cgroup.c

	lockdep_assert_held(&cgroup_mutex);
	lockdep_assert_held(&css_set_rwsem);

	src_cgrp = cset_cgroup_from_root(src_cset, dst_cgrp->root);

	if (!list_empty(&src_cset->mg_preload_node))
		return;

	WARN_ON(src_cset->mg_src_cgrp);
	WARN_ON(!list_empty(&src_cset->mg_tasks));
	WARN_ON(!list_empty(&src_cset->mg_node));

	src_cset->mg_src_cgrp = src_cgrp;
	get_css_set(src_cset);
	list_add(&src_cset->mg_preload_node, preloaded_csets);
}

/**
 * cgroup_migrate_prepare_dst - prepare destination css_sets for migration
 * @dst_cgrp: the destination cgroup (may be %NULL)
 * @preloaded_csets: list of preloaded source css_sets
 *
 * Tasks are about to be moved to @dst_cgrp and all the source css_sets
 * have been preloaded to @preloaded_csets.  This function looks up and
 * pins all destination css_sets, links each to its source, and append them
 * to @preloaded_csets.  If @dst_cgrp is %NULL, the destination of each
 * source css_set is assumed to be its cgroup on the default hierarchy.
 *
 * This function must be called after cgroup_migrate_add_src() has been
 * called on each migration source css_set.  After migration is performed
 * using cgroup_migrate(), cgroup_migrate_finish() must be called on
 * @preloaded_csets.
 */
static int cgroup_migrate_prepare_dst(struct cgroup *dst_cgrp,
				      struct list_head *preloaded_csets)
{
	LIST_HEAD(csets);
	struct css_set *src_cset, *tmp_cset;

	lockdep_assert_held(&cgroup_mutex);

	/*
	 * Except for the root, child_subsys_mask must be zero for a cgroup
	 * with tasks so that child cgroups don't compete against tasks.
	 */
	if (dst_cgrp && cgroup_on_dfl(dst_cgrp) && cgroup_parent(dst_cgrp) &&
	    dst_cgrp->child_subsys_mask)
		return -EBUSY;

	/* look up the dst cset for each src cset and link it to src */
	list_for_each_entry_safe(src_cset, tmp_cset, preloaded_csets, mg_preload_node) {
		struct css_set *dst_cset;

		dst_cset = find_css_set(src_cset,
					dst_cgrp ?: src_cset->dfl_cgrp);
		if (!dst_cset)
			goto err;

		WARN_ON_ONCE(src_cset->mg_dst_cset || dst_cset->mg_dst_cset);

		/*
		 * If src cset equals dst, it's noop.  Drop the src.
		 * cgroup_migrate() will skip the cset too.  Note that we
		 * can't handle src == dst as some nodes are used by both.
		 */
		if (src_cset == dst_cset) {
			src_cset->mg_src_cgrp = NULL;
			list_del_init(&src_cset->mg_preload_node);
			put_css_set(src_cset, false);
			put_css_set(dst_cset, false);
			continue;
		}

		src_cset->mg_dst_cset = dst_cset;

		if (list_empty(&dst_cset->mg_preload_node))
			list_add(&dst_cset->mg_preload_node, &csets);
		else
			put_css_set(dst_cset, false);
	}

	list_splice_tail(&csets, preloaded_csets);
	return 0;
err:
	cgroup_migrate_finish(&csets);
	return -ENOMEM;
}

/**
 * cgroup_migrate - migrate a process or task to a cgroup
 * @cgrp: the destination cgroup
 * @leader: the leader of the process or the task to migrate
 * @threadgroup: whether @leader points to the whole process or a single task
 *
 * Migrate a process or task denoted by @leader to @cgrp.  If migrating a
 * process, the caller must be holding threadgroup_lock of @leader.  The
 * caller is also responsible for invoking cgroup_migrate_add_src() and
 * cgroup_migrate_prepare_dst() on the targets before invoking this
 * function and following up with cgroup_migrate_finish().
 *
 * As long as a controller's ->can_attach() doesn't fail, this function is
 * guaranteed to succeed.  This means that, excluding ->can_attach()
 * failure, when migrating multiple targets, the success or failure can be
 * decided for all targets by invoking group_migrate_prepare_dst() before
 * actually starting migrating.
 */
static int cgroup_migrate(struct cgroup *cgrp, struct task_struct *leader,
			  bool threadgroup)
{
	struct cgroup_taskset tset = {
		.src_csets	= LIST_HEAD_INIT(tset.src_csets),
		.dst_csets	= LIST_HEAD_INIT(tset.dst_csets),
		.csets		= &tset.src_csets,
	};
	struct cgroup_subsys_state *css, *failed_css = NULL;
	struct css_set *cset, *tmp_cset;
	struct task_struct *task, *tmp_task;
	int i, ret;

