[RFC][PATCH 3/4]: Enable multiple mounts of /dev/pts

[sukadev-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org]

From: Sukadev Bhattiprolu <sukadev-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>
Subject: [RFC][PATCH 3/4]: 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_root' and uses current_pts_mnt() to access 'devpts_mnt'

Changelog:
	- Version 0: Based on earlier versions from Serge Hallyn and
	  Matt Helsley.

Signed-off-by: Sukadev Bhattiprolu <sukadev-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>
---
 fs/devpts/inode.c |  120 +++++++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 101 insertions(+), 19 deletions(-)

Index: linux-2.6.24/fs/devpts/inode.c
===================================================================
--- linux-2.6.24.orig/fs/devpts/inode.c	2008-02-05 17:30:52.000000000 -0800
+++ linux-2.6.24/fs/devpts/inode.c	2008-02-05 19:16:39.000000000 -0800
@@ -34,7 +34,10 @@ static inline struct idr *current_pts_ns
 }
 
 static struct vfsmount *devpts_mnt;
-static struct dentry *devpts_root;
+static inline struct vfsmount *current_pts_ns_mnt(void)
+{
+	return devpts_mnt;
+}
 
 static struct {
 	int setuid;
@@ -130,7 +133,7 @@ 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;
 	
@@ -140,10 +143,53 @@ 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;
+	int err;
+
+	/* hereafter we're very simlar to get_sb_nodev */
+	sb = sget(fs_type, devpts_test_sb, devpts_set_sb, data);
+	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;
+	devpts_mnt = mnt;
+
+	return simple_set_mnt(mnt, sb);
 }
 
 static struct file_system_type devpts_fs_type = {
@@ -158,10 +204,9 @@ 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));
 }
@@ -207,12 +252,28 @@ 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;
+
 
 	/* 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 = current_pts_ns_mnt();
+	if (!mnt)
+		return -ENOSYS;
+	root = mnt->mnt_root;
+
+	mutex_lock(&root->d_inode->i_mutex);
+	inode = idr_find(current_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;
 
@@ -222,23 +283,31 @@ 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(current_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 = current_pts_ns_mnt();
+	if (!mnt)
+		return NULL;
+
+	dentry = get_node(mnt->mnt_root, number);
+
 	tty = NULL;
 	if (!IS_ERR(dentry)) {
 		if (dentry->d_inode)
@@ -246,14 +315,21 @@ 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;
+	struct vfsmount *mnt;
+
+	mnt = current_pts_ns_mnt();
+	root = mnt->mnt_root;
+
+	dentry = get_node(root, number);
 
 	if (!IS_ERR(dentry)) {
 		struct inode *inode = dentry->d_inode;
@@ -264,17 +340,23 @@ 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, NULL);
+	if (IS_ERR(mnt))
+		err = PTR_ERR(mnt);
+	else
+		devpts_mnt = mnt;
 	return err;
 }