text: // SPDX-License-Identifier: GPL-2.0
  /*
   * device_cgroup.c - device cgroup subsystem
   *
   * Copyright 2007 IBM Corp
   */
  
  #include <linux/device_cgroup.h>
  #include <linux/cgroup.h>
  #include <linux/ctype.h>
  #include <linux/list.h>
  #include <linux/uaccess.h>
  #include <linux/seq_file.h>
  #include <linux/slab.h>
  #include <linux/rcupdate.h>
  #include <linux/mutex.h>
  
  static DEFINE_MUTEX(devcgroup_mutex);
  
  enum devcg_behavior {
  	DEVCG_DEFAULT_NONE,
  	DEVCG_DEFAULT_ALLOW,
  	DEVCG_DEFAULT_DENY,
  };
  
  /*
   * exception list locking rules:
   * hold devcgroup_mutex for update/read.
   * hold rcu_read_lock() for read.
   */
  
  struct dev_exception_item {
  	u32 major, minor;
  	short type;
  	short access;
  	struct list_head list;
  	struct rcu_head rcu;
  };
  
  struct dev_cgroup {
  	struct cgroup_subsys_state css;
  	struct list_head exceptions;
  	enum devcg_behavior behavior;
  };
  
  static inline struct dev_cgroup *css_to_devcgroup(struct cgroup_subsys_state *s)
  {
  	return s ? container_of(s, struct dev_cgroup, css) : NULL;
  }
  
  static inline struct dev_cgroup *task_devcgroup(struct task_struct *task)
  {
  	return css_to_devcgroup(task_css(task, devices_cgrp_id));
  }
  
  /*
   * called under devcgroup_mutex
   */
  static int dev_exceptions_copy(struct list_head *dest, struct list_head *orig)
  {
  	struct dev_exception_item *ex, *tmp, *new;
  
  	lockdep_assert_held(&devcgroup_mutex);
  
  	list_for_each_entry(ex, orig, list) {
  		new = kmemdup(ex, sizeof(*ex), GFP_KERNEL);
  		if (!new)
  			goto free_and_exit;
  		list_add_tail(&new->list, dest);
  	}
  
  	return 0;
  
  free_and_exit:
  	list_for_each_entry_safe(ex, tmp, dest, list) {
  		list_del(&ex->list);
  		kfree(ex);
  	}
  	return -ENOMEM;
  }
  
  /*
   * called under devcgroup_mutex
   */
  static int dev_exception_add(struct dev_cgroup *dev_cgroup,
  			     struct dev_exception_item *ex)
  {
  	struct dev_exception_item *excopy, *walk;
  
  	lockdep_assert_held(&devcgroup_mutex);
  
  	excopy = kmemdup(ex, sizeof(*ex), GFP_KERNEL);
  	if (!excopy)
  		return -ENOMEM;
  
  	list_for_each_entry(walk, &dev_cgroup->exceptions, list) {
  		if (walk->type != ex->type)
  			continue;
  		if (walk->major != ex->major)
  			continue;
  		if (walk->minor != ex->minor)
  			continue;
  
  		walk->access |= ex->access;
  		kfree(excopy);
  		excopy = NULL;
  	}
  
  	if (excopy != NULL)
  		list_add_tail_rcu(&excopy->list, &dev_cgroup->exceptions);
  	return 0;
  }
  
  /*
   * called under devcgroup_mutex
   */
  static void dev_exception_rm(struct dev_cgroup *dev_cgroup,
  			     struct dev_exception_item *ex)
  {
  	struct dev_exception_item *walk, *tmp;
  
  	lockdep_assert_held(&devcgroup_mutex);
  
  	list_for_each_entry_safe(walk, tmp, &dev_cgroup->exceptions, list) {
  		if (walk->type != ex->type)
  			continue;
  		if (walk->major != ex->major)
  			continue;
  		if (walk->minor != ex->minor)
  			continue;
  
  		walk->access &= ~ex->access;
  		if (!walk->access) {
  			list_del_rcu(&walk->list);
  			kfree_rcu(walk, rcu);
  		}
  	}
  }
  
  static void __dev_exception_clean(struct dev_cgroup *dev_cgroup)
  {
  	struct dev_exception_item *ex, *tmp;
  
  	list_for_each_entry_safe(ex, tmp, &dev_cgroup->exceptions, list) {
  		list_del_rcu(&ex->list);
  		kfree_rcu(ex, rcu);
  	}
  }
  