	/*
	 * Prevent freeing of tasks while we take a snapshot. Tasks that are
	 * already PF_EXITING could be freed from underneath us unless we
	 * take an rcu_read_lock.
	 */
	down_write(&css_set_rwsem);
	rcu_read_lock();
	task = leader;
	do {
		/* @task either already exited or can't exit until the end */
		if (task->flags & PF_EXITING)
			goto next;

		/* leave @task alone if post_fork() hasn't linked it yet */
		if (list_empty(&task->cg_list))
			goto next;

		cset = task_css_set(task);
		if (!cset->mg_src_cgrp)
			goto next;

		/*
		 * cgroup_taskset_first() must always return the leader.
		 * Take care to avoid disturbing the ordering.
		 */
		list_move_tail(&task->cg_list, &cset->mg_tasks);
		if (list_empty(&cset->mg_node))
			list_add_tail(&cset->mg_node, &tset.src_csets);
		if (list_empty(&cset->mg_dst_cset->mg_node))
			list_move_tail(&cset->mg_dst_cset->mg_node,
				       &tset.dst_csets);
	next:
		if (!threadgroup)
			break;
	} while_each_thread(leader, task);
	rcu_read_unlock();
	up_write(&css_set_rwsem);

	/* methods shouldn't be called if no task is actually migrating */
	if (list_empty(&tset.src_csets))
		return 0;

	/* check that we can legitimately attach to the cgroup */
	for_each_e_css(css, i, cgrp) {
		if (css->ss->can_attach) {
			ret = css->ss->can_attach(css, &tset);
			if (ret) {
				failed_css = css;
				goto out_cancel_attach;
			}
		}
	}

	/*
	 * Now that we're guaranteed success, proceed to move all tasks to
	 * the new cgroup.  There are no failure cases after here, so this
	 * is the commit point.
	 */
	down_write(&css_set_rwsem);
	list_for_each_entry(cset, &tset.src_csets, mg_node) {
		list_for_each_entry_safe(task, tmp_task, &cset->mg_tasks, cg_list)
			cgroup_task_migrate(cset->mg_src_cgrp, task,
					    cset->mg_dst_cset);
	}
	up_write(&css_set_rwsem);

	/*
	 * Migration is committed, all target tasks are now on dst_csets.
	 * Nothing is sensitive to fork() after this point.  Notify
	 * controllers that migration is complete.
	 */
	tset.csets = &tset.dst_csets;

	for_each_e_css(css, i, cgrp)
		if (css->ss->attach)
			css->ss->attach(css, &tset);

	ret = 0;
	goto out_release_tset;

out_cancel_attach:
	for_each_e_css(css, i, cgrp) {
		if (css == failed_css)
			break;
		if (css->ss->cancel_attach)
			css->ss->cancel_attach(css, &tset);
	}
out_release_tset:
	down_write(&css_set_rwsem);
	list_splice_init(&tset.dst_csets, &tset.src_csets);
	list_for_each_entry_safe(cset, tmp_cset, &tset.src_csets, mg_node) {
		list_splice_tail_init(&cset->mg_tasks, &cset->tasks);
		list_del_init(&cset->mg_node);
	}
	up_write(&css_set_rwsem);
	return ret;
}

