[RFC][PATCH 3/7]: Enable multiple mounts of /dev/pts

[sukadev@us.ibm.com]

From: Sukadev Bhattiprolu <sukadev@us.ibm.com>
Subject:[RFC][PATCH 3/7]: Enable multiple mounts of /dev/pts

To support multiple PTY namespaces, we should be allow multiple mounts of
/dev/pts, once within each PTY namespace.

This patch removes the get_sb_single() in devpts_get_sb() and uses test and
set sb interfaces to allow remounting /dev/pts.  The patch also removes the
globals, 'devpts_mnt', 'devpts_root' and uses a skeletal 'init_pts_ns' to
store the vfsmount.

Changelog [v3]:
	- Removed some unnecessary comments from devpts_set_sb()

Changelog [v2]:

	- (Pavel Emelianov/Serge Hallyn) Remove reference to pts_ns from
	  sb->s_fs_info to fix the circular reference (/dev/pts is not
	  unmounted unless the pts_ns is destroyed, so we don't need a
	  reference to the pts_ns).

Signed-off-by: Sukadev Bhattiprolu <sukadev@us.ibm.com>
Signed-off-by: Serge Hallyn <serue@us.ibm.com>
Signed-off-by: Matt Helsley <matthltc@us.ibm.com>
---
 fs/devpts/inode.c         |  151 +++++++++++++++++++++++++++++++++++++---------
 include/linux/devpts_fs.h |   11 +++
 2 files changed, 134 insertions(+), 28 deletions(-)

Index: 2.6.25-rc5-mm1/include/linux/devpts_fs.h
===================================================================
--- 2.6.25-rc5-mm1.orig/include/linux/devpts_fs.h	2008-03-24 20:04:26.000000000 -0700
+++ 2.6.25-rc5-mm1/include/linux/devpts_fs.h	2008-04-01 18:08:42.000000000 -0700
@@ -14,6 +14,17 @@
 #define _LINUX_DEVPTS_FS_H
 
 #include <linux/errno.h>
+#include <linux/nsproxy.h>
+#include <linux/kref.h>
+#include <linux/idr.h>
+
+struct pts_namespace {
+	struct kref kref;
+	struct idr allocated_ptys;
+	struct vfsmount *mnt;
+};
+
+extern struct pts_namespace init_pts_ns;
 
 #ifdef CONFIG_UNIX98_PTYS
 
Index: 2.6.25-rc5-mm1/fs/devpts/inode.c
===================================================================
--- 2.6.25-rc5-mm1.orig/fs/devpts/inode.c	2008-03-24 20:04:26.000000000 -0700
+++ 2.6.25-rc5-mm1/fs/devpts/inode.c	2008-04-01 18:08:41.000000000 -0700
@@ -28,12 +28,8 @@
 #define DEVPTS_DEFAULT_MODE 0600
 
 extern int pty_limit;			/* Config limit on Unix98 ptys */
-static DEFINE_IDR(allocated_ptys);
 static DECLARE_MUTEX(allocated_ptys_lock);
 
-static struct vfsmount *devpts_mnt;
-static struct dentry *devpts_root;
-
 static struct {
 	int setuid;
 	int setgid;
@@ -54,6 +50,15 @@ static match_table_t tokens = {
 	{Opt_err, NULL}
 };
 
+struct pts_namespace init_pts_ns = {
+	.kref = {
+		.refcount = ATOMIC_INIT(2),
+	},
+	.allocated_ptys = IDR_INIT(init_pts_ns.allocated_ptys),
+	.mnt = NULL,
+};
+
+
 static int devpts_remount(struct super_block *sb, int *flags, char *data)
 {
 	char *p;
@@ -140,7 +145,7 @@ devpts_fill_super(struct super_block *s,
 	inode->i_fop = &simple_dir_operations;
 	inode->i_nlink = 2;
 
-	devpts_root = s->s_root = d_alloc_root(inode);
+	s->s_root = d_alloc_root(inode);
 	if (s->s_root)
 		return 0;
 	
