kernel/cgroup/pids.c - linux - Git at Google

 // SPDX-License-Identifier: GPL-2.0-only
 /*
  * Process number limiting controller for cgroups.
  *
  * Used to allow a cgroup hierarchy to stop any new processes from fork()ing
  * after a certain limit is reached.
  *
  * Since it is trivial to hit the task limit without hitting any kmemcg limits
  * in place, PIDs are a fundamental resource. As such, PID exhaustion must be
  * preventable in the scope of a cgroup hierarchy by allowing resource limiting
  * of the number of tasks in a cgroup.
  *
  * In order to use the `pids` controller, set the maximum number of tasks in
  * pids.max (this is not available in the root cgroup for obvious reasons). The
  * number of processes currently in the cgroup is given by pids.current.
  * Organisational operations are not blocked by cgroup policies, so it is
  * possible to have pids.current > pids.max. However, it is not possible to
  * violate a cgroup policy through fork(). fork() will return -EAGAIN if forking
  * would cause a cgroup policy to be violated.
  *
  * To set a cgroup to have no limit, set pids.max to "max". This is the default
  * for all new cgroups (N.B. that PID limits are hierarchical, so the most
  * stringent limit in the hierarchy is followed).
  *
  * pids.current tracks all child cgroup hierarchies, so parent/pids.current is
  * a superset of parent/child/pids.current.
  *
  * Copyright (C) 2015 Aleksa Sarai <cyphar@cyphar.com>
  */

 #include <linux/kernel.h>
 #include <linux/threads.h>
 #include <linux/atomic.h>
 #include <linux/cgroup.h>
 #include <linux/slab.h>
 #include <linux/sched/task.h>

 #define PIDS_MAX (PID_MAX_LIMIT + 1ULL)
 #define PIDS_MAX_STR "max"

 enum pidcg_event {
 	/* Fork failed in subtree because this pids_cgroup limit was hit. */
 	PIDCG_MAX,
 	/* Fork failed in this pids_cgroup because ancestor limit was hit. */
 	PIDCG_FORKFAIL,
 	NR_PIDCG_EVENTS,
 };

 struct pids_cgroup {
 	struct cgroup_subsys_state	css;

 	/*
 	 * Use 64-bit types so that we can safely represent "max" as
 	 * %PIDS_MAX = (%PID_MAX_LIMIT + 1).
 	 */
 	atomic64_t			counter;
 	atomic64_t			limit;
 	int64_t				watermark;

 	/* Handles for pids.events[.local] */
 	struct cgroup_file		events_file;
 	struct cgroup_file		events_local_file;

 	atomic64_t			events[NR_PIDCG_EVENTS];
 	atomic64_t			events_local[NR_PIDCG_EVENTS];
 };

 static struct pids_cgroup *css_pids(struct cgroup_subsys_state *css)
 {
 	return container_of(css, struct pids_cgroup, css);
 }

 static struct pids_cgroup *parent_pids(struct pids_cgroup *pids)
 {
 	return css_pids(pids->css.parent);
 }

 static struct cgroup_subsys_state *
 pids_css_alloc(struct cgroup_subsys_state *parent)
 {
 	struct pids_cgroup *pids;

 	pids = kzalloc(sizeof(struct pids_cgroup), GFP_KERNEL);
 	if (!pids)
 		return ERR_PTR(-ENOMEM);

 	atomic64_set(&pids->limit, PIDS_MAX);
 	return &pids->css;
 }

 static void pids_css_free(struct cgroup_subsys_state *css)
 {
 	kfree(css_pids(css));
 }

 static void pids_update_watermark(struct pids_cgroup *p, int64_t nr_pids)
 {
 	/*
 	 * This is racy, but we don't need perfectly accurate tallying of
 	 * the watermark, and this lets us avoid extra atomic overhead.
 	 */
 	if (nr_pids > READ_ONCE(p->watermark))
 		WRITE_ONCE(p->watermark, nr_pids);
 }

 /**
  * pids_cancel - uncharge the local pid count
  * @pids: the pid cgroup state
  * @num: the number of pids to cancel
  *
  * This function will WARN if the pid count goes under 0, because such a case is
  * a bug in the pids controller proper.
  */
 static void pids_cancel(struct pids_cgroup *pids, int num)
 {
 	/*
 	 * A negative count (or overflow for that matter) is invalid,
 	 * and indicates a bug in the `pids` controller proper.
 	 */
 	WARN_ON_ONCE(atomic64_add_negative(-num, &pids->counter));
 }