/**
 * cgroup_attach_task - attach a task or a whole threadgroup to a cgroup
 * @dst_cgrp: the cgroup to attach to
 * @leader: the task or the leader of the threadgroup to be attached
 * @threadgroup: attach the whole threadgroup?
 *
 * Call holding cgroup_mutex and threadgroup_lock of @leader.
 */
static int cgroup_attach_task(struct cgroup *dst_cgrp,
			      struct task_struct *leader, bool threadgroup)
{
	LIST_HEAD(preloaded_csets);
	struct task_struct *task;
	int ret;

	/* look up all src csets */
	down_read(&css_set_rwsem);
	rcu_read_lock();
	task = leader;
	do {
		cgroup_migrate_add_src(task_css_set(task), dst_cgrp,
				       &preloaded_csets);
		if (!threadgroup)
			break;
	} while_each_thread(leader, task);
	rcu_read_unlock();
	up_read(&css_set_rwsem);

	/* prepare dst csets and commit */
	ret = cgroup_migrate_prepare_dst(dst_cgrp, &preloaded_csets);
	if (!ret)
		ret = cgroup_migrate(dst_cgrp, leader, threadgroup);

	cgroup_migrate_finish(&preloaded_csets);
	return ret;
}

/*
 * Find the task_struct of the task to attach by vpid and pass it along to the
 * function to attach either it or all tasks in its threadgroup. Will lock
 * cgroup_mutex and threadgroup.
 */
static ssize_t __cgroup_procs_write(struct kernfs_open_file *of, char *buf,
				    size_t nbytes, loff_t off, bool threadgroup)
{
	struct task_struct *tsk;
	const struct cred *cred = current_cred(), *tcred;
	struct cgroup *cgrp;
	pid_t pid;
	int ret;

	if (kstrtoint(strstrip(buf), 0, &pid) || pid < 0)
		return -EINVAL;

	cgrp = cgroup_kn_lock_live(of->kn);
	if (!cgrp)
		return -ENODEV;

retry_find_task:
	rcu_read_lock();
	if (pid) {
		tsk = find_task_by_vpid(pid);
		if (!tsk) {
			rcu_read_unlock();
			ret = -ESRCH;
			goto out_unlock_cgroup;
		}
		/*
		 * even if we're attaching all tasks in the thread group, we
		 * only need to check permissions on one of them.
		 */
		tcred = __task_cred(tsk);
		if (!uid_eq(cred->euid, GLOBAL_ROOT_UID) &&
		    !uid_eq(cred->euid, tcred->uid) &&
		    !uid_eq(cred->euid, tcred->suid)) {
			rcu_read_unlock();
			ret = -EACCES;
			goto out_unlock_cgroup;
		}
	} else
		tsk = current;

	if (threadgroup)
		tsk = tsk->group_leader;

	/*
	 * Workqueue threads may acquire PF_NO_SETAFFINITY and become
	 * trapped in a cpuset, or RT worker may be born in a cgroup
	 * with no rt_runtime allocated.  Just say no.
	 */
	if (tsk == kthreadd_task || (tsk->flags & PF_NO_SETAFFINITY)) {
		ret = -EINVAL;
		rcu_read_unlock();
		goto out_unlock_cgroup;
	}

	get_task_struct(tsk);
	rcu_read_unlock();

	threadgroup_lock(tsk);
	if (threadgroup) {
		if (!thread_group_leader(tsk)) {
			/*
			 * a race with de_thread from another thread's exec()
			 * may strip us of our leadership, if this happens,
			 * there is no choice but to throw this task away and
			 * try again; this is
			 * "double-double-toil-and-trouble-check locking".
			 */
			threadgroup_unlock(tsk);
			put_task_struct(tsk);
			goto retry_find_task;
		}
	}

	ret = cgroup_attach_task(cgrp, tsk, threadgroup);

	threadgroup_unlock(tsk);

	put_task_struct(tsk);
out_unlock_cgroup:
	cgroup_kn_unlock(of->kn);
	return ret ?: nbytes;
}

