[RFC][PATCH 6/7]: Determine pts_ns from a pty's inode

[sukadev@us.ibm.com]

From: Sukadev Bhattiprolu <sukadev@us.ibm.com>
Subject: [RFC][PATCH 6/7]: Determine pts_ns from a pty's inode.

The devpts interfaces currently operate on a specific pts namespace
which they get from the 'current' task.

With implementation of containers and cloning of PTS namespaces, we want
to be able to access PTYs in a child-pts-ns from a parent-pts-ns. For
instance we could bind-mount and pivot-root the child container on
'/vserver/vserver1' and then access the "pts/0" of 'vserver1' using 

	$ echo foo > /vserver/vserver1/dev/pts/0
	
The task doing the above 'echo' could be in parent-pts-ns. So we find
the 'pts-ns' of the above file from the inode representing the device
rather than from the 'current' task.

Note that we need to find and hold a reference to the pts_ns to prevent
the pts_ns from being freed while it is being accessed from 'outside'.

This patch implements, 'pts_ns_from_inode()' which returns the pts_ns
using 'inode->i_sb->s_fs_info'.

Since, the 'inode' information is not visible inside devpts code itself,
this patch modifies the tty driver code to determine the pts_ns and passes
it into devpts.

Changelog [v2]:
	[Serge Hallyn] Use rcu to access sb->s_fs_info.

	[Serge Hallyn] Simplify handling of ptmx and tty devices by expecting
		user to create them in /dev/pts (see also devpts-mknod patch)


Signed-off-by: Sukadev Bhattiprolu <sukadev@us.ibm.com>
---
 drivers/char/pty.c        |   13 +++++-
 drivers/char/tty_io.c     |   96 +++++++++++++++++++++++++++++++++++++++-------
 fs/devpts/inode.c         |   19 +++------
 include/linux/devpts_fs.h |   38 +++++++++++++++---
 4 files changed, 134 insertions(+), 32 deletions(-)

Index: 2.6.25-rc8-mm1/include/linux/devpts_fs.h
===================================================================
--- 2.6.25-rc8-mm1.orig/include/linux/devpts_fs.h	2008-04-08 13:36:31.000000000 -0700
+++ 2.6.25-rc8-mm1/include/linux/devpts_fs.h	2008-04-08 13:38:08.000000000 -0700
@@ -17,6 +17,7 @@
 #include <linux/nsproxy.h>
 #include <linux/kref.h>
 #include <linux/idr.h>
+#include <linux/fs.h>
 
 struct pts_namespace {
 	struct kref kref;
@@ -26,12 +27,39 @@ struct pts_namespace {
 
 extern struct pts_namespace init_pts_ns;
 
+#define DEVPTS_SUPER_MAGIC 	0x1cd1
+
+static inline struct pts_namespace *current_pts_ns(void)
+{
+	return &init_pts_ns;
+}
+
+static inline struct pts_namespace *pts_ns_from_inode(struct inode *inode)
+{
+	/*
+	 * If this file exists on devpts, return the pts_ns from the
+	 * devpts super-block. Otherwise just use the pts-ns of the
+	 * calling task.
+	 */
+	if(inode->i_sb->s_magic == DEVPTS_SUPER_MAGIC)
+		return rcu_dereference(inode->i_sb->s_fs_info);
+
+	return current_pts_ns();
+}
+
+
 #ifdef CONFIG_UNIX98_PTYS
-int devpts_new_index(void);
-void devpts_kill_index(int idx);
-int devpts_pty_new(struct tty_struct *tty);      /* mknod in devpts */
-struct tty_struct *devpts_get_tty(int number);	 /* get tty structure */
-void devpts_pty_kill(int number);		 /* unlink */
+int devpts_new_index(struct pts_namespace *pts_ns);
+void devpts_kill_index(struct pts_namespace *pts_ns, int idx);
+
+/* mknod in devpts */
+int devpts_pty_new(struct pts_namespace *pts_ns, struct tty_struct *tty);
+
+/* get tty structure */
+struct tty_struct *devpts_get_tty(struct pts_namespace *pts_ns, int number);
+
+/* unlink */
+void devpts_pty_kill(struct pts_namespace *pts_ns, int number);
 