  /**
   * dev_exception_clean - frees all entries of the exception list
   * @dev_cgroup: dev_cgroup with the exception list to be cleaned
   *
   * called under devcgroup_mutex
   */
  static void dev_exception_clean(struct dev_cgroup *dev_cgroup)
  {
  	lockdep_assert_held(&devcgroup_mutex);
  
  	__dev_exception_clean(dev_cgroup);
  }
  
  static inline bool is_devcg_online(const struct dev_cgroup *devcg)
  {
  	return (devcg->behavior != DEVCG_DEFAULT_NONE);
  }
  
  /**
   * devcgroup_online - initializes devcgroup's behavior and exceptions based on
   * 		      parent's
   * @css: css getting online
   * returns 0 in case of success, error code otherwise
   */
  static int devcgroup_online(struct cgroup_subsys_state *css)
  {
  	struct dev_cgroup *dev_cgroup = css_to_devcgroup(css);
  	struct dev_cgroup *parent_dev_cgroup = css_to_devcgroup(css->parent);
  	int ret = 0;
  
  	mutex_lock(&devcgroup_mutex);
  
  	if (parent_dev_cgroup == NULL)
  		dev_cgroup->behavior = DEVCG_DEFAULT_ALLOW;
  	else {
  		ret = dev_exceptions_copy(&dev_cgroup->exceptions,
  					  &parent_dev_cgroup->exceptions);
  		if (!ret)
  			dev_cgroup->behavior = parent_dev_cgroup->behavior;
  	}
  	mutex_unlock(&devcgroup_mutex);
  
  	return ret;
  }
  
  static void devcgroup_offline(struct cgroup_subsys_state *css)
  {
  	struct dev_cgroup *dev_cgroup = css_to_devcgroup(css);
  
  	mutex_lock(&devcgroup_mutex);
  	dev_cgroup->behavior = DEVCG_DEFAULT_NONE;
  	mutex_unlock(&devcgroup_mutex);
  }
  
  /*
   * called from kernel/cgroup.c with cgroup_lock() held.
   */
  static struct cgroup_subsys_state *
  devcgroup_css_alloc(struct cgroup_subsys_state *parent_css)
  {
  	struct dev_cgroup *dev_cgroup;
  
  	dev_cgroup = kzalloc(sizeof(*dev_cgroup), GFP_KERNEL);
  	if (!dev_cgroup)
  		return ERR_PTR(-ENOMEM);
  	INIT_LIST_HEAD(&dev_cgroup->exceptions);
  	dev_cgroup->behavior = DEVCG_DEFAULT_NONE;
  
  	return &dev_cgroup->css;
  }
  
  static void devcgroup_css_free(struct cgroup_subsys_state *css)
  {
  	struct dev_cgroup *dev_cgroup = css_to_devcgroup(css);
  
  	__dev_exception_clean(dev_cgroup);
  	kfree(dev_cgroup);
  }
  
  #define DEVCG_ALLOW 1
  #define DEVCG_DENY 2
  #define DEVCG_LIST 3
  
  #define MAJMINLEN 13
  #define ACCLEN 4
  
  static void set_access(char *acc, short access)
  {
  	int idx = 0;
  	memset(acc, 0, ACCLEN);
  	if (access & DEVCG_ACC_READ)
  		acc[idx++] = 'r';
  	if (access & DEVCG_ACC_WRITE)
  		acc[idx++] = 'w';
  	if (access & DEVCG_ACC_MKNOD)
  		acc[idx++] = 'm';
  }
  