/**
 * cgroup_attach_task_all - attach task 'tsk' to all cgroups of task 'from'
 * @from: attach to all cgroups of a given task
 * @tsk: the task to be attached
 */
int cgroup_attach_task_all(struct task_struct *from, struct task_struct *tsk)
{
	struct cgroup_root *root;
	int retval = 0;

	mutex_lock(&cgroup_mutex);
	for_each_root(root) {
		struct cgroup *from_cgrp;

		if (root == &cgrp_dfl_root)
			continue;

		down_read(&css_set_rwsem);
		from_cgrp = task_cgroup_from_root(from, root);
		up_read(&css_set_rwsem);

		retval = cgroup_attach_task(from_cgrp, tsk, false);
		if (retval)
			break;
	}
	mutex_unlock(&cgroup_mutex);

	return retval;
}
EXPORT_SYMBOL_GPL(cgroup_attach_task_all);

static ssize_t cgroup_tasks_write(struct kernfs_open_file *of,
				  char *buf, size_t nbytes, loff_t off)
{
	return __cgroup_procs_write(of, buf, nbytes, off, false);
}

static ssize_t cgroup_procs_write(struct kernfs_open_file *of,
				  char *buf, size_t nbytes, loff_t off)
{
	return __cgroup_procs_write(of, buf, nbytes, off, true);
}

static ssize_t cgroup_release_agent_write(struct kernfs_open_file *of,
					  char *buf, size_t nbytes, loff_t off)
{
	struct cgroup *cgrp;

	BUILD_BUG_ON(sizeof(cgrp->root->release_agent_path) < PATH_MAX);

	cgrp = cgroup_kn_lock_live(of->kn);
	if (!cgrp)
		return -ENODEV;
	spin_lock(&release_agent_path_lock);
	strlcpy(cgrp->root->release_agent_path, strstrip(buf),
		sizeof(cgrp->root->release_agent_path));
	spin_unlock(&release_agent_path_lock);
	cgroup_kn_unlock(of->kn);
	return nbytes;
}

static int cgroup_release_agent_show(struct seq_file *seq, void *v)
{
	struct cgroup *cgrp = seq_css(seq)->cgroup;

	spin_lock(&release_agent_path_lock);
	seq_puts(seq, cgrp->root->release_agent_path);
	spin_unlock(&release_agent_path_lock);
	seq_putc(seq, '\n');
	return 0;
}

static int cgroup_sane_behavior_show(struct seq_file *seq, void *v)
{
	struct cgroup *cgrp = seq_css(seq)->cgroup;

	seq_printf(seq, "%d\n", cgroup_sane_behavior(cgrp));
	return 0;
}

static void cgroup_print_ss_mask(struct seq_file *seq, unsigned int ss_mask)
{
	struct cgroup_subsys *ss;
	bool printed = false;
	int ssid;

	for_each_subsys(ss, ssid) {
		if (ss_mask & (1 << ssid)) {
			if (printed)
				seq_putc(seq, ' ');
			seq_printf(seq, "%s", ss->name);
			printed = true;
		}
	}
	if (printed)
		seq_putc(seq, '\n');
}

/* show controllers which are currently attached to the default hierarchy */
static int cgroup_root_controllers_show(struct seq_file *seq, void *v)
{
	struct cgroup *cgrp = seq_css(seq)->cgroup;

	cgroup_print_ss_mask(seq, cgrp->root->subsys_mask &
			     ~cgrp_dfl_root_inhibit_ss_mask);
	return 0;
}

/* show controllers which are enabled from the parent */
static int cgroup_controllers_show(struct seq_file *seq, void *v)
{
	struct cgroup *cgrp = seq_css(seq)->cgroup;

	cgroup_print_ss_mask(seq, cgroup_parent(cgrp)->child_subsys_mask);
	return 0;
}

/* show controllers which are enabled for a given cgroup's children */
static int cgroup_subtree_control_show(struct seq_file *seq, void *v)
{
	struct cgroup *cgrp = seq_css(seq)->cgroup;

	cgroup_print_ss_mask(seq, cgrp->child_subsys_mask);
	return 0;
}

/**
 * cgroup_update_dfl_csses - update css assoc of a subtree in default hierarchy
 * @cgrp: root of the subtree to update csses for
 *
 * @cgrp's child_subsys_mask has changed and its subtree's (self excluded)
 * css associations need to be updated accordingly.  This function looks up
 * all css_sets which are attached to the subtree, creates the matching
 * updated css_sets and migrates the tasks to the new ones.
 */
static int cgroup_update_dfl_csses(struct cgroup *cgrp)
{
	LIST_HEAD(preloaded_csets);
	struct cgroup_subsys_state *css;
	struct css_set *src_cset;
	int ret;

	lockdep_assert_held(&cgroup_mutex);

