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

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;

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