  static char type_to_char(short type)
  {
  	if (type == DEVCG_DEV_ALL)
  		return 'a';
  	if (type == DEVCG_DEV_CHAR)
  		return 'c';
  	if (type == DEVCG_DEV_BLOCK)
  		return 'b';
  	return 'X';
  }
  
  static void set_majmin(char *str, unsigned m)
  {
  	if (m == ~0)
  		strcpy(str, "*");
  	else
  		sprintf(str, "%u", m);
  }
  
  static int devcgroup_seq_show(struct seq_file *m, void *v)
  {
  	struct dev_cgroup *devcgroup = css_to_devcgroup(seq_css(m));
  	struct dev_exception_item *ex;
  	char maj[MAJMINLEN], min[MAJMINLEN], acc[ACCLEN];
  
  	rcu_read_lock();
  	/*
  	 * To preserve the compatibility:
  	 * - Only show the "all devices" when the default policy is to allow
  	 * - List the exceptions in case the default policy is to deny
  	 * This way, the file remains as a "whitelist of devices"
  	 */
  	if (devcgroup->behavior == DEVCG_DEFAULT_ALLOW) {
  		set_access(acc, DEVCG_ACC_MASK);
  		set_majmin(maj, ~0);
  		set_majmin(min, ~0);
  		seq_printf(m, "%c %s:%s %s\n", type_to_char(DEVCG_DEV_ALL),
  			   maj, min, acc);
  	} else {
  		list_for_each_entry_rcu(ex, &devcgroup->exceptions, list) {
  			set_access(acc, ex->access);
  			set_majmin(maj, ex->major);
  			set_majmin(min, ex->minor);
  			seq_printf(m, "%c %s:%s %s\n", type_to_char(ex->type),
  				   maj, min, acc);
  		}
  	}
  	rcu_read_unlock();
  
  	return 0;
  }
  
  /**
   * match_exception	- iterates the exception list trying to find a complete match
   * @exceptions: list of exceptions
   * @type: device type (DEVCG_DEV_BLOCK or DEVCG_DEV_CHAR)
   * @major: device file major number, ~0 to match all
   * @minor: device file minor number, ~0 to match all
   * @access: permission mask (DEVCG_ACC_READ, DEVCG_ACC_WRITE, DEVCG_ACC_MKNOD)
   *
   * It is considered a complete match if an exception is found that will
   * contain the entire range of provided parameters.
   *
   * Return: true in case it matches an exception completely
   */
  static bool match_exception(struct list_head *exceptions, short type,
  			    u32 major, u32 minor, short access)
  {
  	struct dev_exception_item *ex;
  
  	list_for_each_entry_rcu(ex, exceptions, list) {
  		if ((type & DEVCG_DEV_BLOCK) && !(ex->type & DEVCG_DEV_BLOCK))
  			continue;
  		if ((type & DEVCG_DEV_CHAR) && !(ex->type & DEVCG_DEV_CHAR))
  			continue;
  		if (ex->major != ~0 && ex->major != major)
  			continue;
  		if (ex->minor != ~0 && ex->minor != minor)
  			continue;
  		/* provided access cannot have more than the exception rule */
  		if (access & (~ex->access))
  			continue;
  		return true;
  	}
  	return false;
  }
  
  /**
   * match_exception_partial - iterates the exception list trying to find a partial match
   * @exceptions: list of exceptions
   * @type: device type (DEVCG_DEV_BLOCK or DEVCG_DEV_CHAR)
   * @major: device file major number, ~0 to match all
   * @minor: device file minor number, ~0 to match all
   * @access: permission mask (DEVCG_ACC_READ, DEVCG_ACC_WRITE, DEVCG_ACC_MKNOD)
   *
   * It is considered a partial match if an exception's range is found to
   * contain *any* of the devices specified by provided parameters. This is
   * used to make sure no extra access is being granted that is forbidden by
   * any of the exception list.
   *
   * Return: true in case the provided range mat matches an exception completely
   */
  static bool match_exception_partial(struct list_head *exceptions, short type,
  				    u32 major, u32 minor, short access)
  {
  	struct dev_exception_item *ex;
  