 /**
  * pids_uncharge - hierarchically uncharge the pid count
  * @pids: the pid cgroup state
  * @num: the number of pids to uncharge
  */
 static void pids_uncharge(struct pids_cgroup *pids, int num)
 {
 	struct pids_cgroup *p;

 	for (p = pids; parent_pids(p); p = parent_pids(p))
 		pids_cancel(p, num);
 }

 /**
  * pids_charge - hierarchically charge the pid count
  * @pids: the pid cgroup state
  * @num: the number of pids to charge
  *
  * This function does *not* follow the pid limit set. It cannot fail and the new
  * pid count may exceed the limit. This is only used for reverting failed
  * attaches, where there is no other way out than violating the limit.
  */
 static void pids_charge(struct pids_cgroup *pids, int num)
 {
 	struct pids_cgroup *p;

 	for (p = pids; parent_pids(p); p = parent_pids(p)) {
 		int64_t new = atomic64_add_return(num, &p->counter);

 		pids_update_watermark(p, new);
 	}
 }

 /**
  * pids_try_charge - hierarchically try to charge the pid count
  * @pids: the pid cgroup state
  * @num: the number of pids to charge
  * @fail: storage of pid cgroup causing the fail
  *
  * This function follows the set limit. It will fail if the charge would cause
  * the new value to exceed the hierarchical limit. Returns 0 if the charge
  * succeeded, otherwise -EAGAIN.
  */
 static int pids_try_charge(struct pids_cgroup *pids, int num, struct pids_cgroup **fail)
 {
 	struct pids_cgroup *p, *q;

 	for (p = pids; parent_pids(p); p = parent_pids(p)) {
 		int64_t new = atomic64_add_return(num, &p->counter);
 		int64_t limit = atomic64_read(&p->limit);

 		/*
 		 * Since new is capped to the maximum number of pid_t, if
 		 * p->limit is %PIDS_MAX then we know that this test will never
 		 * fail.
 		 */
 		if (new > limit) {
 			*fail = p;
 			goto revert;
 		}
 		/*
 		 * Not technically accurate if we go over limit somewhere up
 		 * the hierarchy, but that's tolerable for the watermark.
 		 */
 		pids_update_watermark(p, new);
 	}

 	return 0;

 revert:
 	for (q = pids; q != p; q = parent_pids(q))
 		pids_cancel(q, num);
 	pids_cancel(p, num);

 	return -EAGAIN;
 }

 static int pids_can_attach(struct cgroup_taskset *tset)
 {
 	struct task_struct *task;
 	struct cgroup_subsys_state *dst_css;

 	cgroup_taskset_for_each(task, dst_css, tset) {
 		struct pids_cgroup *pids = css_pids(dst_css);
 		struct cgroup_subsys_state *old_css;
 		struct pids_cgroup *old_pids;

 		/*
 		 * No need to pin @old_css between here and cancel_attach()
 		 * because cgroup core protects it from being freed before
 		 * the migration completes or fails.
 		 */
 		old_css = task_css(task, pids_cgrp_id);
 		old_pids = css_pids(old_css);

 		pids_charge(pids, 1);
 		pids_uncharge(old_pids, 1);
 	}

 	return 0;
 }

 static void pids_cancel_attach(struct cgroup_taskset *tset)
 {
 	struct task_struct *task;
 	struct cgroup_subsys_state *dst_css;

 	cgroup_taskset_for_each(task, dst_css, tset) {
 		struct pids_cgroup *pids = css_pids(dst_css);
 		struct cgroup_subsys_state *old_css;
 		struct pids_cgroup *old_pids;

 		old_css = task_css(task, pids_cgrp_id);
 		old_pids = css_pids(old_css);

 		pids_charge(old_pids, 1);
 		pids_uncharge(pids, 1);
 	}
 }

 static void pids_event(struct pids_cgroup *pids_forking,
 		       struct pids_cgroup *pids_over_limit)
 {
 	struct pids_cgroup *p = pids_forking;

