Quoting Pavel Emelyanov (xemul@openvz.org):
> 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.

Hmm,

I was thinking we would use LSM for this.  Mostly it should suffice
to set up a reasonable /dev for the container to start with, and
hook security_mknod() to prevent it creating devices not on it's
whitelist.  If deemed necessary, read/write could be controlled
by hooking security_permission() and checking whether
file->f_path.dentry->d_inode is a device on the read or write
whitelist.

It would still be a device controller, so it can be composed with an
ns_proxy controller, and the whitelist is modified using the
devs_controller.whitelist file, but it registers a security_ops
with these two hooks.

I haven't implemented that yet, though, whereas you already have code :)
As for handling blkdevs with your code, would just hooking
fs/block_dev.c:do_open() not work?  Or is that not what you are
asking?

thanks,
-serge

> 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 = c