Systemd/src/basic/cgroup-util.c

/* SPDX-License-Identifier: LGPL-2.1+ */

#include <dirent.h>
#include <errno.h>
#include <ftw.h>
#include <limits.h>
#include <signal.h>
#include <stddef.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/statfs.h>
#include <sys/types.h>
#include <sys/utsname.h>
#include <sys/xattr.h>
#include <unistd.h>

#include "alloc-util.h"
#include "cgroup-util.h"
#include "def.h"
#include "dirent-util.h"
#include "extract-word.h"
#include "fd-util.h"
#include "fileio.h"
#include "format-util.h"
#include "fs-util.h"
#include "log.h"
#include "login-util.h"
#include "macro.h"
#include "missing.h"
#include "mkdir.h"
#include "parse-util.h"
#include "path-util.h"
#include "proc-cmdline.h"
#include "process-util.h"
#include "set.h"
#include "special.h"
#include "stat-util.h"
#include "stdio-util.h"
#include "string-table.h"
#include "string-util.h"
#include "strv.h"
#include "unit-name.h"
#include "user-util.h"

static int cg_enumerate_items(const char *controller, const char *path, FILE **_f, const char *item) {
        _cleanup_free_ char *fs = NULL;
        FILE *f;
        int r;

        assert(_f);

        r = cg_get_path(controller, path, item, &fs);
        if (r < 0)
                return r;

        f = fopen(fs, "re");
        if (!f)
                return -errno;

        *_f = f;
        return 0;
}

int cg_enumerate_processes(const char *controller, const char *path, FILE **_f) {
        return cg_enumerate_items(controller, path, _f, "cgroup.procs");
}

int cg_read_pid(FILE *f, pid_t *_pid) {
        unsigned long ul;

        /* Note that the cgroup.procs might contain duplicates! See
         * cgroups.txt for details. */

        assert(f);
        assert(_pid);

        errno = 0;
        if (fscanf(f, "%lu", &ul) != 1) {

                if (feof(f))
                        return 0;

                return errno_or_else(EIO);
        }

        if (ul <= 0)
                return -EIO;

        *_pid = (pid_t) ul;
        return 1;
}

int cg_read_event(
                const char *controller,
                const char *path,
                const char *event,
                char **ret) {

        _cleanup_free_ char *events = NULL, *content = NULL;
        int r;

        r = cg_get_path(controller, path, "cgroup.events", &events);
        if (r < 0)
                return r;

        r = read_full_file(events, &content, NULL);
        if (r < 0)
                return r;

        for (const char *p = content;;) {
                _cleanup_free_ char *line = NULL, *key = NULL, *val = NULL;
                const char *q;

                r = extract_first_word(&p, &line, "\n", 0);
                if (r < 0)
                        return r;
                if (r == 0)
                        return -ENOENT;

                q = line;
                r = extract_first_word(&q, &key, " ", 0);
                if (r < 0)
                        return r;
                if (r == 0)
                        return -EINVAL;

                if (!streq(key, event))
                        continue;

                val = strdup(q);
                if (!val)
                        return -ENOMEM;

                *ret = TAKE_PTR(val);
                return 0;
        }
}

bool cg_ns_supported(void) {
        static thread_local int enabled = -1;

        if (enabled >= 0)
                return enabled;

        if (access("/proc/self/ns/cgroup", F_OK) < 0) {
                if (errno != ENOENT)
                        log_debug_errno(errno, "Failed to check whether /proc/self/ns/cgroup is available, assuming not: %m");
                enabled = false;
        } else
                enabled = true;

        return enabled;
}

int cg_enumerate_subgroups(const char *controller, const char *path, DIR **_d) {
        _cleanup_free_ char *fs = NULL;
        int r;
        DIR *d;

        assert(_d);

        /* This is not recursive! */

        r = cg_get_path(controller, path, NULL, &fs);
        if (r < 0)
                return r;

        d = opendir(fs);
        if (!d)
                return -errno;

        *_d = d;
        return 0;
}

int cg_read_subgroup(DIR *d, char **fn) {
        struct dirent *de;

        assert(d);
        assert(fn);