 	/* Only log the first time limit is hit. */
 	if (atomic64_inc_return(&p->events_local[PIDCG_FORKFAIL]) == 1) {
 		pr_info("cgroup: fork rejected by pids controller in ");
 		pr_cont_cgroup_path(p->css.cgroup);
 		pr_cont("\n");
 	}
 	if (!cgroup_subsys_on_dfl(pids_cgrp_subsys) ||
 	    cgrp_dfl_root.flags & CGRP_ROOT_PIDS_LOCAL_EVENTS) {
 		cgroup_file_notify(&p->events_local_file);
 		return;
 	}

 	atomic64_inc(&pids_over_limit->events_local[PIDCG_MAX]);
 	cgroup_file_notify(&pids_over_limit->events_local_file);

 	for (p = pids_over_limit; parent_pids(p); p = parent_pids(p)) {
 		atomic64_inc(&p->events[PIDCG_MAX]);
 		cgroup_file_notify(&p->events_file);
 	}
 }

 /*
  * task_css_check(true) in pids_can_fork() and pids_cancel_fork() relies
  * on cgroup_threadgroup_change_begin() held by the copy_process().
  */
 static int pids_can_fork(struct task_struct *task, struct css_set *cset)
 {
 	struct pids_cgroup *pids, *pids_over_limit;
 	int err;

 	pids = css_pids(cset->subsys[pids_cgrp_id]);
 	err = pids_try_charge(pids, 1, &pids_over_limit);
 	if (err)
 		pids_event(pids, pids_over_limit);

 	return err;
 }

 static void pids_cancel_fork(struct task_struct *task, struct css_set *cset)
 {
 	struct pids_cgroup *pids;

 	pids = css_pids(cset->subsys[pids_cgrp_id]);
 	pids_uncharge(pids, 1);
 }

 static void pids_release(struct task_struct *task)
 {
 	struct pids_cgroup *pids = css_pids(task_css(task, pids_cgrp_id));

 	pids_uncharge(pids, 1);
 }

 static ssize_t pids_max_write(struct kernfs_open_file *of, char *buf,
 			      size_t nbytes, loff_t off)
 {
 	struct cgroup_subsys_state *css = of_css(of);
 	struct pids_cgroup *pids = css_pids(css);
 	int64_t limit;
 	int err;

 	buf = strstrip(buf);
 	if (!strcmp(buf, PIDS_MAX_STR)) {
 		limit = PIDS_MAX;
 		goto set_limit;
 	}

 	err = kstrtoll(buf, 0, &limit);
 	if (err)
 		return err;

 	if (limit < 0 || limit >= PIDS_MAX)
 		return -EINVAL;

 set_limit:
 	/*
 	 * Limit updates don't need to be mutex'd, since it isn't
 	 * critical that any racing fork()s follow the new limit.
 	 */
 	atomic64_set(&pids->limit, limit);
 	return nbytes;
 }

 static int pids_max_show(struct seq_file *sf, void *v)
 {
 	struct cgroup_subsys_state *css = seq_css(sf);
 	struct pids_cgroup *pids = css_pids(css);
 	int64_t limit = atomic64_read(&pids->limit);

 	if (limit >= PIDS_MAX)
 		seq_printf(sf, "%s\n", PIDS_MAX_STR);
 	else
 		seq_printf(sf, "%lld\n", limit);

 	return 0;
 }

 static s64 pids_current_read(struct cgroup_subsys_state *css,
 			     struct cftype *cft)
 {
 	struct pids_cgroup *pids = css_pids(css);

 	return atomic64_read(&pids->counter);
 }

 static s64 pids_peak_read(struct cgroup_subsys_state *css,
 			  struct cftype *cft)
 {
 	struct pids_cgroup *pids = css_pids(css);

 	return READ_ONCE(pids->watermark);
 }

 static int __pids_events_show(struct seq_file *sf, bool local)
 {
 	struct pids_cgroup *pids = css_pids(seq_css(sf));
 	enum pidcg_event pe = PIDCG_MAX;
 	atomic64_t *events;

 	if (!cgroup_subsys_on_dfl(pids_cgrp_subsys) ||
 	    cgrp_dfl_root.flags & CGRP_ROOT_PIDS_LOCAL_EVENTS) {
 		pe = PIDCG_FORKFAIL;
 		local = true;
 	}
 	events = local ? pids->events_local : pids->events;

 	seq_printf(sf, "max %lld\n", (s64)atomic64_read(&events[pe]));
 	return 0;
 }

 static int pids_events_show(struct seq_file *sf, void *v)
 {
 	__pids_events_show(sf, false);
 	return 0;
 }

