projects/linux·blob v4.8 · refs

348 lines · 8.9 KiB

1/*
2 * Process number limiting controller for cgroups.
3 *
4 * Used to allow a cgroup hierarchy to stop any new processes from fork()ing
5 * after a certain limit is reached.
6 *
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.
11 *
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.
19 *
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).
23 *
24 * pids.current tracks all child cgroup hierarchies, so parent/pids.current is
25 * a superset of parent/child/pids.current.
26 *
27 * Copyright (C) 2015 Aleksa Sarai <[email protected]>
28 *
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.
32 */
33
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>
39
40#define PIDS_MAX (PID_MAX_LIMIT + 1ULL)
41#define PIDS_MAX_STR "max"
42
43struct pids_cgroup {
44 struct cgroup_subsys_state css;
45
46 /*
47 * Use 64-bit types so that we can safely represent "max" as
48 * %PIDS_MAX = (%PID_MAX_LIMIT + 1).
49 */
50 atomic64_t counter;
51 int64_t limit;
52
53 /* Handle for "pids.events" */
54 struct cgroup_file events_file;
55
56 /* Number of times fork failed because limit was hit. */
57 atomic64_t events_limit;
58};
59
60static struct pids_cgroup *css_pids(struct cgroup_subsys_state *css)
61{
62 return container_of(css, struct pids_cgroup, css);
63}
64
65static struct pids_cgroup *parent_pids(struct pids_cgroup *pids)
66{
67 return css_pids(pids->css.parent);
68}
69
70static struct cgroup_subsys_state *
71pids_css_alloc(struct cgroup_subsys_state *parent)
72{
73 struct pids_cgroup *pids;
74
75 pids = kzalloc(sizeof(struct pids_cgroup), GFP_KERNEL);
76 if (!pids)
77 return ERR_PTR(-ENOMEM);
78
79 pids->limit = PIDS_MAX;
80 atomic64_set(&pids->counter, 0);
81 atomic64_set(&pids->events_limit, 0);
82 return &pids->css;
83}
84
85static void pids_css_free(struct cgroup_subsys_state *css)
86{
87 kfree(css_pids(css));
88}
89
90/**
91 * pids_cancel - uncharge the local pid count
92 * @pids: the pid cgroup state
93 * @num: the number of pids to cancel
94 *
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.
97 */
98static void pids_cancel(struct pids_cgroup *pids, int num)
99{
100 /*
101 * A negative count (or overflow for that matter) is invalid,
102 * and indicates a bug in the `pids` controller proper.
103 */
104 WARN_ON_ONCE(atomic64_add_negative(-num, &pids->counter));
105}
106
107/**
108 * pids_uncharge - hierarchically uncharge the pid count
109 * @pids: the pid cgroup state
110 * @num: the number of pids to uncharge
111 */
112static void pids_uncharge(struct pids_cgroup *pids, int num)
113{
114 struct pids_cgroup *p;
115
116 for (p = pids; parent_pids(p); p = parent_pids(p))
117 pids_cancel(p, num);
118}
119
120/**
121 * pids_charge - hierarchically charge the pid count
122 * @pids: the pid cgroup state
123 * @num: the number of pids to charge
124 *
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.
128 */
129static void pids_charge(struct pids_cgroup *pids, int num)
130{
131 struct pids_cgroup *p;
132
133 for (p = pids; parent_pids(p); p = parent_pids(p))
134 atomic64_add(num, &p->counter);
135}
136
137/**
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
141 *
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.
145 */
146static int pids_try_charge(struct pids_cgroup *pids, int num)
147{
148 struct pids_cgroup *p, *q;
149
150 for (p = pids; parent_pids(p); p = parent_pids(p)) {
151 int64_t new = atomic64_add_return(num, &p->counter);
152
153 /*
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
156 * fail.
157 */
158 if (new > p->limit)
159 goto revert;
160 }
161
162 return 0;
163
164revert:
165 for (q = pids; q != p; q = parent_pids(q))
166 pids_cancel(q, num);
167 pids_cancel(p, num);
168
169 return -EAGAIN;
170}
171
172static int pids_can_attach(struct cgroup_taskset *tset)
173{
174 struct task_struct *task;
175 struct cgroup_subsys_state *dst_css;
176
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;
181
182 /*
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.