        FOREACH_DIRENT_ALL(de, d, return -errno) {
                char *b;

                if (de->d_type != DT_DIR)
                        continue;

                if (dot_or_dot_dot(de->d_name))
                        continue;

                b = strdup(de->d_name);
                if (!b)
                        return -ENOMEM;

                *fn = b;
                return 1;
        }

        return 0;
}

int cg_rmdir(const char *controller, const char *path) {
        _cleanup_free_ char *p = NULL;
        int r;

        r = cg_get_path(controller, path, NULL, &p);
        if (r < 0)
                return r;

        r = rmdir(p);
        if (r < 0 && errno != ENOENT)
                return -errno;

        r = cg_hybrid_unified();
        if (r <= 0)
                return r;

        if (streq(controller, SYSTEMD_CGROUP_CONTROLLER)) {
                r = cg_rmdir(SYSTEMD_CGROUP_CONTROLLER_LEGACY, path);
                if (r < 0)
                        log_warning_errno(r, "Failed to remove compat systemd cgroup %s: %m", path);
        }

        return 0;
}

static int cg_kill_items(
                const char *controller,
                const char *path,
                int sig,
                CGroupFlags flags,
                Set *s,
                cg_kill_log_func_t log_kill,
                void *userdata,
                const char *item) {

        _cleanup_set_free_ Set *allocated_set = NULL;
        bool done = false;
        int r, ret = 0, ret_log_kill = 0;
        pid_t my_pid;

        assert(sig >= 0);

         /* Don't send SIGCONT twice. Also, SIGKILL always works even when process is suspended, hence don't send
          * SIGCONT on SIGKILL. */
        if (IN_SET(sig, SIGCONT, SIGKILL))
                flags &= ~CGROUP_SIGCONT;

        /* This goes through the tasks list and kills them all. This
         * is repeated until no further processes are added to the
         * tasks list, to properly handle forking processes */

        if (!s) {
                s = allocated_set = set_new(NULL);
                if (!s)
                        return -ENOMEM;
        }

        my_pid = getpid_cached();

        do {
                _cleanup_fclose_ FILE *f = NULL;
                pid_t pid = 0;
                done = true;

                r = cg_enumerate_items(controller, path, &f, item);
                if (r < 0) {
                        if (ret >= 0 && r != -ENOENT)
                                return r;

                        return ret;
                }

                while ((r = cg_read_pid(f, &pid)) > 0) {

                        if ((flags & CGROUP_IGNORE_SELF) && pid == my_pid)
                                continue;

                        if (set_get(s, PID_TO_PTR(pid)) == PID_TO_PTR(pid))
                                continue;

                        if (log_kill)
                                ret_log_kill = log_kill(pid, sig, userdata);

                        /* If we haven't killed this process yet, kill
                         * it */
                        if (kill(pid, sig) < 0) {
                                if (ret >= 0 && errno != ESRCH)
                                        ret = -errno;
                        } else {
                                if (flags & CGROUP_SIGCONT)
                                        (void) kill(pid, SIGCONT);

                                if (ret == 0) {
                                        if (log_kill)
                                                ret = ret_log_kill;
                                        else
                                                ret = 1;
                                }
                        }

                        done = false;

                        r = set_put(s, PID_TO_PTR(pid));
                        if (r < 0) {
                                if (ret >= 0)
                                        return r;

                                return ret;
                        }
                }

                if (r < 0) {
                        if (ret >= 0)
                                return r;

                        return ret;
                }

                /* To avoid racing against processes which fork
                 * quicker than we can kill them we repeat this until
                 * no new pids need to be killed. */

        } while (!done);

        return ret;
}

int cg_kill(
                const char *controller,
                const char *path,
                int sig,
                CGroupFlags flags,
                Set *s,
                cg_kill_log_func_t log_kill,
                void *userdata) {
        int r;

        r = cg_kill_items(controller, path, sig, flags, s, log_kill, userdata, "cgroup.procs");
        if (r < 0 || sig != SIGKILL)
                return r;

        /* Only in case of killing with SIGKILL and when using cgroupsv2, kill remaining threads manually as
           a workaround for kernel bug. It was fixed in 5.2-rc5 (c03cd7738a83), backported to 4.19.66
           (4340d175b898) and 4.14.138 (feb6b123b7dd). */
        r = cg_unified_controller(controller);
        if (r < 0)
                return r;
        if (r == 0) /* doesn't apply to legacy hierarchy */
                return 0;

        return cg_kill_items(controller, path, sig, flags, s, log_kill, userdata, "cgroup.threads");
}

int cg_kill_recursive(
                const char *controller,
                const char *path,
                int sig,
                CGroupFlags flags,
                Set *s,
                cg_kill_log_func_t log_kill,
                void *userdata) {