 static int pids_events_local_show(struct seq_file *sf, void *v)
 {
 	__pids_events_show(sf, true);
 	return 0;
 }

 static struct cftype pids_files[] = {
 	{
 		.name = "max",
 		.write = pids_max_write,
 		.seq_show = pids_max_show,
 		.flags = CFTYPE_NOT_ON_ROOT,
 	},
 	{
 		.name = "current",
 		.read_s64 = pids_current_read,
 		.flags = CFTYPE_NOT_ON_ROOT,
 	},
 	{
 		.name = "peak",
 		.flags = CFTYPE_NOT_ON_ROOT,
 		.read_s64 = pids_peak_read,
 	},
 	{
 		.name = "events",
 		.seq_show = pids_events_show,
 		.file_offset = offsetof(struct pids_cgroup, events_file),
 		.flags = CFTYPE_NOT_ON_ROOT,
 	},
 	{
 		.name = "events.local",
 		.seq_show = pids_events_local_show,
 		.file_offset = offsetof(struct pids_cgroup, events_local_file),
 		.flags = CFTYPE_NOT_ON_ROOT,
 	},
 	{ }	/* terminate */
 };

 static struct cftype pids_files_legacy[] = {
 	{
 		.name = "max",
 		.write = pids_max_write,
 		.seq_show = pids_max_show,
 		.flags = CFTYPE_NOT_ON_ROOT,
 	},
 	{
 		.name = "current",
 		.read_s64 = pids_current_read,
 		.flags = CFTYPE_NOT_ON_ROOT,
 	},
 	{
 		.name = "peak",
 		.flags = CFTYPE_NOT_ON_ROOT,
 		.read_s64 = pids_peak_read,
 	},
 	{
 		.name = "events",
 		.seq_show = pids_events_show,
 		.file_offset = offsetof(struct pids_cgroup, events_file),
 		.flags = CFTYPE_NOT_ON_ROOT,
 	},
 	{ }	/* terminate */
 };


 struct cgroup_subsys pids_cgrp_subsys = {
 	.css_alloc	= pids_css_alloc,
 	.css_free	= pids_css_free,
 	.can_attach 	= pids_can_attach,
 	.cancel_attach 	= pids_cancel_attach,
 	.can_fork	= pids_can_fork,
 	.cancel_fork	= pids_cancel_fork,
 	.release	= pids_release,
 	.legacy_cftypes = pids_files_legacy,
 	.dfl_cftypes	= pids_files,
 	.threaded	= true,
 };
	// SPDX-License-Identifier: GPL-2.0-only
	/*
	* Process number limiting controller for cgroups.
	*
	* Used to allow a cgroup hierarchy to stop any new processes from fork()ing
	* after a certain limit is reached.
	*
	* Since it is trivial to hit the task limit without hitting any kmemcg limits
	* in place, PIDs are a fundamental resource. As such, PID exhaustion must be
	* preventable in the scope of a cgroup hierarchy by allowing resource limiting
	* of the number of tasks in a cgroup.
	*
	* In order to use the `pids` controller, set the maximum number of tasks in
	* pids.max (this is not available in the root cgroup for obvious reasons). The
	* number of processes currently in the cgroup is given by pids.current.
	* Organisational operations are not blocked by cgroup policies, so it is
	* possible to have pids.current > pids.max. However, it is not possible to
	* violate a cgroup policy through fork(). fork() will return -EAGAIN if forking
	* would cause a cgroup policy to be violated.
	*
	* To set a cgroup to have no limit, set pids.max to "max". This is the default
	* for all new cgroups (N.B. that PID limits are hierarchical, so the most
	* stringent limit in the hierarchy is followed).
	*
	* pids.current tracks all child cgroup hierarchies, so parent/pids.current is
	* a superset of parent/child/pids.current.
	*
	* Copyright (C) 2015 Aleksa Sarai <cyphar@cyphar.com>
	*/

	#include <linux/kernel.h>
	#include <linux/threads.h>
	#include <linux/atomic.h>
	#include <linux/cgroup.h>
	#include <linux/slab.h>
	#include <linux/sched/task.h>

