sukadev@us.ibm.com wrote:
> From: Sukadev Bhattiprolu <sukadev@us.ibm.com>
> Subject: [RFC][PATCH 4/4]: Enable cloning PTY namespaces
> 
> Enable cloning PTY namespaces.
> 
> TODO:
> 	This version temporarily uses the clone flag '0x80000000' which
> 	is unused in mainline atm, but used for CLONE_IO in -mm. 
> 	While we must extend clone() (urgently) to solve this, it hopefully
> 	does not affect review of the rest of this patchset.
> 
> Changelog:
> 	- Version 0: Based on earlier versions from Serge Hallyn and
> 	  Matt Helsley.
> 
> Signed-off-by: Sukadev Bhattiprolu <sukadev@us.ibm.com>
> ---
>  fs/devpts/inode.c         |   84 +++++++++++++++++++++++++++++++++++++++-------
>  include/linux/devpts_fs.h |   52 ++++++++++++++++++++++++++++
>  include/linux/init_task.h |    1 
>  include/linux/nsproxy.h   |    2 +
>  include/linux/sched.h     |    2 +
>  kernel/fork.c             |    2 -
>  kernel/nsproxy.c          |   17 ++++++++-
>  7 files changed, 146 insertions(+), 14 deletions(-)
> 
> Index: linux-2.6.24/fs/devpts/inode.c
> ===================================================================
> --- linux-2.6.24.orig/fs/devpts/inode.c	2008-02-05 19:16:39.000000000 -0800
> +++ linux-2.6.24/fs/devpts/inode.c	2008-02-05 20:27:41.000000000 -0800
> @@ -25,18 +25,25 @@
>  #define DEVPTS_SUPER_MAGIC 0x1cd1
>  
>  extern int pty_limit;		/* Config limit on Unix98 ptys */
> -static DEFINE_IDR(allocated_ptys);
>  static DECLARE_MUTEX(allocated_ptys_lock);
> +static struct file_system_type devpts_fs_type;
> +
> +struct pts_namespace init_pts_ns = {
> +	.kref = {
> +		.refcount = ATOMIC_INIT(2),
> +	},
> +	.allocated_ptys = IDR_INIT(init_pts_ns.allocated_ptys),
> +	.mnt = NULL,
> +};
>  
>  static inline struct idr *current_pts_ns_allocated_ptys(void)
>  {
> -	return &allocated_ptys;
> +	return &current->nsproxy->pts_ns->allocated_ptys;
>  }
>  
> -static struct vfsmount *devpts_mnt;
>  static inline struct vfsmount *current_pts_ns_mnt(void)
>  {
> -	return devpts_mnt;
> +	return current->nsproxy->pts_ns->mnt;
>  }
>  
>  static struct {
> @@ -59,6 +66,42 @@ static match_table_t tokens = {
>  	{Opt_err, NULL}
>  };
>  
> +struct pts_namespace *new_pts_ns(void)
> +{
> +	struct pts_namespace *ns;
> +
> +	ns = kmalloc(sizeof(*ns), GFP_KERNEL);
> +	if (!ns)
> +		return ERR_PTR(-ENOMEM);
> +
> +	ns->mnt = kern_mount_data(&devpts_fs_type, ns);

You create a circular references here - the namespace
holds the vfsmnt, the vfsmnt holds a superblock, a superblock
holds the namespace.

> +	if (IS_ERR(ns->mnt)) {
> +		kfree(ns);
> +		return ERR_PTR(PTR_ERR(ns->mnt));
> +	}
> +
> +	idr_init(&ns->allocated_ptys);
> +	kref_init(&ns->kref);
> +
> +	return ns;
> +}
> +
> +void free_pts_ns(struct kref *ns_kref)
> +{
> +	struct pts_namespace *ns;
> +
> +	ns = container_of(ns_kref, struct pts_namespace, kref);
> +	BUG_ON(ns == &init_pts_ns);
> +
> +	mntput(ns->mnt);
> +	/*
> +	 * TODO:
> +	 *      idr_remove_all(&ns->allocated_ptys); introduced in 2.6.23
> +	 */
> +	idr_destroy(&ns->allocated_ptys);
> +	kfree(ns);
> +}
> +
>  static int devpts_remount(struct super_block *sb, int *flags, char *data)
>  {
>  	char *p;
> @@ -160,18 +203,27 @@ static int devpts_test_sb(struct super_b
>  
>  static int devpts_set_sb(struct super_block *sb, void *data)
>  {
> -	sb->s_fs_info = data;
> +	struct pts_namespace *ns = data;
> +
> +	sb->s_fs_info = get_pts_ns(ns);
>  	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)
>  {
> +	struct pts_namespace *ns;
>  	struct super_block *sb;
>  	int err;
>  
> +	/* hereafter we're very similar to proc_get_sb */
> +	if (flags & MS_KERNMOUNT)
> +		ns = data;
> +	else
> +		ns = current->nsproxy->pts_ns;
> +
>  	/* hereafter we're very simlar to get_sb_nodev */
> -	sb = sget(fs_type, devpts_test_sb, devpts_set_sb, data);
> +	sb = sget(fs_type, devpts_test_sb, devpts_set_sb, ns);
>  	if (IS_ERR(sb))
>  		return PTR_ERR(sb);
>  
> @@ -187,16 +239,25 @@ static int devpts_get_sb(struct file_sys
>  	}
>  
>  	sb->s_flags |= MS_ACTIVE;
> -	devpts_mnt = mnt;
> +	ns->mnt = mnt;
>  
>  	return simple_set_mnt(mnt, sb);