        _cleanup_set_free_ Set *allocated_set = NULL;
        _cleanup_closedir_ DIR *d = NULL;
        int r, ret;
        char *fn;

        assert(path);
        assert(sig >= 0);

        if (!s) {
                s = allocated_set = set_new(NULL);
                if (!s)
                        return -ENOMEM;
        }

        ret = cg_kill(controller, path, sig, flags, s, log_kill, userdata);

        r = cg_enumerate_subgroups(controller, path, &d);
        if (r < 0) {
                if (ret >= 0 && r != -ENOENT)
                        return r;

                return ret;
        }

        while ((r = cg_read_subgroup(d, &fn)) > 0) {
                _cleanup_free_ char *p = NULL;

                p = path_join(empty_to_root(path), fn);
                free(fn);
                if (!p)
                        return -ENOMEM;

                r = cg_kill_recursive(controller, p, sig, flags, s, log_kill, userdata);
                if (r != 0 && ret >= 0)
                        ret = r;
        }
        if (ret >= 0 && r < 0)
                ret = r;

        if (flags & CGROUP_REMOVE) {
                r = cg_rmdir(controller, path);
                if (r < 0 && ret >= 0 && !IN_SET(r, -ENOENT, -EBUSY))
                        return r;
        }

        return ret;
}

int cg_migrate(
                const char *cfrom,
                const char *pfrom,
                const char *cto,
                const char *pto,
                CGroupFlags flags) {

        bool done = false;
        _cleanup_set_free_ Set *s = NULL;
        int r, ret = 0;
        pid_t my_pid;

        assert(cfrom);
        assert(pfrom);
        assert(cto);
        assert(pto);

        s = set_new(NULL);
        if (!s)
                return -ENOMEM;

        my_pid = getpid_cached();

        do {
                _cleanup_fclose_ FILE *f = NULL;
                pid_t pid = 0;
                done = true;

                r = cg_enumerate_processes(cfrom, pfrom, &f);
                if (r < 0) {
                        if (ret >= 0 && r != -ENOENT)
                                return r;

                        return ret;
                }

                while ((r = cg_read_pid(f, &pid)) > 0) {

                        /* This might do weird stuff if we aren't a
                         * single-threaded program. However, we
                         * luckily know we are not */
                        if ((flags & CGROUP_IGNORE_SELF) && pid == my_pid)
                                continue;

                        if (set_get(s, PID_TO_PTR(pid)) == PID_TO_PTR(pid))
                                continue;

                        /* Ignore kernel threads. Since they can only
                         * exist in the root cgroup, we only check for
                         * them there. */
                        if (cfrom &&
                            empty_or_root(pfrom) &&
                            is_kernel_thread(pid) > 0)
                                continue;

                        r = cg_attach(cto, pto, pid);
                        if (r < 0) {
                                if (ret >= 0 && r != -ESRCH)
                                        ret = r;
                        } else if (ret == 0)
                                ret = 1;

                        done = false;

                        r = set_put(s, PID_TO_PTR(pid));
                        if (r < 0) {
                                if (ret >= 0)
                                        return r;

                                return ret;
                        }
                }

                if (r < 0) {
                        if (ret >= 0)
                                return r;

                        return ret;
                }
        } while (!done);

        return ret;
}

int cg_migrate_recursive(
                const char *cfrom,
                const char *pfrom,
                const char *cto,
                const char *pto,
                CGroupFlags flags) {