 static inline void free_pts_ns(struct kref *ns_kref) { }
 
Index: 2.6.25-rc8-mm1/drivers/char/tty_io.c
===================================================================
--- 2.6.25-rc8-mm1.orig/drivers/char/tty_io.c	2008-04-08 09:15:56.000000000 -0700
+++ 2.6.25-rc8-mm1/drivers/char/tty_io.c	2008-04-08 14:25:11.000000000 -0700
@@ -2064,8 +2064,8 @@ static void tty_line_name(struct tty_dri
  * relaxed for the (most common) case of reopening a tty.
  */
 
-static int init_dev(struct tty_driver *driver, int idx,
-	struct tty_struct **ret_tty)
+static int init_dev(struct tty_driver *driver, struct pts_namespace *pts_ns,
+		int idx, struct tty_struct **ret_tty)
 {
 	struct tty_struct *tty, *o_tty;
 	struct ktermios *tp, **tp_loc, *o_tp, **o_tp_loc;
@@ -2074,7 +2074,11 @@ static int init_dev(struct tty_driver *d
 
 	/* check whether we're reopening an existing tty */
 	if (driver->flags & TTY_DRIVER_DEVPTS_MEM) {
-		tty = devpts_get_tty(idx);
+		tty = devpts_get_tty(pts_ns, idx);
+		if (IS_ERR(tty)) {
+			retval = PTR_ERR(tty);
+			goto end_init;
+		}
 		/*
 		 * If we don't have a tty here on a slave open, it's because
 		 * the master already started the close process and there's
@@ -2361,6 +2365,21 @@ static void release_tty(struct tty_struc
 }
 
 /*
+ * If the inode belongs to a device in devpts fs, return the pts-namespace
+ * associated with the device. Return NULL otherwise.
+ */
+struct pts_namespace *pty_pts_ns(struct tty_driver *driver, struct inode *inode)
+{
+	struct pts_namespace *pts_ns;
+
+	pts_ns = NULL;
+	if (driver->flags & TTY_DRIVER_DEVPTS_MEM)
+		pts_ns = pts_ns_from_inode(inode);
+
+	return pts_ns;
+}
+
+/*
  * Even releasing the tty structures is a tricky business.. We have
  * to be very careful that the structures are all released at the
  * same time, as interrupts might otherwise get the wrong pointers.
@@ -2376,10 +2395,12 @@ static void release_dev(struct file *fil
 	int	idx;
 	char	buf[64];
 	unsigned long flags;
+	struct pts_namespace *pts_ns;
+	struct inode *inode;
 
+	inode = filp->f_path.dentry->d_inode;
 	tty = (struct tty_struct *)filp->private_data;
-	if (tty_paranoia_check(tty, filp->f_path.dentry->d_inode,
-							"release_dev"))
+	if (tty_paranoia_check(tty, inode, "release_dev"))
 		return;
 
 	check_tty_count(tty, "release_dev");
@@ -2392,6 +2413,12 @@ static void release_dev(struct file *fil
 	devpts = (tty->driver->flags & TTY_DRIVER_DEVPTS_MEM) != 0;
 	o_tty = tty->link;
 
+	/*
+	 * We already have a reference to pts_ns here, so it cannot
+	 * be going away.
+	 */
+	pts_ns = pty_pts_ns(tty->driver, inode);
+
 #ifdef TTY_PARANOIA_CHECK
 	if (idx < 0 || idx >= tty->driver->num) {
 		printk(KERN_DEBUG "release_dev: bad idx when trying to "
@@ -2569,6 +2596,10 @@ static void release_dev(struct file *fil
 
 	mutex_unlock(&tty_mutex);
 
+	/* drop the reference from ptmx_open/tty_open() */
+	if (devpts)
+		put_pts_ns(pts_ns);
+
 	/* check whether both sides are closing ... */
 	if (!tty_closing || (o_tty && !o_tty_closing))
 		return;
@@ -2634,7 +2665,7 @@ static void release_dev(struct file *fil
 