>  }
>  
> +static void devpts_kill_sb(struct super_block *sb)
> +{
> +       struct pts_namespace *ns;
> +
> +       ns = sb->s_fs_info;
> +       kill_anon_super(sb);
> +       put_pts_ns(ns);
> +}
> +
>  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,
>  };
>  
>  /*
> @@ -352,18 +413,19 @@ static int __init init_devpts_fs(void)
>  	if (err)
>  		return err;
>  
> -	mnt = kern_mount_data(&devpts_fs_type, NULL);
> +	mnt = kern_mount_data(&devpts_fs_type, &init_pts_ns);
>  	if (IS_ERR(mnt))
>  		err = PTR_ERR(mnt);
>  	else
> -		devpts_mnt = mnt;
> +		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)
> Index: linux-2.6.24/include/linux/devpts_fs.h
> ===================================================================
> --- linux-2.6.24.orig/include/linux/devpts_fs.h	2008-02-05 19:16:39.000000000 -0800
> +++ linux-2.6.24/include/linux/devpts_fs.h	2008-02-05 20:21:08.000000000 -0800
> @@ -14,9 +14,45 @@
>  #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
>  
> +extern struct pts_namespace *new_pts_ns(void);
> +extern void free_pts_ns(struct kref *kref);
> +
> +static inline struct pts_namespace *get_pts_ns(struct pts_namespace *ns)
> +{
> +	if (ns)
> +		kref_get(&ns->kref);
> +	return ns;
> +}
> +
> +static inline void put_pts_ns(struct pts_namespace *ns)
> +{
> +	if (ns)
> +		kref_put(&ns->kref, free_pts_ns);
> +}
> +
> +static inline struct pts_namespace *copy_pts_ns(unsigned long flags,
> +		struct pts_namespace *old_ns)
> +{
> +	if (flags & CLONE_NEWPTS)
> +		return new_pts_ns();
> +	else
> +		return get_pts_ns(old_ns);
> +}
> +
>  int devpts_new_index(void);
>  void devpts_kill_index(int idx);
>  int devpts_pty_new(struct tty_struct *tty);      /* mknod in devpts */
> @@ -26,6 +62,22 @@ void devpts_pty_kill(int number);		 /* u
>  #else
>  
>  /* Dummy stubs in the no-pty case */
> +
> +static inline struct pts_namespace *get_pts_ns(struct pts_namespace *ns)
> +{
> +	return &init_pts_ns;
> +}
> +
> +static inline void put_pts_ns(struct pts_namespace *ns) { }
> +
> +static inline struct pts_namespace *copy_pts_ns(unsigned long flags,
> +		struct pts_namespace *old_ns)
> +{
> +	if (flags & CLONE_NEWPTS)
> +		return ERR_PTR(-EINVAL);
> +	return old_ns;
> +}
> +
>  static inline int devpts_new_index(void) { return -EINVAL; }
>  static inline void devpts_kill_index(int idx) { }
>  static inline int devpts_pty_new(struct tty_struct *tty) { return -EINVAL; }
> Index: linux-2.6.24/include/linux/init_task.h
> ===================================================================
> --- linux-2.6.24.orig/include/linux/init_task.h	2008-02-05 19:16:39.000000000 -0800
> +++ linux-2.6.24/include/linux/init_task.h	2008-02-05 19:18:00.000000000 -0800
> @@ -77,6 +77,7 @@ extern struct nsproxy init_nsproxy;
>  	.mnt_ns		= NULL,						\
>  	INIT_NET_NS(net_ns)                                             \
>  	INIT_IPC_NS(ipc_ns)						\
> +	.pts_ns		= &init_pts_ns,					\
>  	.user_ns	= &init_user_ns,				\
>  }
>  
> Index: linux-2.6.24/include/linux/nsproxy.h
> ===================================================================
> --- linux-2.6.24.orig/include/linux/nsproxy.h	2008-02-05 19:16:39.000000000 -0800
> +++ linux-2.6.24/include/linux/nsproxy.h	2008-02-05 19:18:00.000000000 -0800
> @@ -8,6 +8,7 @@ struct mnt_namespace;
>  struct uts_namespace;
>  struct ipc_namespace;
>  struct pid_namespace;
> +struct pts_namespace;
>  
>  /*
>   * A structure to contain pointers to all per-process
> @@ -29,6 +30,7 @@ struct nsproxy {
>  	struct pid_namespace *pid_ns;
>  	struct user_namespace *user_ns;
>  	struct net 	     *net_ns;
> +	struct pts_namespace *pts_ns;
>  };
>  extern struct nsproxy init_nsproxy;
>  
> Index: linux-2.6.24/include/linux/sched.h
> ===================================================================
> --- linux-2.6.24.orig/include/linux/sched.h	2008-02-05 19:16:39.000000000 -0800
> +++ linux-2.6.24/include/linux/sched.h	2008-02-05 19:54:05.000000000 -0800
> @@ -27,6 +27,8 @@
>  #define CLONE_NEWUSER		0x10000000	/* New user namespace */
>  #define CLONE_NEWPID		0x20000000	/* New pid namespace */
>