        _cleanup_closedir_ DIR *d = NULL;
        int r, ret = 0;
        char *fn;

        assert(cfrom);
        assert(pfrom);
        assert(cto);
        assert(pto);

        ret = cg_migrate(cfrom, pfrom, cto, pto, flags);

        r = cg_enumerate_subgroups(cfrom, pfrom, &d);
        if (r < 0) {
                if (ret >= 0 && r != -ENOENT)
                        return r;

                return ret;
        }

        while ((r = cg_read_subgroup(d, &fn)) > 0) {
                _cleanup_free_ char *p = NULL;

                p = path_join(empty_to_root(pfrom), fn);
                free(fn);
                if (!p)
                        return -ENOMEM;

                r = cg_migrate_recursive(cfrom, p, cto, pto, flags);
                if (r != 0 && ret >= 0)
                        ret = r;
        }

        if (r < 0 && ret >= 0)
                ret = r;

        if (flags & CGROUP_REMOVE) {
                r = cg_rmdir(cfrom, pfrom);
                if (r < 0 && ret >= 0 && !IN_SET(r, -ENOENT, -EBUSY))
                        return r;
        }

        return ret;
}

int cg_migrate_recursive_fallback(
                const char *cfrom,
                const char *pfrom,
                const char *cto,
                const char *pto,
                CGroupFlags flags) {

        int r;

        assert(cfrom);
        assert(pfrom);
        assert(cto);
        assert(pto);

        r = cg_migrate_recursive(cfrom, pfrom, cto, pto, flags);
        if (r < 0) {
                char prefix[strlen(pto) + 1];

                /* This didn't work? Then let's try all prefixes of the destination */

                PATH_FOREACH_PREFIX(prefix, pto) {
                        int q;

                        q = cg_migrate_recursive(cfrom, pfrom, cto, prefix, flags);
                        if (q >= 0)
                                return q;
                }
        }

        return r;
}

static const char *controller_to_dirname(const char *controller) {
        const char *e;

        assert(controller);

        /* Converts a controller name to the directory name below
         * /sys/fs/cgroup/ we want to mount it to. Effectively, this
         * just cuts off the name= prefixed used for named
         * hierarchies, if it is specified. */

        if (streq(controller, SYSTEMD_CGROUP_CONTROLLER)) {
                if (cg_hybrid_unified() > 0)
                        controller = SYSTEMD_CGROUP_CONTROLLER_HYBRID;
                else
                        controller = SYSTEMD_CGROUP_CONTROLLER_LEGACY;
        }

        e = startswith(controller, "name=");
        if (e)
                return e;

        return controller;
}

static int join_path_legacy(const char *controller, const char *path, const char *suffix, char **fs) {
        const char *dn;
        char *t = NULL;

        assert(fs);
        assert(controller);

        dn = controller_to_dirname(controller);

        if (isempty(path) && isempty(suffix))
                t = path_join("/sys/fs/cgroup", dn);
        else if (isempty(path))
                t = path_join("/sys/fs/cgroup", dn, suffix);
        else if (isempty(suffix))
                t = path_join("/sys/fs/cgroup", dn, path);
        else
                t = path_join("/sys/fs/cgroup", dn, path, suffix);
        if (!t)
                return -ENOMEM;

        *fs = t;
        return 0;
}

static int join_path_unified(const char *path, const char *suffix, char **fs) {
        char *t;

        assert(fs);

        if (isempty(path) && isempty(suffix))
                t = strdup("/sys/fs/cgroup");
        else if (isempty(path))
                t = path_join("/sys/fs/cgroup", suffix);
        else if (isempty(suffix))
                t = path_join("/sys/fs/cgroup", path);
        else
                t = path_join("/sys/fs/cgroup", path, suffix);
        if (!t)
                return -ENOMEM;

        *fs = t;
        return 0;
}

int cg_get_path(const char *controller, const char *path, const char *suffix, char **fs) {
        int r;

        assert(fs);

        if (!controller) {
                char *t;

                /* If no controller is specified, we return the path
                 * *below* the controllers, without any prefix. */

                if (!path && !suffix)
                        return -EINVAL;

                if (!suffix)
                        t = strdup(path);
                else if (!path)
                        t = strdup(suffix);
                else
                        t = path_join(path, suffix);
                if (!t)
                        return -ENOMEM;

                *fs = path_simplify(t, false);
                return 0;
        }

        if (!cg_controller_is_valid(controller))
                return -EINVAL;

        r = cg_all_unified();
        if (r < 0)
                return r;
        if (r > 0)
                r = join_path_unified(path, suffix, fs);
        else
                r = join_path_legacy(controller, path, suffix, fs);
        if (r < 0)
                return r;

        path_simplify(*fs, false);
        return 0;
}

static int controller_is_accessible(const char *controller) {
        int r;

        assert(controller);