@@ -150,17 +155,73 @@ fail:
 	return -ENOMEM;
 }
 
+/*
+ * We use test and set super-block operations to help determine whether we
+ * need a new super-block for this namespace. get_sb() walks the list of
+ * existing devpts supers, comparing them with the @data ptr. Since we
+ * passed 'current's namespace as the @data pointer we can compare the
+ * namespace pointer in the super-block's 's_fs_info'.  If the test is
+ * TRUE then get_sb() returns a new active reference to the super block.
+ * Otherwise, it helps us build an active reference to a new one.
+ */
+
+static int devpts_test_sb(struct super_block *sb, void *data)
+{
+	return sb->s_fs_info == data;
+}
+
+static int devpts_set_sb(struct super_block *sb, void *data)
+{
+	sb->s_fs_info = data;
+	return set_anon_super(sb, NULL);
+}
+
 static int devpts_get_sb(struct file_system_type *fs_type,
 	int flags, const char *dev_name, void *data, struct vfsmount *mnt)
 {
-	return get_sb_single(fs_type, flags, data, devpts_fill_super, mnt);
+	struct super_block *sb;
+	struct pts_namespace *ns;
+	int err;
+
+	/* hereafter we're very similar to proc_get_sb */
+	if (flags & MS_KERNMOUNT)
+		ns = data;
+	else
+		ns = &init_pts_ns;
+
+	/* hereafter we're very simlar to get_sb_nodev */
+	sb = sget(fs_type, devpts_test_sb, devpts_set_sb, ns);
+	if (IS_ERR(sb))
+		return PTR_ERR(sb);
+
+	if (sb->s_root)
+		return simple_set_mnt(mnt, sb);
+
+	sb->s_flags = flags;
+	err = devpts_fill_super(sb, data, flags & MS_SILENT ? 1 : 0);
+	if (err) {
+		up_write(&sb->s_umount);
+		deactivate_super(sb);
+		return err;
+	}
+
+	sb->s_flags |= MS_ACTIVE;
+	ns->mnt = mnt;
+
+	return simple_set_mnt(mnt, sb);
+}
+
+static void devpts_kill_sb(struct super_block *sb)
+{
+	sb->s_fs_info = NULL;
+	kill_anon_super(sb);
 }
 
 static struct file_system_type devpts_fs_type = {
 	.owner		= THIS_MODULE,
 	.name		= "devpts",
 	.get_sb		= devpts_get_sb,
-	.kill_sb	= kill_anon_super,
+	.kill_sb	= devpts_kill_sb,
 };
 
 /*
@@ -168,10 +229,9 @@ static struct file_system_type devpts_fs
  * to the System V naming convention
  */
 
-static struct dentry *get_node(int num)
+static struct dentry *get_node(struct dentry *root, int num)
 {
 	char s[12];
-	struct dentry *root = devpts_root;
 	mutex_lock(&root->d_inode->i_mutex);
 	return lookup_one_len(s, root, sprintf(s, "%d", num));
 }
@@ -180,14 +240,15 @@ int devpts_new_index(void)
 {
 	int index;
 	int idr_ret;
+	struct pts_namespace *pts_ns = &init_pts_ns;
 
 retry:
-	if (!idr_pre_get(&allocated_ptys, GFP_KERNEL)) {
+	if (!idr_pre_get(&pts_ns->allocated_ptys, GFP_KERNEL)) {
 		return -ENOMEM;
 	}
 
 	down(&allocated_ptys_lock);
-	idr_ret = idr_get_new(&allocated_ptys, NULL, &index);
+	idr_ret = idr_get_new(&pts_ns->allocated_ptys, NULL, &index);
 	if (idr_ret < 0) {
 		up(&allocated_ptys_lock);
 		if (idr_ret == -EAGAIN)
@@ -196,7 +257,7 @@ retry:
 	}
 
 	if (index >= pty_limit) {
-		idr_remove(&allocated_ptys, index);
+		idr_remove(&pts_ns->allocated_ptys, index);
 		up(&allocated_ptys_lock);
 		return -EIO;
 	}
@@ -206,8 +267,10 @@ retry:
 
 void devpts_kill_index(int idx)
 {
+	struct pts_namespace *pts_ns = &init_pts_ns;
+
 	down(&allocated_ptys_lock);
-	idr_remove(&allocated_ptys, idx);
+	idr_remove(&pts_ns->allocated_ptys, idx);
 	up(&allocated_ptys_lock);
 }
 