	#define PIDS_MAX (PID_MAX_LIMIT + 1ULL)
	#define PIDS_MAX_STR "max"

	enum pidcg_event {
	/* Fork failed in subtree because this pids_cgroup limit was hit. */
	PIDCG_MAX,
	/* Fork failed in this pids_cgroup because ancestor limit was hit. */
	PIDCG_FORKFAIL,
	NR_PIDCG_EVENTS,
	};

	struct pids_cgroup {
	struct cgroup_subsys_state css;

	/*
	* Use 64-bit types so that we can safely represent "max" as
	* %PIDS_MAX = (%PID_MAX_LIMIT + 1).
	*/
	atomic64_t counter;
	atomic64_t limit;
	int64_t watermark;

	/* Handles for pids.events[.local] */
	struct cgroup_file events_file;
	struct cgroup_file events_local_file;

	atomic64_t events[NR_PIDCG_EVENTS];
	atomic64_t events_local[NR_PIDCG_EVENTS];
	};

	static struct pids_cgroup css_pids(struct cgroup_subsys_state css)
	{
	return container_of(css, struct pids_cgroup, css);
	}

	static struct pids_cgroup parent_pids(struct pids_cgroup pids)
	{
	return css_pids(pids->css.parent);
	}

	static struct cgroup_subsys_state *
	pids_css_alloc(struct cgroup_subsys_state *parent)
	{
	struct pids_cgroup *pids;

	pids = kzalloc(sizeof(struct pids_cgroup), GFP_KERNEL);
	if (!pids)
	return ERR_PTR(-ENOMEM);

	atomic64_set(&pids->limit, PIDS_MAX);
	return &pids->css;
	}

	static void pids_css_free(struct cgroup_subsys_state *css)
	{
	kfree(css_pids(css));
	}

	static void pids_update_watermark(struct pids_cgroup *p, int64_t nr_pids)
	{
	/*
	* This is racy, but we don't need perfectly accurate tallying of
	* the watermark, and this lets us avoid extra atomic overhead.
	*/
	if (nr_pids > READ_ONCE(p->watermark))
	WRITE_ONCE(p->watermark, nr_pids);
	}

	/**
	* pids_cancel - uncharge the local pid count
	* @pids: the pid cgroup state
	* @num: the number of pids to cancel
	*
	* This function will WARN if the pid count goes under 0, because such a case is
	* a bug in the pids controller proper.
	*/
	static void pids_cancel(struct pids_cgroup *pids, int num)
	{
	/*
	* A negative count (or overflow for that matter) is invalid,
	* and indicates a bug in the `pids` controller proper.
	*/
	WARN_ON_ONCE(atomic64_add_negative(-num, &pids->counter));
	}

	/**
	* pids_uncharge - hierarchically uncharge the pid count
	* @pids: the pid cgroup state
	* @num: the number of pids to uncharge
	*/
	static void pids_uncharge(struct pids_cgroup *pids, int num)
	{
	struct pids_cgroup *p;

	for (p = pids; parent_pids(p); p = parent_pids(p))
	pids_cancel(p, num);
	}

	/**
	* pids_charge - hierarchically charge the pid count
	* @pids: the pid cgroup state
	* @num: the number of pids to charge
	*
	* This function does not follow the pid limit set. It cannot fail and the new
	* pid count may exceed the limit. This is only used for reverting failed
	* attaches, where there is no other way out than violating the limit.
	*/
	static void pids_charge(struct pids_cgroup *pids, int num)
	{
	struct pids_cgroup *p;

	for (p = pids; parent_pids(p); p = parent_pids(p)) {
	int64_t new = atomic64_add_return(num, &p->counter);

	pids_update_watermark(p, new);
	}
	}

	/**
	* pids_try_charge - hierarchically try to charge the pid count
	* @pids: the pid cgroup state
	* @num: the number of pids to charge
	* @fail: storage of pid cgroup causing the fail
	*
	* This function follows the set limit. It will fail if the charge would cause
	* the new value to exceed the hierarchical limit. Returns 0 if the charge
	* succeeded, otherwise -EAGAIN.
	*/
	static int pids_try_charge(struct pids_cgroup pids, int num, struct pids_cgroup *fail)
	{
	struct pids_cgroup p, q;

	for (p = pids; parent_pids(p); p = parent_pids(p)) {
	int64_t new = atomic64_add_return(num, &p->counter);
	int64_t limit = atomic64_read(&p->limit);