        /* Checks whether a specific controller is accessible,
         * i.e. its hierarchy mounted. In the unified hierarchy all
         * controllers are considered accessible, except for the named
         * hierarchies */

        if (!cg_controller_is_valid(controller))
                return -EINVAL;

        r = cg_all_unified();
        if (r < 0)
                return r;
        if (r > 0) {
                /* We don't support named hierarchies if we are using
                 * the unified hierarchy. */

                if (streq(controller, SYSTEMD_CGROUP_CONTROLLER))
                        return 0;

                if (startswith(controller, "name="))
                        return -EOPNOTSUPP;

        } else {
                const char *cc, *dn;

                dn = controller_to_dirname(controller);
                cc = strjoina("/sys/fs/cgroup/", dn);

                if (laccess(cc, F_OK) < 0)
                        return -errno;
        }

        return 0;
}

int cg_get_path_and_check(const char *controller, const char *path, const char *suffix, char **fs) {
        int r;

        assert(controller);
        assert(fs);

        /* Check if the specified controller is actually accessible */
        r = controller_is_accessible(controller);
        if (r < 0)
                return r;

        return cg_get_path(controller, path, suffix, fs);
}

static int trim_cb(const char *path, const struct stat *sb, int typeflag, struct FTW *ftwbuf) {
        assert(path);
        assert(sb);
        assert(ftwbuf);

        if (typeflag != FTW_DP)
                return 0;

        if (ftwbuf->level < 1)
                return 0;

        (void) rmdir(path);
        return 0;
}

int cg_trim(const char *controller, const char *path, bool delete_root) {
        _cleanup_free_ char *fs = NULL;
        int r = 0, q;

        assert(path);

        r = cg_get_path(controller, path, NULL, &fs);
        if (r < 0)
                return r;

        errno = 0;
        if (nftw(fs, trim_cb, 64, FTW_DEPTH|FTW_MOUNT|FTW_PHYS) != 0) {
                if (errno == ENOENT)
                        r = 0;
                else
                        r = errno_or_else(EIO);
        }

        if (delete_root) {
                if (rmdir(fs) < 0 && errno != ENOENT)
                        return -errno;
        }

        q = cg_hybrid_unified();
        if (q < 0)
                return q;
        if (q > 0 && streq(controller, SYSTEMD_CGROUP_CONTROLLER)) {
                q = cg_trim(SYSTEMD_CGROUP_CONTROLLER_LEGACY, path, delete_root);
                if (q < 0)
                        log_warning_errno(q, "Failed to trim compat systemd cgroup %s: %m", path);
        }

        return r;
}

/* Create a cgroup in the hierarchy of controller.
 * Returns 0 if the group already existed, 1 on success, negative otherwise.
 */
int cg_create(const char *controller, const char *path) {
        _cleanup_free_ char *fs = NULL;
        int r;

        r = cg_get_path_and_check(controller, path, NULL, &fs);
        if (r < 0)
                return r;

        r = mkdir_parents(fs, 0755);
        if (r < 0)
                return r;

        r = mkdir_errno_wrapper(fs, 0755);
        if (r == -EEXIST)
                return 0;
        if (r < 0)
                return r;

        r = cg_hybrid_unified();
        if (r < 0)
                return r;

        if (r > 0 && streq(controller, SYSTEMD_CGROUP_CONTROLLER)) {
                r = cg_create(SYSTEMD_CGROUP_CONTROLLER_LEGACY, path);
                if (r < 0)
                        log_warning_errno(r, "Failed to create compat systemd cgroup %s: %m", path);
        }

        return 1;
}

int cg_create_and_attach(const char *controller, const char *path, pid_t pid) {
        int r, q;

        assert(pid >= 0);

        r = cg_create(controller, path);
        if (r < 0)
                return r;

        q = cg_attach(controller, path, pid);
        if (q < 0)
                return q;

