proc/tty/consoles

["Dr. Werner Fink" <werner-l3A5Bk7waGM@public.gmane.org>]

 Add a new file /proc/tty/consoles to be able to
 determine the registered system console lines.
 If the reading process holds /dev/console open at
 the regular standard input stream the active device
 will be marked by an asterisk.

Signed-off-by: Werner Fink <werner-l3A5Bk7waGM@public.gmane.org>
---
 Documentation/filesystems/proc.txt |   16 ++++
 fs/proc/proc_tty.c                 |  138 ++++++++++++++++++++++++++++++++++++
 2 files changed, 154 insertions(+), 0 deletions(-)

diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
index 9fb6cbe..a5b6dd5 100644
--- a/Documentation/filesystems/proc.txt
+++ b/Documentation/filesystems/proc.txt
@@ -1074,6 +1074,7 @@ Table 1-11: Files in /proc/tty
  drivers       list of drivers and their usage                
  ldiscs        registered line disciplines                    
  driver/serial usage statistic and status of single tty lines 
+ consoles      registered system console lines
 ..............................................................................
 
 To see  which  tty's  are  currently in use, you can simply look into the file
@@ -1092,6 +1093,21 @@ To see  which  tty's  are  currently in use, you can simply look into the file
   /dev/tty             /dev/tty        5       0 system:/dev/tty 
   unknown              /dev/tty        4    1-63 console 
 
+To see which character device lines are currently used for the system console
+/dev/console, you may simply look into the file /proc/tty/consoles:
+
+  > cat /proc/tty/consoles
+  tty0                 4:7
+  ttyS0                4:64
+
+If the reading process holds /dev/console open at the regular standard input
+stream the active device will be marked by an asterisk:
+
+  > cat /proc/tty/consoles < /dev/console
+  *tty0                4:7
+  > tty
+  /dev/pts/3
+
 
 1.8 Miscellaneous kernel statistics in /proc/stat
 -------------------------------------------------
diff --git a/fs/proc/proc_tty.c b/fs/proc/proc_tty.c
index 83adcc8..f717e84 100644
--- a/fs/proc/proc_tty.c
+++ b/fs/proc/proc_tty.c
@@ -12,7 +12,10 @@
 #include <linux/proc_fs.h>
 #include <linux/stat.h>
 #include <linux/tty.h>
+#include <linux/tty_driver.h>
+#include <linux/console.h>
 #include <linux/seq_file.h>
+#include <linux/fdtable.h>
 #include <linux/bitops.h>
 
 /*
@@ -137,6 +140,140 @@ static const struct file_operations proc_tty_drivers_operations = {
 };
 
 /*
+ * The device ID of file descriptor 0 of the current reading
+ * task if a character device...
+ */
+static dev_t current_dev;
+
+/*
+ * This is the handler for /proc/tty/consoles
+ */
+static int show_console_dev(struct seq_file *m, void *v)
+{
+	const struct tty_driver * driver;
+	struct console * con;
+	int index, len;
+	dev_t dev;
+
+	if (v == SEQ_START_TOKEN)
+		return 0;
+	con = (struct console *)v;
+	if (!con)
+		return 0;
+	driver = con->device(con, &index);
+	if (!driver)
+		return 0;
+
+	dev = MKDEV(driver->major, driver->minor_start) + index;
+
+	seq_printf(m, "%s%s%d%n", (current_dev == dev) ? "*" : "",
+			con->name, con->index, &len);
+	len = 19 - len;
+	if (len < 1)
+		len = 1;
+	seq_printf(m, "%*c%d:%d\n", len, ' ', MAJOR(dev), MINOR(dev));
+
+	return 0;
+}
+
+/* iterator for consoles */
+static void *c_start(struct seq_file *m, loff_t *pos)
+{
+	struct console * con;
+	loff_t off = 0;
+
+	if (*pos == 0)
+		return SEQ_START_TOKEN;
+
+	acquire_console_sem();
+	for (con = console_drivers; con; con = con->next) {
+		if (!con->device)
+			continue;
+		if (!con->write)
+			continue;
+		if ((con->flags & CON_ENABLED) == 0)
+			continue;
+		if (++off == *pos)
+			break;
+	}
+	release_console_sem();
+
+	return con;
+}
+
+static void *c_next(struct seq_file *m, void *v, loff_t *pos)
+{
+	struct console * con;
+
+	acquire_console_sem();
+ 	if (v == SEQ_START_TOKEN)
+		con = console_drivers;
+	else
+		con = ((struct console *)v)->next;
+	for (; con; con = con->next) {
+		if (!con->device)
+			continue;
+		if (!con->write)
+			continue;
+		if ((con->flags & CON_ENABLED) == 0)
+			continue;
+		++*pos;
+		break;
+	}
+	release_console_sem();
+
+	return con;
+}
+
+static void c_stop(struct seq_file *m, void *v)
+{
+}
+
+static const struct seq_operations tty_consoles_op = {
+	.start	= c_start,
+	.next	= c_next,
+	.stop	= c_stop,
+	.show	= show_console_dev
+};
+
+static int tty_consoles_open(struct inode *inode, struct file *file)
+{
+	struct files_struct * curfiles;
+
+	current_dev = 0;
+	curfiles = get_files_struct(current);
+	if (curfiles) {
+		const struct file * curfp;
+		spin_lock(&curfiles->file_lock);
+		curfp = fcheck_files(curfiles, 0);
+		if (curfp && curfp->private_data) {
+			const struct inode * inode;
+			dget(curfp->f_dentry);
+			inode = curfp->f_dentry->d_inode;
+			if (S_ISCHR(inode->i_mode)) {
+				struct tty_struct * tty;
+				tty = (struct tty_struct *)curfp->private_data;
+				if (tty && tty->magic == TTY_MAGIC) {
+					tty = tty_pair_get_tty(tty);
+					current_dev = tty_devnum(tty);
+				}
+			}
+			dput(curfp->f_dentry);
+		}
+		spin_unlock(&curfiles->file_lock);
+		put_files_struct(curfiles);
+	}
+	return seq_open(file, &tty_consoles_op);
+}
+
+static const struct file_operations proc_tty_consoles_operations = {
+	.open		= tty_consoles_open,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.release	= seq_release,
+};
+
+/*
  * This function is called by tty_register_driver() to handle
  * registering the driver's /proc handler into /proc/tty/driver/<foo>
  */
@@ -186,4 +323,5 @@ void __init proc_tty_init(void)
 	proc_tty_driver = proc_mkdir_mode("tty/driver", S_IRUSR|S_IXUSR, NULL);
 	proc_create("tty/ldiscs", 0, NULL, &tty_ldiscs_proc_fops);
 	proc_create("tty/drivers", 0, NULL, &proc_tty_drivers_operations);
+	proc_create("tty/consoles", 0, NULL, &proc_tty_consoles_operations);
 }
-- 
1.6.0.2