Hi.

At KS we have pointed out the need in some container, that allows
to limit the visibility of some devices to task within it. I.e.
allow for /dev/null, /dev/zero etc, but disable (by default) some
IDE devices or SCSI discs and so on.

Here's the beta of the container. Currently this only allows to
hide the _character_ devices only from the living tasks. To play 
with it you just create the container like this

 # mount -t container none /cont/devs -o devices
 # mkdir /cont/devs/0

it will have two specific files

 # ls /cont/devs
devices.block  devices.char  notify_on_release  releasable  release_agent  tasks

then move a task into it

 # /bin/echo -n $$ > /cont/devs/0/tasks

after this you won't be able to read from even /dev/zero

 # hexdump /dev/zero 
hexdump: /dev/zero: No such device or address
hexdump: /dev/zero: Bad file descriptor

meanwhile from another ssh session you will. You may allow access
to /dev/zero like this

 # /bin/echo -n '+1:5' > /cont/devs/0/devices.char

More generally, the '+<major>:<minor>' string grants access to
some device, and '-<major>:<minor>' disables one.

The TODO list now looks like this:
* add the block devices support :) don't know how to make it yet;
* make /proc/devices show relevant info depending on who is
  reading it. currently even if major 1 is disabled for task,
  it will be listed in this file;
* make it possible to enable/disable not just individual major:minor
  pair, but something more flexible, e.g. major:* for all minors
  for given major or major:m1-m2 for minor range, etc;
* add the ability to restrict the read/write permissions for a 
  container. currently one may just control the visible-invisible
  state for a device in a container, but maybe just readable or
  just writable would be better.

This patch is minimally tested, because I just want to know your
opinion on whether it worths developing the container in such a way or not.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>

---

diff --git a/drivers/base/map.c b/drivers/base/map.c
index e87017f..0188053 100644
--- a/drivers/base/map.c
+++ b/drivers/base/map.c
@@ -153,3 +153,21 @@ struct kobj_map *kobj_map_init(kobj_prob
 	p->lock = lock;
 	return p;
 }
+
+void kobj_map_fini(struct kobj_map *map)
+{
+	int i;
+	struct probe *p, *next;
+
+	for (i = 0; i < 256; i++) {
+		p = map->probes[i];
+		while (p->next != NULL) {
+			next = p->next;
+			kfree(p);
+			p = next;
+		}
+	}
+
+	kfree(p);
+	kfree(map);
+}
diff --git a/fs/Makefile b/fs/Makefile
index 2661ef9..837c731 100644
--- a/fs/Makefile
+++ b/fs/Makefile
@@ -64,6 +64,8 @@ obj-y				+= devpts/
 
 obj-$(CONFIG_PROFILING)		+= dcookies.o
 obj-$(CONFIG_DLM)		+= dlm/
+
+obj-$(CONFIG_CONTAINER_DEVS)	+= devscontrol.o
  
 # Do not add any filesystems before this line
 obj-$(CONFIG_REISERFS_FS)	+= reiserfs/
diff --git a/fs/char_dev.c b/fs/char_dev.c
index c3bfa76..1b0e4da 100644
--- a/fs/char_dev.c
+++ b/fs/char_dev.c
@@ -22,6 +22,8 @@
 #include <linux/mutex.h>
 #include <linux/backing-dev.h>
 
+#include <linux/devscontrol.h>
+
 #ifdef CONFIG_KMOD
 #include <linux/kmod.h>
 #endif
@@ -362,17 +364,24 @@ int chrdev_open(struct inode * inode, st
 	struct cdev *p;
 	struct cdev *new = NULL;
 	int ret = 0;
+	struct kobj_map *map;
+
+	map = task_cdev_map(current);
+	if (map == NULL)
+		map = cdev_map;
 