	/* look up all csses currently attached to @cgrp's subtree */
	down_read(&css_set_rwsem);
	css_for_each_descendant_pre(css, cgroup_css(cgrp, NULL)) {
		struct cgrp_cset_link *link;

		/* self is not affected by child_subsys_mask change */
		if (css->cgroup == cgrp)
			continue;

		list_for_each_entry(link, &css->cgroup->cset_links, cset_link)
			cgroup_migrate_add_src(link->cset, cgrp,
					       &preloaded_csets);
	}
	up_read(&css_set_rwsem);

	/* NULL dst indicates self on default hierarchy */
	ret = cgroup_migrate_prepare_dst(NULL, &preloaded_csets);
	if (ret)
		goto out_finish;

	list_for_each_entry(src_cset, &preloaded_csets, mg_preload_node) {
		struct task_struct *last_task = NULL, *task;

		/* src_csets precede dst_csets, break on the first dst_cset */
		if (!src_cset->mg_src_cgrp)
			break;

		/*
		 * All tasks in src_cset need to be migrated to the
		 * matching dst_cset.  Empty it process by process.  We
		 * walk tasks but migrate processes.  The leader might even
		 * belong to a different cset but such src_cset would also
		 * be among the target src_csets because the default
		 * hierarchy enforces per-process membership.
		 */
		while (true) {
			down_read(&css_set_rwsem);
			task = list_first_entry_or_null(&src_cset->tasks,
						struct task_struct, cg_list);
			if (task) {
				task = task->group_leader;
				WARN_ON_ONCE(!task_css_set(task)->mg_src_cgrp);
				get_task_struct(task);
			}
			up_read(&css_set_rwsem);

			if (!task)
				break;

			/* guard against possible infinite loop */
			if (WARN(last_task == task,
				 "cgroup: update_dfl_csses failed to make progress, aborting in inconsistent state\n"))
				goto out_finish;
			last_task = task;

			threadgroup_lock(task);
			/* raced against de_thread() from another thread? */
			if (!thread_group_leader(task)) {
				threadgroup_unlock(task);
				put_task_struct(task);
				continue;
			}

			ret = cgroup_migrate(src_cset->dfl_cgrp, task, true);

			threadgroup_unlock(task);
			put_task_struct(task);

			if (WARN(ret, "cgroup: failed to update controllers for the default hierarchy (%d), further operations may crash or hang\n", ret))
				goto out_finish;
		}
	}

out_finish:
	cgroup_migrate_finish(&preloaded_csets);
	return ret;
}

/* change the enabled child controllers for a cgroup in the default hierarchy */
static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
					    char *buf, size_t nbytes,
					    loff_t off)
{
	unsigned int enable = 0, disable = 0;
	struct cgroup *cgrp, *child;
	struct cgroup_subsys *ss;
	char *tok;
	int ssid, ret;

	/*
	 * Parse input - space separated list of subsystem names prefixed
	 * with either + or -.
	 */
	buf = strstrip(buf);
	while ((tok = strsep(&buf, " "))) {
		if (tok[0] == '\0')
			continue;
		for_each_subsys(ss, ssid) {
			if (ss->disabled || strcmp(tok + 1, ss->name) ||
			    ((1 << ss->id) & cgrp_dfl_root_inhibit_ss_mask))
				continue;

			if (*tok == '+') {
				enable |= 1 << ssid;
				disable &= ~(1 << ssid);
			} else if (*tok == '-') {
				disable |= 1 << ssid;
				enable &= ~(1 << ssid);
			} else {
				return -EINVAL;
			}
			break;
		}
		if (ssid == CGROUP_SUBSYS_COUNT)
			return -EINVAL;
	}

	cgrp = cgroup_kn_lock_live(of->kn);
	if (!cgrp)
		return -ENODEV;

	for_each_subsys(ss, ssid) {
		if (enable & (1 << ssid)) {
			if (cgrp->child_subsys_mask & (1 << ssid)) {
				enable &= ~(1 << ssid);
				continue;
			}