@@ -217,12 +280,26 @@ int devpts_pty_new(struct tty_struct *tt
 	struct tty_driver *driver = tty->driver;
 	dev_t device = MKDEV(driver->major, driver->minor_start+number);
 	struct dentry *dentry;
-	struct inode *inode = new_inode(devpts_mnt->mnt_sb);
+	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);
 	BUG_ON(driver->subtype != PTY_TYPE_SLAVE);
 
+	mnt = pts_ns->mnt;
+	root = mnt->mnt_root;
+
+	mutex_lock(&root->d_inode->i_mutex);
+	inode = idr_find(&pts_ns->allocated_ptys, number);
+	mutex_unlock(&root->d_inode->i_mutex);
+
+	if (inode && !IS_ERR(inode))
+		return -EEXIST;
+
+	inode = new_inode(mnt->mnt_sb);
 	if (!inode)
 		return -ENOMEM;
 
@@ -232,23 +309,29 @@ int devpts_pty_new(struct tty_struct *tt
 	inode->i_mtime = inode->i_atime = inode->i_ctime = CURRENT_TIME;
 	init_special_inode(inode, S_IFCHR|config.mode, device);
 	inode->i_private = tty;
+	idr_replace(&pts_ns->allocated_ptys, inode, number);
 
-	dentry = get_node(number);
+	dentry = get_node(root, number);
 	if (!IS_ERR(dentry) && !dentry->d_inode) {
 		d_instantiate(dentry, inode);
-		fsnotify_create(devpts_root->d_inode, dentry);
+		fsnotify_create(root->d_inode, dentry);
 	}
 
-	mutex_unlock(&devpts_root->d_inode->i_mutex);
+	mutex_unlock(&root->d_inode->i_mutex);
 
 	return 0;
 }
 
 struct tty_struct *devpts_get_tty(int number)
 {
-	struct dentry *dentry = get_node(number);
+	struct vfsmount *mnt;
+	struct dentry *dentry;
 	struct tty_struct *tty;
 
+	mnt = init_pts_ns.mnt;
+
+	dentry = get_node(mnt->mnt_root, number);
+
 	tty = NULL;
 	if (!IS_ERR(dentry)) {
 		if (dentry->d_inode)
@@ -256,14 +339,19 @@ struct tty_struct *devpts_get_tty(int nu
 		dput(dentry);
 	}
 
-	mutex_unlock(&devpts_root->d_inode->i_mutex);
+	mutex_unlock(&mnt->mnt_root->d_inode->i_mutex);
 
 	return tty;
 }
 
 void devpts_pty_kill(int number)
 {
-	struct dentry *dentry = get_node(number);
+	struct dentry *dentry;
+	struct dentry *root;
+
+	root = init_pts_ns.mnt->mnt_root;
+
+	dentry = get_node(root, number);
 
 	if (!IS_ERR(dentry)) {
 		struct inode *inode = dentry->d_inode;
@@ -274,24 +362,31 @@ void devpts_pty_kill(int number)
 		}
 		dput(dentry);
 	}
-	mutex_unlock(&devpts_root->d_inode->i_mutex);
+	mutex_unlock(&root->d_inode->i_mutex);
 }
 
 static int __init init_devpts_fs(void)
 {
-	int err = register_filesystem(&devpts_fs_type);
-	if (!err) {
-		devpts_mnt = kern_mount(&devpts_fs_type);
-		if (IS_ERR(devpts_mnt))
-			err = PTR_ERR(devpts_mnt);
-	}
+	struct vfsmount *mnt;
+	int err;
+
+	err = register_filesystem(&devpts_fs_type);
+	if (err)
+		return err;
+
+	mnt = kern_mount_data(&devpts_fs_type, &init_pts_ns);
+	if (IS_ERR(mnt))
+		err = PTR_ERR(mnt);
+	else
+		init_pts_ns.mnt = mnt;
 	return err;
 }
 
 static void __exit exit_devpts_fs(void)
 {
 	unregister_filesystem(&devpts_fs_type);
-	mntput(devpts_mnt);
+	mntput(init_pts_ns.mnt);
+	init_pts_ns.mnt = NULL;
 }
 
 module_init(init_devpts_fs)