        /* This does not remove the cgroup on failure */
        return r;
}

int cg_attach(const char *controller, const char *path, pid_t pid) {
        _cleanup_free_ char *fs = NULL;
        char c[DECIMAL_STR_MAX(pid_t) + 2];
        int r;

        assert(path);
        assert(pid >= 0);

        r = cg_get_path_and_check(controller, path, "cgroup.procs", &fs);
        if (r < 0)
                return r;

        if (pid == 0)
                pid = getpid_cached();

        xsprintf(c, PID_FMT "\n", pid);

        r = write_string_file(fs, c, WRITE_STRING_FILE_DISABLE_BUFFER);
        if (r < 0)
                return r;

        r = cg_hybrid_unified();
        if (r < 0)
                return r;

        if (r > 0 && streq(controller, SYSTEMD_CGROUP_CONTROLLER)) {
                r = cg_attach(SYSTEMD_CGROUP_CONTROLLER_LEGACY, path, pid);
                if (r < 0)
                        log_warning_errno(r, "Failed to attach "PID_FMT" to compat systemd cgroup %s: %m", pid, path);
        }

        return 0;
}

int cg_attach_fallback(const char *controller, const char *path, pid_t pid) {
        int r;

        assert(controller);
        assert(path);
        assert(pid >= 0);

        r = cg_attach(controller, path, pid);
        if (r < 0) {
                char prefix[strlen(path) + 1];

                /* This didn't work? Then let's try all prefixes of
                 * the destination */

                PATH_FOREACH_PREFIX(prefix, path) {
                        int q;

                        q = cg_attach(controller, prefix, pid);
                        if (q >= 0)
                                return q;
                }
        }

        return r;
}

int cg_set_access(
                const char *controller,
                const char *path,
                uid_t uid,
                gid_t gid) {

        struct Attribute {
                const char *name;
                bool fatal;
        };

        /* cgroup v1, aka legacy/non-unified */
        static const struct Attribute legacy_attributes[] = {
                { "cgroup.procs",           true  },
                { "tasks",                  false },
                { "cgroup.clone_children",  false },
                {},
        };

        /* cgroup v2, aka unified */
        static const struct Attribute unified_attributes[] = {
                { "cgroup.procs",           true  },
                { "cgroup.subtree_control", true  },
                { "cgroup.threads",         false },
                {},
        };

        static const struct Attribute* const attributes[] = {
                [false] = legacy_attributes,
                [true]  = unified_attributes,
        };

        _cleanup_free_ char *fs = NULL;
        const struct Attribute *i;
        int r, unified;

        assert(path);

        if (uid == UID_INVALID && gid == GID_INVALID)
                return 0;

        unified = cg_unified_controller(controller);
        if (unified < 0)
                return unified;

        /* Configure access to the cgroup itself */
        r = cg_get_path(controller, path, NULL, &fs);
        if (r < 0)
                return r;

        r = chmod_and_chown(fs, 0755, uid, gid);
        if (r < 0)
                return r;

        /* Configure access to the cgroup's attributes */
        for (i = attributes[unified]; i->name; i++) {
                fs = mfree(fs);

                r = cg_get_path(controller, path, i->name, &fs);
                if (r < 0)
                        return r;

                r = chmod_and_chown(fs, 0644, uid, gid);
                if (r < 0) {
                        if (i->fatal)
                                return r;

                        log_debug_errno(r, "Failed to set access on cgroup %s, ignoring: %m", fs);
                }
        }

        if (streq(controller, SYSTEMD_CGROUP_CONTROLLER)) {
                r = cg_hybrid_unified();
                if (r < 0)
                        return r;
                if (r > 0) {
                        /* Always propagate access mode from unified to legacy controller */
                        r = cg_set_access(SYSTEMD_CGROUP_CONTROLLER_LEGACY, path, uid, gid);
                        if (r < 0)
                                log_debug_errno(r, "Failed to set access on compatibility systemd cgroup %s, ignoring: %m", path);
                }
        }

        return 0;
}

int cg_set_xattr(const char *controller, const char *path, const char *name, const void *value, size_t size, int flags) {
        _cleanup_free_ char *fs = NULL;
        int r;

        assert(path);
        assert(name);
        assert(value || size <= 0);

        r = cg_get_path(controller, path, NULL, &fs);
        if (r < 0)
                return r;

        if (setxattr(fs, name, value, size, flags) < 0)
                return -errno;

        return 0;
}

int cg_get_xattr(const char *controller, const char *path, const char *name, void *value, size_t size) {
        _cleanup_free_ char *fs = NULL;
        ssize_t n;
        int r;