	/*
	* Since new is capped to the maximum number of pid_t, if
	* p->limit is %PIDS_MAX then we know that this test will never
	* fail.
	*/
	if (new > limit) {
	*fail = p;
	goto revert;
	}
	/*
	* Not technically accurate if we go over limit somewhere up
	* the hierarchy, but that's tolerable for the watermark.
	*/
	pids_update_watermark(p, new);
	}

	return 0;

	revert:
	for (q = pids; q != p; q = parent_pids(q))
	pids_cancel(q, num);
	pids_cancel(p, num);

	return -EAGAIN;
	}

	static int pids_can_attach(struct cgroup_taskset *tset)
	{
	struct task_struct *task;
	struct cgroup_subsys_state *dst_css;

	cgroup_taskset_for_each(task, dst_css, tset) {
	struct pids_cgroup *pids = css_pids(dst_css);
	struct cgroup_subsys_state *old_css;
	struct pids_cgroup *old_pids;

	/*
	* No need to pin @old_css between here and cancel_attach()
	* because cgroup core protects it from being freed before
	* the migration completes or fails.
	*/
	old_css = task_css(task, pids_cgrp_id);
	old_pids = css_pids(old_css);

	pids_charge(pids, 1);
	pids_uncharge(old_pids, 1);
	}

	return 0;
	}

	static void pids_cancel_attach(struct cgroup_taskset *tset)
	{
	struct task_struct *task;
	struct cgroup_subsys_state *dst_css;

	cgroup_taskset_for_each(task, dst_css, tset) {
	struct pids_cgroup *pids = css_pids(dst_css);
	struct cgroup_subsys_state *old_css;
	struct pids_cgroup *old_pids;

	old_css = task_css(task, pids_cgrp_id);
	old_pids = css_pids(old_css);

	pids_charge(old_pids, 1);
	pids_uncharge(pids, 1);
	}
	}

	static void pids_event(struct pids_cgroup *pids_forking,
	struct pids_cgroup *pids_over_limit)
	{
	struct pids_cgroup *p = pids_forking;

	/* Only log the first time limit is hit. */
	if (atomic64_inc_return(&p->events_local[PIDCG_FORKFAIL]) == 1) {
	pr_info("cgroup: fork rejected by pids controller in ");
	pr_cont_cgroup_path(p->css.cgroup);
	pr_cont("\n");
	}
	if (!cgroup_subsys_on_dfl(pids_cgrp_subsys) \|\|
	cgrp_dfl_root.flags & CGRP_ROOT_PIDS_LOCAL_EVENTS) {
	cgroup_file_notify(&p->events_local_file);
	return;
	}

	atomic64_inc(&pids_over_limit->events_local[PIDCG_MAX]);
	cgroup_file_notify(&pids_over_limit->events_local_file);

	for (p = pids_over_limit; parent_pids(p); p = parent_pids(p)) {
	atomic64_inc(&p->events[PIDCG_MAX]);
	cgroup_file_notify(&p->events_file);
	}
	}

	/*
	* task_css_check(true) in pids_can_fork() and pids_cancel_fork() relies
	* on cgroup_threadgroup_change_begin() held by the copy_process().
	*/
	static int pids_can_fork(struct task_struct task, struct css_set cset)
	{
	struct pids_cgroup pids, pids_over_limit;
	int err;

	pids = css_pids(cset->subsys[pids_cgrp_id]);
	err = pids_try_charge(pids, 1, &pids_over_limit);
	if (err)
	pids_event(pids, pids_over_limit);

	return err;
	}

	static void pids_cancel_fork(struct task_struct task, struct css_set cset)
	{
	struct pids_cgroup *pids;

	pids = css_pids(cset->subsys[pids_cgrp_id]);
	pids_uncharge(pids, 1);
	}

	static void pids_release(struct task_struct *task)
	{
	struct pids_cgroup *pids = css_pids(task_css(task, pids_cgrp_id));

	pids_uncharge(pids, 1);
	}

	static ssize_t pids_max_write(struct kernfs_open_file of, char buf,
	size_t nbytes, loff_t off)
	{
	struct cgroup_subsys_state *css = of_css(of);
	struct pids_cgroup *pids = css_pids(css);
	int64_t limit;
	int err;

	buf = strstrip(buf);
	if (!strcmp(buf, PIDS_MAX_STR)) {
	limit = PIDS_MAX;
	goto set_limit;
	}

