Quoting Miklos Szeredi (miklos@szeredi.hu):
quoted text
> From: Miklos Szeredi <mszeredi@suse.cz>
> 
> Add sysctl variables for accounting and limiting the number of user
> mounts.
> 
> The maximum number of user mounts is set to 1024 by default.  This
> won't in itself enable user mounts, setting a mount to be owned by a
> user is first needed
> 
> [akpm]
>  - don't use enumerated sysctls
> 
> Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

Seems sane enough, given your responses to Dave.

Acked-by: Serge Hallyn <serue@us.ibm.com>

quoted text
> ---
> 
> Index: linux/Documentation/filesystems/proc.txt
> ===================================================================
> --- linux.orig/Documentation/filesystems/proc.txt	2008-01-03 17:12:58.000000000 +0100
> +++ linux/Documentation/filesystems/proc.txt	2008-01-03 21:15:35.000000000 +0100
> @@ -1012,6 +1012,15 @@ reaches aio-max-nr then io_setup will fa
>  raising aio-max-nr does not result in the pre-allocation or re-sizing
>  of any kernel data structures.
> 
> +nr_user_mounts and max_user_mounts
> +----------------------------------
> +
> +These represent the number of "user" mounts and the maximum number of
> +"user" mounts respectively.  User mounts may be created by
> +unprivileged users.  User mounts may also be created with sysadmin
> +privileges on behalf of a user, in which case nr_user_mounts may
> +exceed max_user_mounts.
> +
>  2.2 /proc/sys/fs/binfmt_misc - Miscellaneous binary formats
>  -----------------------------------------------------------
> 
> Index: linux/fs/namespace.c
> ===================================================================
> --- linux.orig/fs/namespace.c	2008-01-03 21:14:16.000000000 +0100
> +++ linux/fs/namespace.c	2008-01-03 21:15:35.000000000 +0100
> @@ -44,6 +44,9 @@ static struct list_head *mount_hashtable
>  static struct kmem_cache *mnt_cache __read_mostly;
>  static struct rw_semaphore namespace_sem;
> 
> +int nr_user_mounts;
> +int max_user_mounts = 1024;
> +
>  /* /sys/fs */
>  struct kobject *fs_kobj;
>  EXPORT_SYMBOL_GPL(fs_kobj);
> @@ -477,11 +480,30 @@ static struct vfsmount *skip_mnt_tree(st
>  	return p;
>  }
> 
> +static void dec_nr_user_mounts(void)
> +{
> +	spin_lock(&vfsmount_lock);
> +	nr_user_mounts--;
> +	spin_unlock(&vfsmount_lock);
> +}
> +
>  static void set_mnt_user(struct vfsmount *mnt)
>  {
>  	BUG_ON(mnt->mnt_flags & MNT_USER);
>  	mnt->mnt_uid = current->fsuid;
>  	mnt->mnt_flags |= MNT_USER;
> +	spin_lock(&vfsmount_lock);
> +	nr_user_mounts++;
> +	spin_unlock(&vfsmount_lock);
> +}
> +
> +static void clear_mnt_user(struct vfsmount *mnt)
> +{
> +	if (mnt->mnt_flags & MNT_USER) {
> +		mnt->mnt_uid = 0;
> +		mnt->mnt_flags &= ~MNT_USER;
> +		dec_nr_user_mounts();
> +	}
>  }
> 
>  static struct vfsmount *clone_mnt(struct vfsmount *old, struct dentry *root,
> @@ -542,6 +564,7 @@ static inline void __mntput(struct vfsmo
>  	 */
>  	WARN_ON(atomic_read(&mnt->__mnt_writers));
>  	dput(mnt->mnt_root);
> +	clear_mnt_user(mnt);
>  	free_vfsmnt(mnt);
>  	deactivate_super(sb);
>  }
> @@ -1306,6 +1329,7 @@ static int do_remount(struct nameidata *
>  	else
>  		err = do_remount_sb(sb, flags, data, 0);
>  	if (!err) {
> +		clear_mnt_user(nd->path.mnt);
>  		nd->path.mnt->mnt_flags = mnt_flags;
>  		if (flags & MS_SETUSER)
>  			set_mnt_user(nd->path.mnt);
> Index: linux/include/linux/fs.h
> ===================================================================
> --- linux.orig/include/linux/fs.h	2008-01-03 20:52:38.000000000 +0100
> +++ linux/include/linux/fs.h	2008-01-03 21:15:35.000000000 +0100
> @@ -50,6 +50,9 @@ extern struct inodes_stat_t inodes_stat;
> 
>  extern int leases_enable, lease_break_time;
> 
> +extern int nr_user_mounts;
> +extern int max_user_mounts;
> +
>  #ifdef CONFIG_DNOTIFY
>  extern int dir_notify_enable;
>  #endif
> Index: linux/kernel/sysctl.c
> ===================================================================
> --- linux.orig/kernel/sysctl.c	2008-01-03 17:13:22.000000000 +0100
> +++ linux/kernel/sysctl.c	2008-01-03 21:15:35.000000000 +0100
> @@ -1288,6 +1288,22 @@ static struct ctl_table fs_table[] = {
>  #endif	
>  #endif
>  	{
> +		.ctl_name	= CTL_UNNUMBERED,
> +		.procname	= "nr_user_mounts",
> +		.data		= &nr_user_mounts,
> +		.maxlen		= sizeof(int),
> +		.mode		= 0444,
> +		.proc_handler	= &proc_dointvec,
> +	},
> +	{
> +		.ctl_name	= CTL_UNNUMBERED,
> +		.procname	= "max_user_mounts",
> +		.data		= &max_user_mounts,
> +		.maxlen		= sizeof(int),
> +		.mode		= 0644,
> +		.proc_handler	= &proc_dointvec,
> +	},
> +	{
>  		.ctl_name	= KERN_SETUID_DUMPABLE,
>  		.procname	= "suid_dumpable",
>  		.data		= &suid_dumpable,
> 
> --
--
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