        assert(path);
        assert(name);

        r = cg_get_path(controller, path, NULL, &fs);
        if (r < 0)
                return r;

        n = getxattr(fs, name, value, size);
        if (n < 0)
                return -errno;

        return (int) n;
}

int cg_pid_get_path(const char *controller, pid_t pid, char **path) {
        _cleanup_fclose_ FILE *f = NULL;
        const char *fs, *controller_str;
        int unified, r;
        size_t cs = 0;

        assert(path);
        assert(pid >= 0);

        if (controller) {
                if (!cg_controller_is_valid(controller))
                        return -EINVAL;
        } else
                controller = SYSTEMD_CGROUP_CONTROLLER;

        unified = cg_unified_controller(controller);
        if (unified < 0)
                return unified;
        if (unified == 0) {
                if (streq(controller, SYSTEMD_CGROUP_CONTROLLER))
                        controller_str = SYSTEMD_CGROUP_CONTROLLER_LEGACY;
                else
                        controller_str = controller;

                cs = strlen(controller_str);
        }

        fs = procfs_file_alloca(pid, "cgroup");
        r = fopen_unlocked(fs, "re", &f);
        if (r == -ENOENT)
                return -ESRCH;
        if (r < 0)
                return r;

        for (;;) {
                _cleanup_free_ char *line = NULL;
                char *e, *p;

                r = read_line(f, LONG_LINE_MAX, &line);
                if (r < 0)
                        return r;
                if (r == 0)
                        break;

                if (unified) {
                        e = startswith(line, "0:");
                        if (!e)
                                continue;

                        e = strchr(e, ':');
                        if (!e)
                                continue;
                } else {
                        char *l;
                        size_t k;
                        const char *word, *state;
                        bool found = false;

                        l = strchr(line, ':');
                        if (!l)
                                continue;

                        l++;
                        e = strchr(l, ':');
                        if (!e)
                                continue;

                        *e = 0;
                        FOREACH_WORD_SEPARATOR(word, k, l, ",", state)
                                if (k == cs && memcmp(word, controller_str, cs) == 0) {
                                        found = true;
                                        break;
                                }
                        if (!found)
                                continue;
                }

                p = strdup(e + 1);
                if (!p)
                        return -ENOMEM;

                /* Truncate suffix indicating the process is a zombie */
                e = endswith(p, " (deleted)");
                if (e)
                        *e = 0;

                *path = p;
                return 0;
        }

        return -ENODATA;
}

int cg_install_release_agent(const char *controller, const char *agent) {
        _cleanup_free_ char *fs = NULL, *contents = NULL;
        const char *sc;
        int r;

        assert(agent);

        r = cg_unified_controller(controller);
        if (r < 0)
                return r;
        if (r > 0) /* doesn't apply to unified hierarchy */
                return -EOPNOTSUPP;

        r = cg_get_path(controller, NULL, "release_agent", &fs);
        if (r < 0)
                return r;

        r = read_one_line_file(fs, &contents);
        if (r < 0)
                return r;

        sc = strstrip(contents);
        if (isempty(sc)) {
                r = write_string_file(fs, agent, WRITE_STRING_FILE_DISABLE_BUFFER);
                if (r < 0)
                        return r;
        } else if (!path_equal(sc, agent))
                return -EEXIST;

        fs = mfree(fs);
        r = cg_get_path(controller, NULL, "notify_on_release", &fs);
        if (r < 0)
                return r;

        contents = mfree(contents);
        r = read_one_line_file(fs, &contents);
        if (r < 0)
                return r;

        sc = strstrip(contents);
        if (streq(sc, "0")) {
                r = write_string_file(fs, "1", WRITE_STRING_FILE_DISABLE_BUFFER);
                if (r < 0)
                        return r;

                return 1;
        }

        if (!streq(sc, "1"))
                return -EIO;

        return 0;
}

int cg_uninstall_release_agent(const char *controller) {
        _cleanup_free_ char *fs = NULL;
        int r;

        r = cg_unified_controller(controller);
        if (r < 0)
                return r;
        if (r > 0) /* Doesn't apply to unified hierarchy */
                return -EOPNOTSUPP;