	err = kstrtoll(buf, 0, &limit);
	if (err)
	return err;

	if (limit < 0 \|\| limit >= PIDS_MAX)
	return -EINVAL;

	set_limit:
	/*
	* Limit updates don't need to be mutex'd, since it isn't
	* critical that any racing fork()s follow the new limit.
	*/
	atomic64_set(&pids->limit, limit);
	return nbytes;
	}

	static int pids_max_show(struct seq_file sf, void v)
	{
	struct cgroup_subsys_state *css = seq_css(sf);
	struct pids_cgroup *pids = css_pids(css);
	int64_t limit = atomic64_read(&pids->limit);

	if (limit >= PIDS_MAX)
	seq_printf(sf, "%s\n", PIDS_MAX_STR);
	else
	seq_printf(sf, "%lld\n", limit);

	return 0;
	}

	static s64 pids_current_read(struct cgroup_subsys_state *css,
	struct cftype *cft)
	{
	struct pids_cgroup *pids = css_pids(css);

	return atomic64_read(&pids->counter);
	}

	static s64 pids_peak_read(struct cgroup_subsys_state *css,
	struct cftype *cft)
	{
	struct pids_cgroup *pids = css_pids(css);

	return READ_ONCE(pids->watermark);
	}

	static int __pids_events_show(struct seq_file *sf, bool local)
	{
	struct pids_cgroup *pids = css_pids(seq_css(sf));
	enum pidcg_event pe = PIDCG_MAX;
	atomic64_t *events;

	if (!cgroup_subsys_on_dfl(pids_cgrp_subsys) \|\|
	cgrp_dfl_root.flags & CGRP_ROOT_PIDS_LOCAL_EVENTS) {
	pe = PIDCG_FORKFAIL;
	local = true;
	}
	events = local ? pids->events_local : pids->events;

	seq_printf(sf, "max %lld\n", (s64)atomic64_read(&events[pe]));
	return 0;
	}

	static int pids_events_show(struct seq_file sf, void v)
	{
	__pids_events_show(sf, false);
	return 0;
	}

	static int pids_events_local_show(struct seq_file sf, void v)
	{
	__pids_events_show(sf, true);
	return 0;
	}

	static struct cftype pids_files[] = {
	{
	.name = "max",
	.write = pids_max_write,
	.seq_show = pids_max_show,
	.flags = CFTYPE_NOT_ON_ROOT,
	},
	{
	.name = "current",
	.read_s64 = pids_current_read,
	.flags = CFTYPE_NOT_ON_ROOT,
	},
	{
	.name = "peak",
	.flags = CFTYPE_NOT_ON_ROOT,
	.read_s64 = pids_peak_read,
	},
	{
	.name = "events",
	.seq_show = pids_events_show,
	.file_offset = offsetof(struct pids_cgroup, events_file),
	.flags = CFTYPE_NOT_ON_ROOT,
	},
	{
	.name = "events.local",
	.seq_show = pids_events_local_show,
	.file_offset = offsetof(struct pids_cgroup, events_local_file),
	.flags = CFTYPE_NOT_ON_ROOT,
	},
	{ } /* terminate */
	};

	static struct cftype pids_files_legacy[] = {
	{
	.name = "max",
	.write = pids_max_write,
	.seq_show = pids_max_show,
	.flags = CFTYPE_NOT_ON_ROOT,
	},
	{
	.name = "current",
	.read_s64 = pids_current_read,
	.flags = CFTYPE_NOT_ON_ROOT,
	},
	{
	.name = "peak",
	.flags = CFTYPE_NOT_ON_ROOT,
	.read_s64 = pids_peak_read,
	},
	{
	.name = "events",
	.seq_show = pids_events_show,
	.file_offset = offsetof(struct pids_cgroup, events_file),
	.flags = CFTYPE_NOT_ON_ROOT,
	},
	{ } /* terminate */
	};


	struct cgroup_subsys pids_cgrp_subsys = {
	.css_alloc = pids_css_alloc,
	.css_free = pids_css_free,
	.can_attach = pids_can_attach,
	.cancel_attach = pids_cancel_attach,
	.can_fork = pids_can_fork,
	.cancel_fork = pids_cancel_fork,
	.release = pids_release,
	.legacy_cftypes = pids_files_legacy,
	.dfl_cftypes = pids_files,
	.threaded = true,
	};