  	list_for_each_entry_rcu(ex, exceptions, list) {
  		if ((type & DEVCG_DEV_BLOCK) && !(ex->type & DEVCG_DEV_BLOCK))
  			continue;
  		if ((type & DEVCG_DEV_CHAR) && !(ex->type & DEVCG_DEV_CHAR))
  			continue;
  		/*
  		 * We must be sure that both the exception and the provided
  		 * range aren't masking all devices
  		 */
  		if (ex->major != ~0 && major != ~0 && ex->major != major)
  			continue;
  		if (ex->minor != ~0 && minor != ~0 && ex->minor != minor)
  			continue;
  		/*
  		 * In order to make sure the provided range isn't matching
  		 * an exception, all its access bits shouldn't match the
  		 * exception's access bits
  		 */
  		if (!(access & ex->access))
  			continue;
  		return true;
  	}
  	return false;
  }
  
  /**
   * verify_new_ex - verifies if a new exception is allowed by parent cgroup's permissions
   * @dev_cgroup: dev cgroup to be tested against
   * @refex: new exception
   * @behavior: behavior of the exception's dev_cgroup
   *
   * This is used to make sure a child cgroup won't have more privileges
   * than its parent
   */
  static bool verify_new_ex(struct dev_cgroup *dev_cgroup,
  		          struct dev_exception_item *refex,
  		          enum devcg_behavior behavior)
  {
  	bool match = false;
  
  	RCU_LOCKDEP_WARN(!rcu_read_lock_held() &&
  			 !lockdep_is_held(&devcgroup_mutex),
  			 "device_cgroup:verify_new_ex called without proper synchronization");
  
  	if (dev_cgroup->behavior == DEVCG_DEFAULT_ALLOW) {
  		if (behavior == DEVCG_DEFAULT_ALLOW) {
  			/*
  			 * new exception in the child doesn't matter, only
  			 * adding extra restrictions
  			 */ 
  			return true;
  		} else {
  			/*
  			 * new exception in the child will add more devices
  			 * that can be acessed, so it can't match any of
  			 * parent's exceptions, even slightly
  			 */ 
  			match = match_exception_partial(&dev_cgroup->exceptions,
  							refex->type,
  							refex->major,
  							refex->minor,
  							refex->access);
  
  			if (match)
  				return false;
  			return true;
  		}
  	} else {
  		/*
  		 * Only behavior == DEVCG_DEFAULT_DENY allowed here, therefore
  		 * the new exception will add access to more devices and must
  		 * be contained completely in an parent's exception to be
  		 * allowed
  		 */
  		match = match_exception(&dev_cgroup->exceptions, refex->type,
  					refex->major, refex->minor,
  					refex->access);
  
  		if (match)
  			/* parent has an exception that matches the proposed */
  			return true;
  		else
  			return false;
  	}
  	return false;
  }
  
  /*
   * parent_has_perm:
   * when adding a new allow rule to a device exception list, the rule
   * must be allowed in the parent device
   */
  static int parent_has_perm(struct dev_cgroup *childcg,
  				  struct dev_exception_item *ex)
  {
  	struct dev_cgroup *parent = css_to_devcgroup(childcg->css.parent);
  
  	if (!parent)
  		return 1;
  	return verify_new_ex(parent, ex, childcg->behavior);
  }
  
  /**
   * parent_allows_removal - verify if it's ok to remove an exception
   * @childcg: child cgroup from where the exception will be removed
   * @ex: exception being removed
   *
   * When removing an exception in cgroups with default ALLOW policy, it must
   * be checked if removing it will give the child cgroup more access than the
   * parent.
   *
   * Return: true if it's ok to remove exception, false otherwise
   */
  static bool parent_allows_removal(struct dev_cgroup *childcg,
  				  struct dev_exception_item *ex)
  {
  	struct dev_cgroup *parent = css_to_devcgroup(childcg->css.parent);
  
  	if (!parent)
  		return true;
  
