/*
 * 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>

#define ACC_MKNOD 1
#define ACC_READ  2
#define ACC_WRITE 4
#define ACC_MASK (ACC_MKNOD | ACC_READ | ACC_WRITE)

#define DEV_BLOCK 1
#define DEV_CHAR  2
#define DEV_ALL   4  /* this represents all devices */

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;
        /* temporary list for pending propagation operations */
        struct list_head propagate_pending;
};

static inline struct dev_cgroup *css_to_devcgroup(struct cgroup_subsys_state *s)
{
        return container_of(s, struct dev_cgroup, css);
}

static inline struct dev_cgroup *cgroup_to_devcgroup(struct cgroup *cgroup)
{
        return css_to_devcgroup(cgroup_subsys_state(cgroup, devices_subsys_id));
}

static inline struct dev_cgroup *task_devcgroup(struct task_struct *task)
{
        return css_to_devcgroup(task_subsys_state(task, devices_subsys_id));
}

struct cgroup_subsys devices_subsys;

static int devcgroup_can_attach(struct cgroup *new_cgrp,
                                struct cgroup_taskset *set)
{
        struct task_struct *task = cgroup_taskset_first(set);

        if (current != task && !capable(CAP_SYS_ADMIN))
                return -EPERM;
        return 0;
}

/*
 * 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
 * @cgroup: cgroup getting online
 * returns 0 in case of success, error code otherwise
 */
static int devcgroup_online(struct cgroup *cgroup)
{
        struct dev_cgroup *dev_cgroup, *parent_dev_cgroup = NULL;
        int ret = 0;

        mutex_lock(&devcgroup_mutex);
        dev_cgroup = cgroup_to_devcgroup(cgroup);
        if (cgroup->parent)
                parent_dev_cgroup = cgroup_to_devcgroup(cgroup->parent);

        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 *cgroup)
{
        struct dev_cgroup *dev_cgroup = cgroup_to_devcgroup(cgroup);

        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 *cgroup)
{
        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);
        INIT_LIST_HEAD(&dev_cgroup->propagate_pending);
        dev_cgroup->behavior = DEVCG_DEFAULT_NONE;

        return &dev_cgroup->css;
}

static void devcgroup_css_free(struct cgroup *cgroup)
{
        struct dev_cgroup *dev_cgroup;

        dev_cgroup = cgroup_to_devcgroup(cgroup);
        __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 & ACC_READ)
                acc[idx++] = 'r';
        if (access & ACC_WRITE)
                acc[idx++] = 'w';
        if (access & ACC_MKNOD)
                acc[idx++] = 'm';
}

static char type_to_char(short type)
{
        if (type == DEV_ALL)
                return 'a';
        if (type == DEV_CHAR)
                return 'c';
        if (type == 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_read(struct cgroup *cgroup, struct cftype *cft,
                                struct seq_file *m)
{
        struct dev_cgroup *devcgroup = cgroup_to_devcgroup(cgroup);
        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, ACC_MASK);
                set_majmin(maj, ~0);
                set_majmin(min, ~0);
                seq_printf(m, "%c %s:%s %s\n", type_to_char(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 match a rule
 *                        based on type, major, minor and access type. It is
 *                        considered a match if an exception is found that
 *                        will contain the entire range of provided parameters.
 * @exceptions: list of exceptions
 * @type: device type (DEV_BLOCK or DEV_CHAR)
 * @major: device file major number, ~0 to match all
 * @minor: device file minor number, ~0 to match all
 * @access: permission mask (ACC_READ, ACC_WRITE, ACC_MKNOD)
 *
 * returns: 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 & DEV_BLOCK) && !(ex->type & DEV_BLOCK))
                        continue;
                if ((type & DEV_CHAR) && !(ex->type & 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 match a rule
 *                           based on type, major, minor and access type. It is
 *                           considered a 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.
 * @exceptions: list of exceptions
 * @type: device type (DEV_BLOCK or DEV_CHAR)
 * @major: device file major number, ~0 to match all
 * @minor: device file minor number, ~0 to match all
 * @access: permission mask (ACC_READ, ACC_WRITE, ACC_MKNOD)
 *
 * returns: 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 & DEV_BLOCK) && !(ex->type & DEV_BLOCK))
                        continue;
                if ((type & DEV_CHAR) && !(ex->type & 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 part of what is allowed
 *                 by a dev cgroup based on the default policy +
 *                 exceptions. This is used to make sure a child cgroup
 *                 won't have more privileges than its parent
 * @dev_cgroup: dev cgroup to be tested against
 * @refex: new exception
 * @behavior: behavior of the exception's dev_cgroup
 */
static bool verify_new_ex(struct dev_cgroup *dev_cgroup,
                          struct dev_exception_item *refex,
                          enum devcg_behavior behavior)
{
        bool match = false;

        rcu_lockdep_assert(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 cgroup *pcg = childcg->css.cgroup->parent;
        struct dev_cgroup *parent;

        if (!pcg)
                return 1;
        parent = cgroup_to_devcgroup(pcg);
        return verify_new_ex(parent, ex, childcg->behavior);
}

