2 * Process number limiting controller for cgroups.
4 * Used to allow a cgroup hierarchy to stop any new processes from fork()ing
5 * after a certain limit is reached.
7 * Since it is trivial to hit the task limit without hitting any kmemcg limits
8 * in place, PIDs are a fundamental resource. As such, PID exhaustion must be
9 * preventable in the scope of a cgroup hierarchy by allowing resource limiting
10 * of the number of tasks in a cgroup.
12 * In order to use the `pids` controller, set the maximum number of tasks in
13 * pids.max (this is not available in the root cgroup for obvious reasons). The
14 * number of processes currently in the cgroup is given by pids.current.
15 * Organisational operations are not blocked by cgroup policies, so it is
16 * possible to have pids.current > pids.max. However, it is not possible to
17 * violate a cgroup policy through fork(). fork() will return -EAGAIN if forking
18 * would cause a cgroup policy to be violated.
20 * To set a cgroup to have no limit, set pids.max to "max". This is the default
21 * for all new cgroups (N.B. that PID limits are hierarchical, so the most
22 * stringent limit in the hierarchy is followed).
24 * pids.current tracks all child cgroup hierarchies, so parent/pids.current is
25 * a superset of parent/child/pids.current.
27 * Copyright (C) 2015 Aleksa Sarai <cyphar@cyphar.com>
29 * This file is subject to the terms and conditions of version 2 of the GNU
30 * General Public License. See the file COPYING in the main directory of the
31 * Linux distribution for more details.
34 #include <linux/kernel.h>
35 #include <linux/threads.h>
36 #include <linux/atomic.h>
37 #include <linux/cgroup.h>
38 #include <linux/slab.h>
40 #define PIDS_MAX (PID_MAX_LIMIT + 1ULL)
41 #define PIDS_MAX_STR "max"
44 struct cgroup_subsys_state css;
47 * Use 64-bit types so that we can safely represent "max" as
48 * %PIDS_MAX = (%PID_MAX_LIMIT + 1).
53 /* Handle for "pids.events" */
54 struct cgroup_file events_file;
56 /* Number of times fork failed because limit was hit. */
57 atomic64_t events_limit;
60 static struct pids_cgroup *css_pids(struct cgroup_subsys_state *css)
62 return container_of(css, struct pids_cgroup, css);
65 static struct pids_cgroup *parent_pids(struct pids_cgroup *pids)
67 return css_pids(pids->css.parent);
70 static struct cgroup_subsys_state *
71 pids_css_alloc(struct cgroup_subsys_state *parent)
73 struct pids_cgroup *pids;
75 pids = kzalloc(sizeof(struct pids_cgroup), GFP_KERNEL);
77 return ERR_PTR(-ENOMEM);
79 pids->limit = PIDS_MAX;
80 atomic64_set(&pids->counter, 0);
81 atomic64_set(&pids->events_limit, 0);
85 static void pids_css_free(struct cgroup_subsys_state *css)
91 * pids_cancel - uncharge the local pid count
92 * @pids: the pid cgroup state
93 * @num: the number of pids to cancel
95 * This function will WARN if the pid count goes under 0, because such a case is
96 * a bug in the pids controller proper.
98 static void pids_cancel(struct pids_cgroup *pids, int num)
101 * A negative count (or overflow for that matter) is invalid,
102 * and indicates a bug in the `pids` controller proper.
104 WARN_ON_ONCE(atomic64_add_negative(-num, &pids->counter));
108 * pids_uncharge - hierarchically uncharge the pid count
109 * @pids: the pid cgroup state
110 * @num: the number of pids to uncharge
112 static void pids_uncharge(struct pids_cgroup *pids, int num)
114 struct pids_cgroup *p;
116 for (p = pids; parent_pids(p); p = parent_pids(p))
121 * pids_charge - hierarchically charge the pid count
122 * @pids: the pid cgroup state
123 * @num: the number of pids to charge
125 * This function does *not* follow the pid limit set. It cannot fail and the new
126 * pid count may exceed the limit. This is only used for reverting failed
127 * attaches, where there is no other way out than violating the limit.
129 static void pids_charge(struct pids_cgroup *pids, int num)
131 struct pids_cgroup *p;
133 for (p = pids; parent_pids(p); p = parent_pids(p))
134 atomic64_add(num, &p->counter);
138 * pids_try_charge - hierarchically try to charge the pid count
139 * @pids: the pid cgroup state
140 * @num: the number of pids to charge
142 * This function follows the set limit. It will fail if the charge would cause
143 * the new value to exceed the hierarchical limit. Returns 0 if the charge
144 * succeeded, otherwise -EAGAIN.