  	/* It's always allowed to remove access to devices */
  	if (childcg->behavior == DEVCG_DEFAULT_DENY)
  		return true;
  
  	/*
  	 * Make sure you're not removing part or a whole exception existing in
  	 * the parent cgroup
  	 */
  	return !match_exception_partial(&parent->exceptions, ex->type,
  					ex->major, ex->minor, ex->access);
  }
  
  /**
   * may_allow_all - checks if it's possible to change the behavior to
   *		   allow based on parent's rules.
   * @parent: device cgroup's parent
   * returns: != 0 in case it's allowed, 0 otherwise
   */
  static inline int may_allow_all(struct dev_cgroup *parent)
  {
  	if (!parent)
  		return 1;
  	return parent->behavior == DEVCG_DEFAULT_ALLOW;
  }
  
  /**
   * revalidate_active_exceptions - walks through the active exception list and
   * 				  revalidates the exceptions based on parent's
   * 				  behavior and exceptions. The exceptions that
   * 				  are no longer valid will be removed.
   * 				  Called with devcgroup_mutex held.
   * @devcg: cgroup which exceptions will be checked
   *
   * This is one of the three key functions for hierarchy implementation.
   * This function is responsible for re-evaluating all the cgroup's active
   * exceptions due to a parent's exception change.
   * Refer to Documentation/cgroup-v1/devices.txt for more details.
   */
  static void revalidate_active_exceptions(struct dev_cgroup *devcg)
  {
  	struct dev_exception_item *ex;
  	struct list_head *this, *tmp;
  
  	list_for_each_safe(this, tmp, &devcg->exceptions) {
  		ex = container_of(this, struct dev_exception_item, list);
  		if (!parent_has_perm(devcg, ex))
  			dev_exception_rm(devcg, ex);
  	}
  }
  
  /**
   * propagate_exception - propagates a new exception to the children
   * @devcg_root: device cgroup that added a new exception
   * @ex: new exception to be propagated
   *
   * returns: 0 in case of success, != 0 in case of error
   */
  static int propagate_exception(struct dev_cgroup *devcg_root,
  			       struct dev_exception_item *ex)
  {
  	struct cgroup_subsys_state *pos;
  	int rc = 0;
  
  	rcu_read_lock();
  
  	css_for_each_descendant_pre(pos, &devcg_root->css) {
  		struct dev_cgroup *devcg = css_to_devcgroup(pos);
  
  		/*
  		 * Because devcgroup_mutex is held, no devcg will become
  		 * online or offline during the tree walk (see on/offline
  		 * methods), and online ones are safe to access outside RCU
  		 * read lock without bumping refcnt.
  		 */
  		if (pos == &devcg_root->css || !is_devcg_online(devcg))
  			continue;
  
  		rcu_read_unlock();
  
  		/*
  		 * in case both root's behavior and devcg is allow, a new
  		 * restriction means adding to the exception list
  		 */
  		if (devcg_root->behavior == DEVCG_DEFAULT_ALLOW &&
  		    devcg->behavior == DEVCG_DEFAULT_ALLOW) {
  			rc = dev_exception_add(devcg, ex);
  			if (rc)
  				break;
  		} else {
  			/*
  			 * in the other possible cases:
  			 * root's behavior: allow, devcg's: deny
  			 * root's behavior: deny, devcg's: deny
  			 * the exception will be removed
  			 */
  			dev_exception_rm(devcg, ex);
  		}
  		revalidate_active_exceptions(devcg);
  