 	/* Make this pty number available for reallocation */
 	if (devpts)
-		devpts_kill_index(idx);
+		devpts_kill_index(pts_ns, idx);
 }
 
 /**
@@ -2666,6 +2697,7 @@ static int tty_open(struct inode *inode,
 	int index;
 	dev_t device = inode->i_rdev;
 	unsigned short saved_flags = filp->f_flags;
+	struct pts_namespace *pts_ns;
 
 	nonseekable_open(inode, filp);
 
@@ -2715,10 +2747,31 @@ retry_open:
 		return -ENODEV;
 	}
 got_driver:
-	retval = init_dev(driver, index, &tty);
+
+	/*
+	 * If this is a pty device, we maybe accessing this device from
+	 * an ancestor pts-namespace. Find the pts-namespace from the
+	 * device's inode and grab a reference.
+	 *
+	 * If pts-namespace is NULL then:
+	 * 	- either this is not a PTY device or
+	 * 	- this open is from an ancestor-pts-ns and the pts-ns has
+	 * 	  just been freed.
+	 *
+	 * If the pts-namespace is NULL for a PTY device (i.e pts-ns has
+	 * been freed), init_dev() will fail the open()).
+	 */
+	rcu_read_lock();
+	pts_ns = pty_pts_ns(driver, inode);
+	get_pts_ns(pts_ns);
+	rcu_read_unlock();
+
+	retval = init_dev(driver, pts_ns, index, &tty);
 	mutex_unlock(&tty_mutex);
-	if (retval)
+	if (retval) {
+		put_pts_ns(pts_ns);
 		return retval;
+	}
 
 	filp->private_data = tty;
 	file_move(filp, &tty->tty_files);
@@ -2790,16 +2843,31 @@ static int ptmx_open(struct inode *inode
 	struct tty_struct *tty;
 	int retval;
 	int index;
+	struct pts_namespace *pts_ns;
 
 	nonseekable_open(inode, filp);
 
+	/*
+	 * We maybe accessing this device from an ancestor pts-namespace.
+	 * Find the pts-namespace from the device's inode and grab a
+	 * reference.
+	 *
+	 * If pts-namespace is NULL, this open is from an ancestor-pts-ns
+	 * and the pts-ns has just been freed and devpts_new_index()
+	 * below will fail the open().
+	 */
+	rcu_read_lock();
+	pts_ns = pts_ns_from_inode(inode);
+	get_pts_ns(pts_ns);
+	rcu_read_unlock();
+
 	/* find a device that is not in use. */
-	index = devpts_new_index();
+	retval = index = devpts_new_index(pts_ns);
 	if (index < 0)
-		return index;
+		goto drop_ns;
 
 	mutex_lock(&tty_mutex);
-	retval = init_dev(ptm_driver, index, &tty);
+	retval = init_dev(ptm_driver, pts_ns, index, &tty);
 	mutex_unlock(&tty_mutex);
 
 	if (retval)
@@ -2809,7 +2877,7 @@ static int ptmx_open(struct inode *inode
 	filp->private_data = tty;
 	file_move(filp, &tty->tty_files);
 
-	retval = devpts_pty_new(tty->link);
+	retval = devpts_pty_new(pts_ns, tty->link);
 	if (retval)
 		goto out1;
 
@@ -2821,7 +2889,9 @@ out1:
 	release_dev(filp);
 	return retval;
 out:
-	devpts_kill_index(index);
+	devpts_kill_index(pts_ns, index);
+drop_ns:
+	put_pts_ns(pts_ns);
 	return retval;
 }
 #endif
Index: 2.6.25-rc8-mm1/fs/devpts/inode.c
===================================================================
--- 2.6.25-rc8-mm1.orig/fs/devpts/inode.c	2008-04-08 13:35:43.000000000 -0700
+++ 2.6.25-rc8-mm1/fs/devpts/inode.c	2008-04-08 13:38:08.000000000 -0700
@@ -23,8 +23,6 @@
 #include <linux/fsnotify.h>
 #include <linux/seq_file.h>
 
