[RFC][PATCH 4/7]: Allow mknod of ptmx and tty in devpts

[sukadev@us.ibm.com]

From: Sukadev Bhattiprolu <sukadev@us.ibm.com>
Subject: [RFC][PATCH 4/7]: Allow mknod of ptmx and tty in devpts

We want to allow administrators to access PTYs in descendant pts-namespaces,
for instance "echo foo > /vserver/vserver1/dev/pts/0". To enable such access
we must hold a reference to the pts-ns in which the device (ptmx or slave pty)
exists. 

Note that we cannot use the pts-ns of the 'current' process since that pts-ns
could be different from the pts-ns in which the PTY device was created. So
we find the pts-ns from the inode of the PTY (inode->i_sb->s_fs_info).

While this would work for the slave PTY devices like /dev/pts/0, it would
not work for either the master PTY device (/dev/ptmx) or controlling terminal
(/dev/tty).

To uniformly handle the master, slave and controlling ttys, we allow creation
of 'ptmx' and 'tty' devices in /dev/pts. When creating containers, the
administrator can then:

In init-pts-ns:

	$ mknod /dev/pts/ptmx c 5 2
	$ mknod /dev/pts/tty c 5 0
	$ rm /dev/ptmx /dev/tty
	$ ln -s /dev/pts/ptmx /dev/ptmx
	$ ln -s /dev/pts/tty /dev/tty

In child-pts-ns:

	$ umount /dev/pts
	$ mount -t devpts lxcpts /dev/pts
	$ mknod /dev/pts/ptmx c 5 2
	$ mknod /dev/pts/tty c 5 0

With this, even if the 'ptmx' is accessed from parent pts-ns we still find
and hold the pts-ns in which 'ptmx' actually belongs.

This patch merely allows creation of /dev/pts/ptmx and /dev/pts/tty. Follow-on
patches will enable cloning the pts namespace and using the pts-ns from
the inode.

TODO:
	- Ability to unlink the /dev/pts/ptmx and /dev/pts/tty nodes.

Note:
	- If /dev/ptmx is a symlink to /vserver/vserver1/dev/pts/ptmx,
	  open("/dev/ptmx") in init-pts-ns will create a PTY in 'vserver1' ! 

Signed-off-by: Sukadev Bhattiprolu <sukadev@us.ibm.com>
---
 fs/devpts/inode.c |   55 ++++++++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 51 insertions(+), 4 deletions(-)

Index: 2.6.25-rc8-mm1/fs/devpts/inode.c
===================================================================
--- 2.6.25-rc8-mm1.orig/fs/devpts/inode.c	2008-04-08 09:18:23.000000000 -0700
+++ 2.6.25-rc8-mm1/fs/devpts/inode.c	2008-04-08 13:35:43.000000000 -0700
@@ -58,7 +58,6 @@ struct pts_namespace init_pts_ns = {
 	.mnt = NULL,
 };
 
-
 static int devpts_remount(struct super_block *sb, int *flags, char *data)
 {
 	char *p;
@@ -122,6 +121,54 @@ static const struct super_operations dev
 	.show_options	= devpts_show_options,
 };
 
+
+static int devpts_mknod(struct inode *dir, struct dentry *dentry,
+			int mode, dev_t rdev)
+{
+	int inum;
+	struct inode *inode;
+	struct super_block *sb = dir->i_sb;
+
+	if (dentry->d_inode)
+		return -EEXIST;
+
+	if (!S_ISCHR(mode))
+		return -EPERM;
+
+	if (rdev == MKDEV(TTYAUX_MAJOR, 0))
+		inum = 2;
+	else if (rdev == MKDEV(TTYAUX_MAJOR, 2))
+		inum = 3;
+	else
+		return -EPERM;
+
+	inode = new_inode(sb);
+	if (!inode)
+		return -ENOMEM;
+
+	inode->i_ino = inum;
+	inode->i_uid = inode->i_gid = 0;
+	inode->i_blocks = 0;
+	inode->i_mtime = inode->i_atime = inode->i_ctime = CURRENT_TIME;
+
+	init_special_inode(inode, mode, rdev);
+
+	d_instantiate(dentry, inode);
+	/*
+	 * Get a reference to the dentry so the device-nodes persist
+	 * even when there are no active references to them. We use
+	 * kill_litter_super() to remove this entry when unmounting
+	 * devpts.
+	 */
+	dget(dentry);
+	return 0;
+}
+
+const struct inode_operations devpts_dir_inode_operations = {
+	.lookup         = simple_lookup,
+	.mknod		= devpts_mknod,
+};
+
 static int
 devpts_fill_super(struct super_block *s, void *data, int silent)
 {
@@ -141,7 +188,7 @@ devpts_fill_super(struct super_block *s,
 	inode->i_blocks = 0;
 	inode->i_uid = inode->i_gid = 0;
 	inode->i_mode = S_IFDIR | S_IRUGO | S_IXUGO | S_IWUSR;
-	inode->i_op = &simple_dir_inode_operations;
+	inode->i_op = &devpts_dir_inode_operations;
 	inode->i_fop = &simple_dir_operations;
 	inode->i_nlink = 2;
 
@@ -214,7 +261,7 @@ static int devpts_get_sb(struct file_sys
 static void devpts_kill_sb(struct super_block *sb)
 {
 	sb->s_fs_info = NULL;
-	kill_anon_super(sb);
+	kill_litter_super(sb);
 }
 
 static struct file_system_type devpts_fs_type = {
@@ -303,7 +350,7 @@ int devpts_pty_new(struct tty_struct *tt
 	if (!inode)
 		return -ENOMEM;
 
-	inode->i_ino = number+2;
+	inode->i_ino = number+4;
 	inode->i_uid = config.setuid ? config.uid : current->fsuid;
 	inode->i_gid = config.setgid ? config.gid : current->fsgid;
 	inode->i_mtime = inode->i_atime = inode->i_ctime = CURRENT_TIME;