  		rcu_read_lock();
  	}
  
  	rcu_read_unlock();
  	return rc;
  }
  
  /*
   * Modify the exception list using allow/deny rules.
   * CAP_SYS_ADMIN is needed for this.  It's at least separate from CAP_MKNOD
   * so we can give a container CAP_MKNOD to let it create devices but not
   * modify the exception list.
   * It seems likely we'll want to add a CAP_CONTAINER capability to allow
   * us to also grant CAP_SYS_ADMIN to containers without giving away the
   * device exception list controls, but for now we'll stick with CAP_SYS_ADMIN
   *
   * Taking rules away is always allowed (given CAP_SYS_ADMIN).  Granting
   * new access is only allowed if you're in the top-level cgroup, or your
   * parent cgroup has the access you're asking for.
   */
  static int devcgroup_update_access(struct dev_cgroup *devcgroup,
  				   int filetype, char *buffer)
  {
  	const char *b;
  	char temp[12];		/* 11 + 1 characters needed for a u32 */
  	int count, rc = 0;
  	struct dev_exception_item ex;
  	struct dev_cgroup *parent = css_to_devcgroup(devcgroup->css.parent);
  
  	if (!capable(CAP_SYS_ADMIN))
  		return -EPERM;
  
  	memset(&ex, 0, sizeof(ex));
  	b = buffer;
  
  	switch (*b) {
  	case 'a':
  		switch (filetype) {
  		case DEVCG_ALLOW:
  			if (css_has_online_children(&devcgroup->css))
  				return -EINVAL;
  
  			if (!may_allow_all(parent))
  				return -EPERM;
  			dev_exception_clean(devcgroup);
  			devcgroup->behavior = DEVCG_DEFAULT_ALLOW;
  			if (!parent)
  				break;
  
  			rc = dev_exceptions_copy(&devcgroup->exceptions,
  						 &parent->exceptions);
  			if (rc)
  				return rc;
  			break;
  		case DEVCG_DENY:
  			if (css_has_online_children(&devcgroup->css))
  				return -EINVAL;
  
  			dev_exception_clean(devcgroup);
  			devcgroup->behavior = DEVCG_DEFAULT_DENY;
  			break;
  		default:
  			return -EINVAL;
  		}
  		return 0;
  	case 'b':
  		ex.type = DEVCG_DEV_BLOCK;
  		break;
  	case 'c':
  		ex.type = DEVCG_DEV_CHAR;
  		break;
  	default:
  		return -EINVAL;
  	}
  	b++;
  	if (!isspace(*b))
  		return -EINVAL;
  	b++;
  	if (*b == '*') {
  		ex.major = ~0;
  		b++;
  	} else if (isdigit(*b)) {
  		memset(temp, 0, sizeof(temp));
  		for (count = 0; count < sizeof(temp) - 1; count++) {
  			temp[count] = *b;
  			b++;
  			if (!isdigit(*b))
  				break;
  		}
  		rc = kstrtou32(temp, 10, &ex.major);
  		if (rc)
  			return -EINVAL;
  	} else {
  		return -EINVAL;
  	}
  	if (*b != ':')
  		return -EINVAL;
  	b++;
  
  	/* read minor */
  	if (*b == '*') {
  		ex.minor = ~0;
  		b++;
  	} else if (isdigit(*b)) {
  		memset(temp, 0, sizeof(temp));
  		for (count = 0; count < sizeof(temp) - 1; count++) {
  			temp[count] = *b;
  			b++;
  			if (!isdigit(*b))
  				break;
  		}
  		rc = kstrtou32(temp, 10, &ex.minor);
  		if (rc)
  			return -EINVAL;
  	} else {
  		return -EINVAL;
  	}
  	if (!isspace(*b))
  		return -EINVAL;
  	for (b++, count = 0; count < 3; count++, b++) {
  		switch (*b) {
  		case 'r':
  			ex.access |= DEVCG_ACC_READ;
  			break;
  		case 'w':
  			ex.access |= DEVCG_ACC_WRITE;
  			break;
  		case 'm':
  			ex.access |= DEVCG_ACC_MKNOD;
  			break;
  		case '\n':
  		case '\0':
  			count = 3;
  			break;
  		default:
  			return -EINVAL;
  		}
  	}
  