/*
 * parent_allows_removal - check if the parent cgroup allows an exception to
 *                         be removed
 * @childcg: child cgroup from where the exception will be removed
 * @ex: exception being removed
 */
static bool parent_allows_removal(struct dev_cgroup *childcg,
                                  struct dev_exception_item *ex)
{
        struct cgroup *pcg = childcg->css.cgroup->parent;
        struct dev_cgroup *parent;

        if (!pcg)
                return true;
        parent = cgroup_to_devcgroup(pcg);

        if (childcg->behavior == DEVCG_DEFAULT_DENY)
                /* It's always allowed to remove access to devices */
                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/cgroups/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);
        }
}

/**
 * get_online_devcg - walks the cgroup tree and fills a list with the online
 *                    groups
 * @root: cgroup used as starting point
 * @online: list that will be filled with online groups
 *
 * Must be called with devcgroup_mutex held. Grabs RCU lock.
 * Because devcgroup_mutex is held, no devcg will become online or offline
 * during the tree walk (see devcgroup_online, devcgroup_offline)
 * A separated list is needed because propagate_behavior() and
 * propagate_exception() need to allocate memory and can block.
 */
static void get_online_devcg(struct cgroup *root, struct list_head *online)
{
        struct cgroup *pos;
        struct dev_cgroup *devcg;

        lockdep_assert_held(&devcgroup_mutex);

        rcu_read_lock();
        cgroup_for_each_descendant_pre(pos, root) {
                devcg = cgroup_to_devcgroup(pos);
                if (is_devcg_online(devcg))
                        list_add_tail(&devcg->propagate_pending, online);
        }
        rcu_read_unlock();
}

/**
 * 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 *root = devcg_root->css.cgroup;
        struct dev_cgroup *devcg, *parent, *tmp;
        int rc = 0;
        LIST_HEAD(pending);

        get_online_devcg(root, &pending);

        list_for_each_entry_safe(devcg, tmp, &pending, propagate_pending) {
                parent = cgroup_to_devcgroup(devcg->css.cgroup->parent);

                /*
                 * 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);

                list_del_init(&devcg->propagate_pending);
        }
        return rc;
}

static inline bool has_children(struct dev_cgroup *devcgroup)
{
        struct cgroup *cgrp = devcgroup->css.cgroup;

        return !list_empty(&cgrp->children);
}

/*
 * 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, const 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 cgroup *p = devcgroup->css.cgroup;
        struct dev_cgroup *parent = NULL;

        if (!capable(CAP_SYS_ADMIN))
                return -EPERM;

        if (p->parent)
                parent = cgroup_to_devcgroup(p->parent);

        memset(&ex, 0, sizeof(ex));
        b = buffer;

        switch (*b) {
        case 'a':
                switch (filetype) {
                case DEVCG_ALLOW:
                        if (has_children(devcgroup))
                                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 (has_children(devcgroup))
                                return -EINVAL;

                        dev_exception_clean(devcgroup);
                        devcgroup->behavior = DEVCG_DEFAULT_DENY;
                        break;
                default:
                        return -EINVAL;
                }
                return 0;
        case 'b':
                ex.type = DEV_BLOCK;
                break;
        case 'c':
                ex.type = 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 |= ACC_READ;
                        break;
                case 'w':
                        ex.access |= ACC_WRITE;
                        break;
                case 'm':
                        ex.access |= 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 int devcgroup_access_write(struct cgroup *cgrp, struct cftype *cft,
                                  const char *buffer)
{
        int retval;

        mutex_lock(&devcgroup_mutex);
        retval = devcgroup_update_access(cgroup_to_devcgroup(cgrp),
                                         cft->private, buffer);
        mutex_unlock(&devcgroup_mutex);
        return retval;
}

static struct cftype dev_cgroup_files[] = {
        {
                .name = "allow",
                .write_string  = devcgroup_access_write,
                .private = DEVCG_ALLOW,
        },
        {
                .name = "deny",
                .write_string = devcgroup_access_write,
                .private = DEVCG_DENY,
        },
        {
                .name = "list",
                .read_seq_string = devcgroup_seq_read,
                .private = DEVCG_LIST,
        },
        { }     /* terminate */
};

struct cgroup_subsys devices_subsys = {
        .name = "devices",
        .can_attach = devcgroup_can_attach,
        .css_alloc = devcgroup_css_alloc,
        .css_free = devcgroup_css_free,
        .css_online = devcgroup_online,
        .css_offline = devcgroup_offline,
        .subsys_id = devices_subsys_id,
        .base_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 ACC_WRITE, ACC_READ and ACC_MKNOD
 *
 * returns 0 on success, -EPERM case the operation is not permitted
 */
static 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;
}

int __devcgroup_inode_permission(struct inode *inode, int mask)
{
        short type, access = 0;

        if (S_ISBLK(inode->i_mode))
                type = DEV_BLOCK;
        if (S_ISCHR(inode->i_mode))
                type = DEV_CHAR;
        if (mask & MAY_WRITE)
                access |= ACC_WRITE;
        if (mask & MAY_READ)
                access |= ACC_READ;

        return __devcgroup_check_permission(type, imajor(inode), iminor(inode),
                        access);
}

int devcgroup_inode_mknod(int mode, dev_t dev)
{
        short type;

        if (!S_ISBLK(mode) && !S_ISCHR(mode))
                return 0;

        if (S_ISBLK(mode))
                type = DEV_BLOCK;
        else
                type = DEV_CHAR;

        return __devcgroup_check_permission(type, MAJOR(dev), MINOR(dev),
                        ACC_MKNOD);

}