			/*
			 * Because css offlining is asynchronous, userland
			 * might try to re-enable the same controller while
			 * the previous instance is still around.  In such
			 * cases, wait till it's gone using offline_waitq.
			 */
			cgroup_for_each_live_child(child, cgrp) {
				DEFINE_WAIT(wait);

				if (!cgroup_css(child, ss))
					continue;

				cgroup_get(child);
				prepare_to_wait(&child->offline_waitq, &wait,
						TASK_UNINTERRUPTIBLE);
				cgroup_kn_unlock(of->kn);
				schedule();
				finish_wait(&child->offline_waitq, &wait);
				cgroup_put(child);

				return restart_syscall();
			}

			/* unavailable or not enabled on the parent? */
			if (!(cgrp_dfl_root.subsys_mask & (1 << ssid)) ||
			    (cgroup_parent(cgrp) &&
			     !(cgroup_parent(cgrp)->child_subsys_mask & (1 << ssid)))) {
				ret = -ENOENT;
				goto out_unlock;
			}
		} else if (disable & (1 << ssid)) {
			if (!(cgrp->child_subsys_mask & (1 << ssid))) {
				disable &= ~(1 << ssid);
				continue;
			}

			/* a child has it enabled? */
			cgroup_for_each_live_child(child, cgrp) {
				if (child->child_subsys_mask & (1 << ssid)) {
					ret = -EBUSY;
					goto out_unlock;
				}
			}
		}
	}

	if (!enable && !disable) {
		ret = 0;
		goto out_unlock;
	}

	/*
	 * Except for the root, child_subsys_mask must be zero for a cgroup
	 * with tasks so that child cgroups don't compete against tasks.
	 */
	if (enable && cgroup_parent(cgrp) && !list_empty(&cgrp->cset_links)) {
		ret = -EBUSY;
		goto out_unlock;
	}

	/*
	 * Create csses for enables and update child_subsys_mask.  This
	 * changes cgroup_e_css() results which in turn makes the
	 * subsequent cgroup_update_dfl_csses() associate all tasks in the
	 * subtree to the updated csses.
	 */
	for_each_subsys(ss, ssid) {
		if (!(enable & (1 << ssid)))
			continue;

		cgroup_for_each_live_child(child, cgrp) {
			ret = create_css(child, ss);
			if (ret)
				goto err_undo_css;
		}
	}

	cgrp->child_subsys_mask |= enable;
	cgrp->child_subsys_mask &= ~disable;

	ret = cgroup_update_dfl_csses(cgrp);
	if (ret)
		goto err_undo_css;

	/* all tasks are now migrated away from the old csses, kill them */
	for_each_subsys(ss, ssid) {
		if (!(disable & (1 << ssid)))
			continue;

		cgroup_for_each_live_child(child, cgrp)
			kill_css(cgroup_css(child, ss));
	}

	kernfs_activate(cgrp->kn);
	ret = 0;
out_unlock:
	cgroup_kn_unlock(of->kn);
	return ret ?: nbytes;

err_undo_css:
	cgrp->child_subsys_mask &= ~enable;
	cgrp->child_subsys_mask |= disable;

	for_each_subsys(ss, ssid) {
		if (!(enable & (1 << ssid)))
			continue;

		cgroup_for_each_live_child(child, cgrp) {
			struct cgroup_subsys_state *css = cgroup_css(child, ss);
			if (css)
				kill_css(css);
		}
	}
	goto out_unlock;
}

static int cgroup_populated_show(struct seq_file *seq, void *v)
{
	seq_printf(seq, "%d\n", (bool)seq_css(seq)->cgroup->populated_cnt);
	return 0;
}

static ssize_t cgroup_file_write(struct kernfs_open_file *of, char *buf,
				 size_t nbytes, loff_t off)
{
	struct cgroup *cgrp = of->kn->parent->priv;
	struct cftype *cft = of->kn->priv;
	struct cgroup_subsys_state *css;
	int ret;

	if (cft->write)
		return cft->write(of, buf, nbytes, off);