 	spin_lock(&cdev_lock);
 	p = inode->i_cdev;
-	if (!p) {
+	if (!p || p->last != map) {
 		struct kobject *kobj;
 		int idx;
+
 		spin_unlock(&cdev_lock);
-		kobj = kobj_lookup(cdev_map, inode->i_rdev, &idx);
+		kobj = kobj_lookup(map, inode->i_rdev, &idx);
 		if (!kobj)
 			return -ENXIO;
 		new = container_of(kobj, struct cdev, kobj);
+		BUG_ON(p != NULL && p != new);
 		spin_lock(&cdev_lock);
 		p = inode->i_cdev;
 		if (!p) {
@@ -384,6 +393,8 @@ int chrdev_open(struct inode * inode, st
 			ret = -ENXIO;
 	} else if (!cdev_get(p))
 		ret = -ENXIO;
+	if (p)
+		p->last = map;
 	spin_unlock(&cdev_lock);
 	cdev_put(new);
 	if (ret)
@@ -461,6 +472,49 @@ int cdev_add(struct cdev *p, dev_t dev, 
 	return kobj_map(cdev_map, dev, count, NULL, exact_match, exact_lock, p);
 }
 
+int cdev_add_to_map(struct kobj_map *map, dev_t dev)
+{
+	int tmp;
+	struct kobject *k;
+	struct cdev *c;
+
+	k = kobj_lookup(cdev_map, dev, &tmp);
+	if (k == NULL)
+		return -ENODEV;
+
+	c = container_of(k, struct cdev, kobj);
+	tmp = kobj_map(map, dev, 1, NULL, exact_match, exact_lock, c);
+	if (tmp < 0) {
+		cdev_put(c);
+		return tmp;
+	}
+
+	return 0;
+}
+
+int cdev_del_from_map(struct kobj_map *map, dev_t dev)
+{
+	int tmp;
+	struct kobject *k;
+	struct cdev *c;
+
+	k = kobj_lookup(cdev_map, dev, &tmp);
+	if (k == NULL)
+		return -ENODEV;
+
+	c = container_of(k, struct cdev, kobj);
+	kobj_unmap(map, dev, 1);
+
+	spin_lock(&cdev_lock);
+	if (c->last == map)
+		c->last = NULL;
+	spin_unlock(&cdev_lock);
+
+	cdev_put(c);
+	cdev_put(c);
+	return 0;
+}
+
 static void cdev_unmap(dev_t dev, unsigned count)
 {
 	kobj_unmap(cdev_map, dev, count);
@@ -542,6 +596,16 @@ static struct kobject *base_probe(dev_t 
 	return NULL;
 }
 
