linuxdebug/kernel/cgroup/pids.c

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2024-07-16 15:50:57 +02:00
// 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"
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;
/* Handle for "pids.events" */
struct cgroup_file events_file;
/* Number of times fork failed because limit was hit. */
atomic64_t events_limit;
};
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->counter, 0);
atomic64_set(&pids->limit, PIDS_MAX);
atomic64_set(&pids->events_limit, 0);
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
*
* 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 *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)
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);
}
}
/*
* 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 cgroup_subsys_state *css;
struct pids_cgroup *pids;
int err;
if (cset)
css = cset->subsys[pids_cgrp_id];
else
css = task_css_check(current, pids_cgrp_id, true);
pids = css_pids(css);
err = pids_try_charge(pids, 1);
if (err) {
/* Only log the first time events_limit is incremented. */
if (atomic64_inc_return(&pids->events_limit) == 1) {
pr_info("cgroup: fork rejected by pids controller in ");
pr_cont_cgroup_path(css->cgroup);
pr_cont("\n");
}
cgroup_file_notify(&pids->events_file);
}
return err;
}
static void pids_cancel_fork(struct task_struct *task, struct css_set *cset)
{
struct cgroup_subsys_state *css;
struct pids_cgroup *pids;
if (cset)
css = cset->subsys[pids_cgrp_id];
else
css = task_css_check(current, pids_cgrp_id, true);
pids = css_pids(css);
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, void *v)
{
struct pids_cgroup *pids = css_pids(seq_css(sf));
seq_printf(sf, "max %lld\n", (s64)atomic64_read(&pids->events_limit));
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,
},
{ } /* 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,
.dfl_cftypes = pids_files,
.threaded = true,
};