	/*
	 * kernfs guarantees that a file isn't deleted with operations in
	 * flight, which means that the matching css is and stays alive and
	 * doesn't need to be pinned.  The RCU locking is not necessary
	 * either.  It's just for the convenience of using cgroup_css().
	 */
	rcu_read_lock();
	css = cgroup_css(cgrp, cft->ss);
	rcu_read_unlock();

	if (cft->write_u64) {
		unsigned long long v;
		ret = kstrtoull(buf, 0, &v);
		if (!ret)
			ret = cft->write_u64(css, cft, v);
	} else if (cft->write_s64) {
		long long v;
		ret = kstrtoll(buf, 0, &v);
		if (!ret)
			ret = cft->write_s64(css, cft, v);
	} else {
		ret = -EINVAL;
	}

	return ret ?: nbytes;
}

static void *cgroup_seqfile_start(struct seq_file *seq, loff_t *ppos)
{
	return seq_cft(seq)->seq_start(seq, ppos);
}

static void *cgroup_seqfile_next(struct seq_file *seq, void *v, loff_t *ppos)
{
	return seq_cft(seq)->seq_next(seq, v, ppos);
}

static void cgroup_seqfile_stop(struct seq_file *seq, void *v)
{
	seq_cft(seq)->seq_stop(seq, v);
}

static int cgroup_seqfile_show(struct seq_file *m, void *arg)
{
	struct cftype *cft = seq_cft(m);
	struct cgroup_subsys_state *css = seq_css(m);

	if (cft->seq_show)
		return cft->seq_show(m, arg);

	if (cft->read_u64)
		seq_printf(m, "%llu\n", cft->read_u64(css, cft));
	else if (cft->read_s64)
		seq_printf(m, "%lld\n", cft->read_s64(css, cft));
	else
		return -EINVAL;
	return 0;
}

static struct kernfs_ops cgroup_kf_single_ops = {
	.atomic_write_len	= PAGE_SIZE,
	.write			= cgroup_file_write,
	.seq_show		= cgroup_seqfile_show,
};

static struct kernfs_ops cgroup_kf_ops = {
	.atomic_write_len	= PAGE_SIZE,
	.write			= cgroup_file_write,
	.seq_start		= cgroup_seqfile_start,
	.seq_next		= cgroup_seqfile_next,
	.seq_stop		= cgroup_seqfile_stop,
	.seq_show		= cgroup_seqfile_show,
};

/*
 * cgroup_rename - Only allow simple rename of directories in place.
 */
static int cgroup_rename(struct kernfs_node *kn, struct kernfs_node *new_parent,
			 const char *new_name_str)
{
	struct cgroup *cgrp = kn->priv;
	int ret;

	if (kernfs_type(kn) != KERNFS_DIR)
		return -ENOTDIR;
	if (kn->parent != new_parent)
		return -EIO;

	/*
	 * This isn't a proper migration and its usefulness is very
	 * limited.  Disallow if sane_behavior.
	 */
	if (cgroup_sane_behavior(cgrp))
		return -EPERM;

	/*
	 * We're gonna grab cgroup_mutex which nests outside kernfs
	 * active_ref.  kernfs_rename() doesn't require active_ref
	 * protection.  Break them before grabbing cgroup_mutex.
	 */
	kernfs_break_active_protection(new_parent);
	kernfs_break_active_protection(kn);

	mutex_lock(&cgroup_mutex);

	ret = kernfs_rename(kn, new_parent, new_name_str);

	mutex_unlock(&cgroup_mutex);

	kernfs_unbreak_active_protection(kn);
	kernfs_unbreak_active_protection(new_parent);
	return ret;
}

/* set uid and gid of cgroup dirs and files to that of the creator */
static int cgroup_kn_set_ugid(struct kernfs_node *kn)
{
	struct iattr iattr = { .ia_valid = ATTR_UID | ATTR_GID,
			       .ia_uid = current_fsuid(),
			       .ia_gid = current_fsgid(), };

	if (uid_eq(iattr.ia_uid, GLOBAL_ROOT_UID) &&
	    gid_eq(iattr.ia_gid, GLOBAL_ROOT_GID))
		return 0;

	return kernfs_setattr(kn, &iattr);
}

static int cgroup_add_file(struct cgroup *cgrp, struct cftype *cft)
{
	char name[CGROUP_FILE_NAME_MAX];
	struct kernfs_node *kn;
	struct lock_class_key *key = NULL;
	int ret;