  	switch (filetype) {
  	case DEVCG_ALLOW:
  		/*
  		 * If the default policy is to allow by default, try to remove
  		 * an matching exception instead. And be silent about it: we
  		 * don't want to break compatibility
  		 */
  		if (devcgroup->behavior == DEVCG_DEFAULT_ALLOW) {
  			/* Check if the parent allows removing it first */
  			if (!parent_allows_removal(devcgroup, &ex))
  				return -EPERM;
  			dev_exception_rm(devcgroup, &ex);
  			break;
  		}
  
  		if (!parent_has_perm(devcgroup, &ex))
  			return -EPERM;
  		rc = dev_exception_add(devcgroup, &ex);
  		break;
  	case DEVCG_DENY:
  		/*
  		 * If the default policy is to deny by default, try to remove
  		 * an matching exception instead. And be silent about it: we
  		 * don't want to break compatibility
  		 */
  		if (devcgroup->behavior == DEVCG_DEFAULT_DENY)
  			dev_exception_rm(devcgroup, &ex);
  		else
  			rc = dev_exception_add(devcgroup, &ex);
  
  		if (rc)
  			break;
  		/* we only propagate new restrictions */
  		rc = propagate_exception(devcgroup, &ex);
  		break;
  	default:
  		rc = -EINVAL;
  	}
  	return rc;
  }
  
  static ssize_t devcgroup_access_write(struct kernfs_open_file *of,
  				      char *buf, size_t nbytes, loff_t off)
  {
  	int retval;
  
  	mutex_lock(&devcgroup_mutex);
  	retval = devcgroup_update_access(css_to_devcgroup(of_css(of)),
  					 of_cft(of)->private, strstrip(buf));
  	mutex_unlock(&devcgroup_mutex);
  	return retval ?: nbytes;
  }
  
  static struct cftype dev_cgroup_files[] = {
  	{
  		.name = "allow",
  		.write = devcgroup_access_write,
  		.private = DEVCG_ALLOW,
  	},
  	{
  		.name = "deny",
  		.write = devcgroup_access_write,
  		.private = DEVCG_DENY,
  	},
  	{
  		.name = "list",
  		.seq_show = devcgroup_seq_show,
  		.private = DEVCG_LIST,
  	},
  	{ }	/* terminate */
  };
  
  struct cgroup_subsys devices_cgrp_subsys = {
  	.css_alloc = devcgroup_css_alloc,
  	.css_free = devcgroup_css_free,
  	.css_online = devcgroup_online,
  	.css_offline = devcgroup_offline,
  	.legacy_cftypes = dev_cgroup_files,
  };
  
  /**
   * __devcgroup_check_permission - checks if an inode operation is permitted
   * @dev_cgroup: the dev cgroup to be tested against
   * @type: device type
   * @major: device major number
   * @minor: device minor number
   * @access: combination of DEVCG_ACC_WRITE, DEVCG_ACC_READ and DEVCG_ACC_MKNOD
   *
   * returns 0 on success, -EPERM case the operation is not permitted
   */
  int __devcgroup_check_permission(short type, u32 major, u32 minor,
  				 short access)
  {
  	struct dev_cgroup *dev_cgroup;
  	bool rc;
  
  	rcu_read_lock();
  	dev_cgroup = task_devcgroup(current);
  	if (dev_cgroup->behavior == DEVCG_DEFAULT_ALLOW)
  		/* Can't match any of the exceptions, even partially */
  		rc = !match_exception_partial(&dev_cgroup->exceptions,
  					      type, major, minor, access);
  	else
  		/* Need to match completely one exception to be allowed */
  		rc = match_exception(&dev_cgroup->exceptions, type, major,
  				     minor, access);
  	rcu_read_unlock();
  
  	if (!rc)
  		return -EPERM;
  
  	return 0;
  }
  