+struct kobj_map *cdev_map_init(void)
+{
+	return kobj_map_init(base_probe, &chrdevs_lock);
+}
+
+void cdev_map_fini(struct kobj_map *map)
+{
+	kobj_map_fini(map);
+}
+
 void __init chrdev_init(void)
 {
 	cdev_map = kobj_map_init(base_probe, &chrdevs_lock);
diff --git a/fs/devscontrol.c b/fs/devscontrol.c
new file mode 100644
index 0000000..6fb5f05
--- /dev/null
+++ b/fs/devscontrol.c
@@ -0,0 +1,170 @@
+/*
+ * devscontrol.c - Device Controller
+ *
+ * Copyright 2007 OpenVZ SWsoft Inc
+ * Author: Pavel Emelyanov <xemul@openvz.org>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ */
+
+#include <linux/container.h>
+#include <linux/cdev.h>
+#include <linux/err.h>
+#include <linux/devscontrol.h>
+#include <linux/uaccess.h>
+
+struct devs_container {
+	struct container_subsys_state css;
+
+	struct kobj_map *cdev_map;
+};
+
+static inline
+struct devs_container *css_to_devs(struct container_subsys_state *css)
+{
+	return container_of(css, struct devs_container, css);
+}
+
+static inline
+struct devs_container *container_to_devs(struct container *cont)
+{
+	return css_to_devs(container_subsys_state(cont, devs_subsys_id));
+}
+
+struct kobj_map *task_cdev_map(struct task_struct *tsk)
+{
+	struct container_subsys_state *css;
+
+	css = task_subsys_state(tsk, devs_subsys_id);
+	if (css->container->parent == NULL)
+		return NULL;
+	else
+		return css_to_devs(css)->cdev_map;
+}
+
+static struct container_subsys_state *
+devs_create(struct container_subsys *ss, struct container *cont)
+{
+	struct devs_container *devs;
+
+	devs = kzalloc(sizeof(struct devs_container), GFP_KERNEL);
+	if (devs == NULL)
+		goto out;
+
+	devs->cdev_map = cdev_map_init();
+	if (devs->cdev_map == NULL)
+		goto out_free;
+
+	return &devs->css;
+
+out_free:
+	kfree(devs);
+out:
+	return ERR_PTR(-ENOMEM);
+}
+
+static void devs_destroy(struct container_subsys *ss, struct container *cont)
+{
+	struct devs_container *devs;
+
+	devs = container_to_devs(cont);
+	cdev_map_fini(devs->cdev_map);
+	kfree(devs);
+}
+
+static int decode_dev_name(char *buf, dev_t *dev)
+{
+	unsigned int major, minor;
+	char *end;
+
+	major = simple_strtoul(buf, &end, 10);
+	if (*end != ':')
+		return -EINVAL;
+
+	minor = simple_strtoul(end + 1, &end, 10);
+	if (*end != '\0')
+		return -EINVAL;
+
+	*dev = MKDEV(major, minor);
+	return 0;
+}
+
+static ssize_t
+devs_char_write(struct container *cont, struct cftype *cft, struct file *f,
+		const char __user *userbuf, size_t nbytes, loff_t *pos)
+{
+	int err;
+	dev_t dev;
+	char buf[64];
+	struct devs_container *devs;
+
+	if (copy_from_user(buf, userbuf, sizeof(buf)))
+		return -EFAULT;
+
+	err = decode_dev_name(buf + 1, &dev);
+	if (err < 0)
+		return err;
+
+	devs = container_to_devs(cont);
+
+	switch (buf[0]) {
+	case '+':
+		err = cdev_add_to_map(devs->cdev_map, dev);
+		if (err < 0)
+			return err;
+
+		css_get(&devs->css);
+		break;
+	case '-':
+		err = cdev_del_from_map(devs->cdev_map, dev);
+		if (err < 0)
+			return err;
+
+		css_put(&devs->css);
+		break;
+	default:
+		return -EINVAL;
+	}
+
+	return nbytes;
+}
+
+static ssize_t
+devs_block_write(struct container *cont, struct cftype *cft, struct file *f,
+		const char __user *userbuf, size_t nbytes, loff_t *pos)
+{
+	return -ENOTTY;
+}
+
+static struct cftype devs_files[] = {
+	{
+		.name = "char",
+		.write = devs_char_write,
+	},
+	{
+		.name = "block",
+		.write = devs_block_write,
+	},
+};
+
+static int devs_populate(struct container_subsys *ss, struct container *cont)
+{
+	return container_add_files(cont, ss,
+			devs_files, ARRAY_SIZE(devs_files));
+}
+
+struct container_subsys devs_subsys = {
+	.name = "devices",
+	.subsys_id = devs_subsys_id,
+	.create = devs_create,
+	.destroy = devs_destroy,
+	.populate = devs_populate,
+};
diff --git a/include/linux/cdev.h b/include/linux/cdev.h
index 1e29b13..0eded40 100644
--- a/include/linux/cdev.h
+++ b/include/linux/cdev.h
@@ -9,6 +9,7 @@
 struct file_operations;
 struct inode;
 struct module;
+struct kobj_map;
 
 struct cdev {
 	struct kobject kobj;
@@ -17,6 +18,7 @@ struct cdev {
 	struct list_head list;
 	dev_t dev;
 	unsigned int count;
+	struct kobj_map *last;
 };
 
 void cdev_init(struct cdev *, const struct file_operations *);
@@ -33,5