#ifdef CONFIG_DEBUG_LOCK_ALLOC
	key = &cft->lockdep_key;
#endif
	kn = __kernfs_create_file(cgrp->kn, cgroup_file_name(cgrp, cft, name),
				  cgroup_file_mode(cft), 0, cft->kf_ops, cft,
				  NULL, false, key);
	if (IS_ERR(kn))
		return PTR_ERR(kn);

	ret = cgroup_kn_set_ugid(kn);
	if (ret) {
		kernfs_remove(kn);
		return ret;
	}

	if (cft->seq_show == cgroup_populated_show)
		cgrp->populated_kn = kn;
	return 0;
}

/**
 * cgroup_addrm_files - add or remove files to a cgroup directory
 * @cgrp: the target cgroup
 * @cfts: array of cftypes to be added
 * @is_add: whether to add or remove
 *
 * Depending on @is_add, add or remove files defined by @cfts on @cgrp.
 * For removals, this function never fails.  If addition fails, this
 * function doesn't remove files already added.  The caller is responsible
 * for cleaning up.
 */
static int cgroup_addrm_files(struct cgroup *cgrp, struct cftype cfts[],
			      bool is_add)
{
	struct cftype *cft;
	int ret;

	lockdep_assert_held(&cgroup_mutex);

	for (cft = cfts; cft->name[0] != '\0'; cft++) {
		/* does cft->flags tell us to skip this file on @cgrp? */
		if ((cft->flags & CFTYPE_ONLY_ON_DFL) && !cgroup_on_dfl(cgrp))
			continue;
		if ((cft->flags & CFTYPE_INSANE) && cgroup_sane_behavior(cgrp))
			continue;
		if ((cft->flags & CFTYPE_NOT_ON_ROOT) && !cgroup_parent(cgrp))
			continue;
		if ((cft->flags & CFTYPE_ONLY_ON_ROOT) && cgroup_parent(cgrp))
			continue;

		if (is_add) {
			ret = cgroup_add_file(cgrp, cft);
			if (ret) {
				pr_warn("%s: failed to add %s, err=%d\n",
					__func__, cft->name, ret);
				return ret;
			}
		} else {
			cgroup_rm_file(cgrp, cft);
		}
	}
	return 0;
}

static int cgroup_apply_cftypes(struct cftype *cfts, bool is_add)
{
	LIST_HEAD(pending);
	struct cgroup_subsys *ss = cfts[0].ss;
	struct cgroup *root = &ss->root->cgrp;
	struct cgroup_subsys_state *css;
	int ret = 0;

	lockdep_assert_held(&cgroup_mutex);

	/* add/rm files for all cgroups created before */
	css_for_each_descendant_pre(css, cgroup_css(root, ss)) {
		struct cgroup *cgrp = css->cgroup;

		if (cgroup_is_dead(cgrp))
			continue;

		ret = cgroup_addrm_files(cgrp, cfts, is_add);
		if (ret)
			break;
	}

	if (is_add && !ret)
		kernfs_activate(root->kn);
	return ret;
}

static void cgroup_exit_cftypes(struct cftype *cfts)
{
	struct cftype *cft;

	for (cft = cfts; cft->name[0] != '\0'; cft++) {
		/* free copy for custom atomic_write_len, see init_cftypes() */
		if (cft->max_write_len && cft->max_write_len != PAGE_SIZE)
			kfree(cft->kf_ops);
		cft->kf_ops = NULL;
		cft->ss = NULL;
	}
}

static int cgroup_init_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
{
	struct cftype *cft;

	for (cft = cfts; cft->name[0] != '\0'; cft++) {
		struct kernfs_ops *kf_ops;

		WARN_ON(cft->ss || cft->kf_ops);

		if (cft->seq_start)
			kf_ops = &cgroup_kf_ops;
		else
			kf_ops = &cgroup_kf_single_ops;