186 */
187 old_css = task_css(task, pids_cgrp_id);
188 old_pids = css_pids(old_css);
189
190 pids_charge(pids, 1);
191 pids_uncharge(old_pids, 1);
192 }
193
194 return 0;
195}
196
197static void pids_cancel_attach(struct cgroup_taskset *tset)
198{
199 struct task_struct *task;
200 struct cgroup_subsys_state *dst_css;
201
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;
206
207 old_css = task_css(task, pids_cgrp_id);
208 old_pids = css_pids(old_css);
209
210 pids_charge(old_pids, 1);
211 pids_uncharge(pids, 1);
212 }
213}
214
215/*
216 * task_css_check(true) in pids_can_fork() and pids_cancel_fork() relies
217 * on threadgroup_change_begin() held by the copy_process().
218 */
219static int pids_can_fork(struct task_struct *task)
220{
221 struct cgroup_subsys_state *css;
222 struct pids_cgroup *pids;
223 int err;
224
225 css = task_css_check(current, pids_cgrp_id, true);
226 pids = css_pids(css);
227 err = pids_try_charge(pids, 1);
228 if (err) {
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));
233 pr_cont("\n");
234 }
235 cgroup_file_notify(&pids->events_file);
236 }
237 return err;
238}
239
240static void pids_cancel_fork(struct task_struct *task)
241{
242 struct cgroup_subsys_state *css;
243 struct pids_cgroup *pids;
244
245 css = task_css_check(current, pids_cgrp_id, true);
246 pids = css_pids(css);
247 pids_uncharge(pids, 1);
248}
249
250static void pids_free(struct task_struct *task)
251{
252 struct pids_cgroup *pids = css_pids(task_css(task, pids_cgrp_id));
253
254 pids_uncharge(pids, 1);
255}
256
257static ssize_t pids_max_write(struct kernfs_open_file *of, char *buf,
258 size_t nbytes, loff_t off)
259{
260 struct cgroup_subsys_state *css = of_css(of);
261 struct pids_cgroup *pids = css_pids(css);
262 int64_t limit;
263 int err;
264
265 buf = strstrip(buf);
266 if (!strcmp(buf, PIDS_MAX_STR)) {
267 limit = PIDS_MAX;
268 goto set_limit;
269 }
270
271 err = kstrtoll(buf, 0, &limit);
272 if (err)
273 return err;
274
275 if (limit < 0 || limit >= PIDS_MAX)
276 return -EINVAL;
277
278set_limit:
279 /*
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.
282 */
283 pids->limit = limit;
284 return nbytes;
285}
286
287static int pids_max_show(struct seq_file *sf, void *v)
288{
289 struct cgroup_subsys_state *css = seq_css(sf);
290 struct pids_cgroup *pids = css_pids(css);
291 int64_t limit = pids->limit;
292
293 if (limit >= PIDS_MAX)
294 seq_printf(sf, "%s\n", PIDS_MAX_STR);
295 else
296 seq_printf(sf, "%lld\n", limit);
297
298 return 0;
299}
300
301static s64 pids_current_read(struct cgroup_subsys_state *css,
302 struct cftype *cft)
303{
304 struct pids_cgroup *pids = css_pids(css);
305
306 return atomic64_read(&pids->counter);
307}
308
309static int pids_events_show(struct seq_file *sf, void *v)
310{
311 struct pids_cgroup *pids = css_pids(seq_css(sf));
312
313 seq_printf(sf, "max %lld\n", (s64)atomic64_read(&pids->events_limit));
314 return 0;
315}
316
317static struct cftype pids_files[] = {
318 {
319 .name = "max",
320 .write = pids_max_write,
321 .seq_show = pids_max_show,
322 .flags = CFTYPE_NOT_ON_ROOT,
323 },
324 {
325 .name = "current",
326 .read_s64 = pids_current_read,
327 .flags = CFTYPE_NOT_ON_ROOT,
328 },
329 {
330 .name = "events",
331 .seq_show = pids_events_show,
332 .file_offset = offsetof(struct pids_cgroup, events_file),
333 .flags = CFTYPE_NOT_ON_ROOT,
334 },
335 { } /* terminate */
336};
337
338struct 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,
345 .free = pids_free,
346 .legacy_cftypes = pids_files,
347 .dfl_cftypes = pids_files,
348};