-#define DEVPTS_SUPER_MAGIC 0x1cd1
-
 #define DEVPTS_DEFAULT_MODE 0600
 
 extern int pty_limit;			/* Config limit on Unix98 ptys */
@@ -283,11 +281,10 @@ static struct dentry *get_node(struct de
 	return lookup_one_len(s, root, sprintf(s, "%d", num));
 }
 
-int devpts_new_index(void)
+int devpts_new_index(struct pts_namespace *pts_ns)
 {
 	int index;
 	int idr_ret;
-	struct pts_namespace *pts_ns = &init_pts_ns;
 
 retry:
 	if (!idr_pre_get(&pts_ns->allocated_ptys, GFP_KERNEL)) {
@@ -312,16 +309,15 @@ retry:
 	return index;
 }
 
-void devpts_kill_index(int idx)
+void devpts_kill_index(struct pts_namespace *pts_ns, int idx)
 {
-	struct pts_namespace *pts_ns = &init_pts_ns;
 
 	down(&allocated_ptys_lock);
 	idr_remove(&pts_ns->allocated_ptys, idx);
 	up(&allocated_ptys_lock);
 }
 
-int devpts_pty_new(struct tty_struct *tty)
+int devpts_pty_new( struct pts_namespace *pts_ns, struct tty_struct *tty)
 {
 	int number = tty->index; /* tty layer puts index from devpts_new_index() in here */
 	struct tty_driver *driver = tty->driver;
@@ -330,7 +326,6 @@ int devpts_pty_new(struct tty_struct *tt
 	struct dentry *root;
 	struct vfsmount *mnt;
 	struct inode *inode;
-	struct pts_namespace *pts_ns = &init_pts_ns;
 
 	/* We're supposed to be given the slave end of a pty */
 	BUG_ON(driver->type != TTY_DRIVER_TYPE_PTY);
@@ -369,13 +364,13 @@ int devpts_pty_new(struct tty_struct *tt
 	return 0;
 }
 
-struct tty_struct *devpts_get_tty(int number)
+struct tty_struct *devpts_get_tty(struct pts_namespace *pts_ns, int number)
 {
 	struct vfsmount *mnt;
 	struct dentry *dentry;
 	struct tty_struct *tty;
 
-	mnt = init_pts_ns.mnt;
+	mnt = pts_ns->mnt;
 
 	dentry = get_node(mnt->mnt_root, number);
 
@@ -391,12 +386,12 @@ struct tty_struct *devpts_get_tty(int nu
 	return tty;
 }
 
-void devpts_pty_kill(int number)
+void devpts_pty_kill(struct pts_namespace *pts_ns, int number)
 {
 	struct dentry *dentry;
 	struct dentry *root;
 
-	root = init_pts_ns.mnt->mnt_root;
+	root = pts_ns->mnt->mnt_root;
 
 	dentry = get_node(root, number);
 
Index: 2.6.25-rc8-mm1/drivers/char/pty.c
===================================================================
--- 2.6.25-rc8-mm1.orig/drivers/char/pty.c	2008-04-08 09:12:54.000000000 -0700
+++ 2.6.25-rc8-mm1/drivers/char/pty.c	2008-04-08 13:37:16.000000000 -0700
@@ -37,6 +37,9 @@ static struct tty_driver *pts_driver;
 
 static void pty_close(struct tty_struct * tty, struct file * filp)
 {
+	struct inode *inode;
+	struct pts_namespace *pts_ns;
+
 	if (!tty)
 		return;
 	if (tty->driver->subtype == PTY_TYPE_MASTER) {
@@ -58,8 +61,14 @@ static void pty_close(struct tty_struct 
 	if (tty->driver->subtype == PTY_TYPE_MASTER) {
 		set_bit(TTY_OTHER_CLOSED, &tty->flags);
 #ifdef CONFIG_UNIX98_PTYS
-		if (tty->driver == ptm_driver)
-			devpts_pty_kill(tty->index);
+		if (tty->driver == ptm_driver) {
+			inode = filp->f_path.dentry->d_inode;
+			rcu_read_lock();
+			pts_ns = pts_ns_from_inode(inode);
+			rcu_read_unlock();
+
+			devpts_pty_kill(pts_ns, tty->index);
+		}
 #endif
 		tty_vhangup(tty->link);
 	}