146 static int pids_try_charge(struct pids_cgroup *pids, int num)
148 struct pids_cgroup *p, *q;
150 for (p = pids; parent_pids(p); p = parent_pids(p)) {
151 int64_t new = atomic64_add_return(num, &p->counter);
154 * Since new is capped to the maximum number of pid_t, if
155 * p->limit is %PIDS_MAX then we know that this test will never
165 for (q = pids; q != p; q = parent_pids(q))
172 static int pids_can_attach(struct cgroup_taskset *tset)
174 struct task_struct *task;
175 struct cgroup_subsys_state *dst_css;
177 cgroup_taskset_for_each(task, dst_css, tset) {
178 struct pids_cgroup *pids = css_pids(dst_css);
179 struct cgroup_subsys_state *old_css;
180 struct pids_cgroup *old_pids;
183 * No need to pin @old_css between here and cancel_attach()
184 * because cgroup core protects it from being freed before
185 * the migration completes or fails.
187 old_css = task_css(task, pids_cgrp_id);
188 old_pids = css_pids(old_css);
190 pids_charge(pids, 1);
191 pids_uncharge(old_pids, 1);
197 static void pids_cancel_attach(struct cgroup_taskset *tset)
199 struct task_struct *task;
200 struct cgroup_subsys_state *dst_css;
202 cgroup_taskset_for_each(task, dst_css, tset) {
203 struct pids_cgroup *pids = css_pids(dst_css);
204 struct cgroup_subsys_state *old_css;
205 struct pids_cgroup *old_pids;
207 old_css = task_css(task, pids_cgrp_id);
208 old_pids = css_pids(old_css);
210 pids_charge(old_pids, 1);
211 pids_uncharge(pids, 1);
216 * task_css_check(true) in pids_can_fork() and pids_cancel_fork() relies
217 * on threadgroup_change_begin() held by the copy_process().
219 static int pids_can_fork(struct task_struct *task)
221 struct cgroup_subsys_state *css;
222 struct pids_cgroup *pids;
225 css = task_css_check(current, pids_cgrp_id, true);
226 pids = css_pids(css);
227 err = pids_try_charge(pids, 1);
229 /* Only log the first time events_limit is incremented. */
230 if (atomic64_inc_return(&pids->events_limit) == 1) {
231 pr_info("cgroup: fork rejected by pids controller in ");
232 pr_cont_cgroup_path(task_cgroup(current, pids_cgrp_id));
235 cgroup_file_notify(&pids->events_file);
240 static void pids_cancel_fork(struct task_struct *task)
242 struct cgroup_subsys_state *css;
243 struct pids_cgroup *pids;
245 css = task_css_check(current, pids_cgrp_id, true);
246 pids = css_pids(css);
247 pids_uncharge(pids, 1);
250 static void pids_free(struct task_struct *task)
252 struct pids_cgroup *pids = css_pids(task_css(task, pids_cgrp_id));
254 pids_uncharge(pids, 1);
257 static ssize_t pids_max_write(struct kernfs_open_file *of, char *buf,
258 size_t nbytes, loff_t off)
260 struct cgroup_subsys_state *css = of_css(of);
261 struct pids_cgroup *pids = css_pids(css);
266 if (!strcmp(buf, PIDS_MAX_STR)) {
271 err = kstrtoll(buf, 0, &limit);
275 if (limit < 0 || limit >= PIDS_MAX)
280 * Limit updates don't need to be mutex'd, since it isn't
281 * critical that any racing fork()s follow the new limit.
287 static int pids_max_show(struct seq_file *sf, void *v)
289 struct cgroup_subsys_state *css = seq_css(sf);
290 struct pids_cgroup *pids = css_pids(css);
291 int64_t limit = pids->limit;
293 if (limit >= PIDS_MAX)
294 seq_printf(sf, "%s\n", PIDS_MAX_STR);
296 seq_printf(sf, "%lld\n", limit);
301 static s64 pids_current_read(struct cgroup_subsys_state *css,
304 struct pids_cgroup *pids = css_pids(css);
306 return atomic64_read(&pids->counter);
309 static int pids_events_show(struct seq_file *sf, void *v)
311 struct pids_cgroup *pids = css_pids(seq_css(sf));
313 seq_printf(sf, "max %lld\n", (s64)atomic64_read(&pids->events_limit));
317 static struct cftype pids_files[] = {
320 .write = pids_max_write,
321 .seq_show = pids_max_show,
322 .flags = CFTYPE_NOT_ON_ROOT,
326 .read_s64 = pids_current_read,
327 .flags = CFTYPE_NOT_ON_ROOT,
331 .seq_show = pids_events_show,
332 .file_offset = offsetof(struct pids_cgroup, events_file),
333 .flags = CFTYPE_NOT_ON_ROOT,
338 struct cgroup_subsys pids_cgrp_subsys = {
339 .css_alloc = pids_css_alloc,
340 .css_free = pids_css_free,
341 .can_attach = pids_can_attach,
342 .cancel_attach = pids_cancel_attach,
343 .can_fork = pids_can_fork,
344 .cancel_fork = pids_cancel_fork,
346 .legacy_cftypes = pids_files,
347 .dfl_cftypes = pids_files,