        r = cg_get_path(controller, NULL, "notify_on_release", &fs);
        if (r < 0)
                return r;

        r = write_string_file(fs, "0", WRITE_STRING_FILE_DISABLE_BUFFER);
        if (r < 0)
                return r;

        fs = mfree(fs);

        r = cg_get_path(controller, NULL, "release_agent", &fs);
        if (r < 0)
                return r;

        r = write_string_file(fs, "", WRITE_STRING_FILE_DISABLE_BUFFER);
        if (r < 0)
                return r;

        return 0;
}

int cg_is_empty(const char *controller, const char *path) {
        _cleanup_fclose_ FILE *f = NULL;
        pid_t pid;
        int r;

        assert(path);

        r = cg_enumerate_processes(controller, path, &f);
        if (r == -ENOENT)
                return true;
        if (r < 0)
                return r;

        r = cg_read_pid(f, &pid);
        if (r < 0)
                return r;

        return r == 0;
}

int cg_is_empty_recursive(const char *controller, const char *path) {
        int r;

        assert(path);

        /* The root cgroup is always populated */
        if (controller && empty_or_root(path))
                return false;

        r = cg_unified_controller(controller);
        if (r < 0)
                return r;
        if (r > 0) {
                _cleanup_free_ char *t = NULL;

                /* On the unified hierarchy we can check empty state
                 * via the "populated" attribute of "cgroup.events". */

                r = cg_read_event(controller, path, "populated", &t);
                if (r == -ENOENT)
                        return true;
                if (r < 0)
                        return r;

                return streq(t, "0");
        } else {
                _cleanup_closedir_ DIR *d = NULL;
                char *fn;

                r = cg_is_empty(controller, path);
                if (r <= 0)
                        return r;

                r = cg_enumerate_subgroups(controller, path, &d);
                if (r == -ENOENT)
                        return true;
                if (r < 0)
                        return r;

                while ((r = cg_read_subgroup(d, &fn)) > 0) {
                        _cleanup_free_ char *p = NULL;

                        p = path_join(path, fn);
                        free(fn);
                        if (!p)
                                return -ENOMEM;

                        r = cg_is_empty_recursive(controller, p);
                        if (r <= 0)
                                return r;
                }
                if (r < 0)
                        return r;

                return true;
        }
}

int cg_split_spec(const char *spec, char **controller, char **path) {
        char *t = NULL, *u = NULL;
        const char *e;

        assert(spec);

        if (*spec == '/') {
                if (!path_is_normalized(spec))
                        return -EINVAL;

                if (path) {
                        t = strdup(spec);
                        if (!t)
                                return -ENOMEM;

                        *path = path_simplify(t, false);
                }

                if (controller)
                        *controller = NULL;

                return 0;
        }

        e = strchr(spec, ':');
        if (!e) {
                if (!cg_controller_is_valid(spec))
                        return -EINVAL;

                if (controller) {
                        t = strdup(spec);
                        if (!t)
                                return -ENOMEM;

                        *controller = t;
                }

                if (path)
                        *path = NULL;

                return 0;
        }

        t = strndup(spec, e-spec);
        if (!t)
                return -ENOMEM;
        if (!cg_controller_is_valid(t)) {
                free(t);
                return -EINVAL;
        }

        if (isempty(e+1))
                u = NULL;
        else {
                u = strdup(e+1);
                if (!u) {
                        free(t);
                        return -ENOMEM;
                }

                if (!path_is_normalized(u) ||
                    !path_is_absolute(u)) {
                        free(t);
                        free(u);
                        return -EINVAL;
                }

                path_simplify(u, false);
        }

        if (controller)
                *controller = t;
        else
                free(t);

        if (path)
                *path = u;
        else
                free(u);

        return 0;
}

int cg_mangle_path(const char *path, char **result) {
        _cleanup_free_ char *c = NULL, *p = NULL;
        char *t;
        int r;

        assert(path);
        assert(result);

        /* First, check if it already is a filesystem path */
        if (path_startswith(path, "/sys/fs/cgroup")) {

                t = strdup(path);
                if (!t)
                        return -ENOMEM;

                *result = path_simplify(t, false);
                return 0;
        }

        /* Otherwise, treat it as cg spec */
        r = cg_split_spec(path, &c, &p);
        if (r < 0)
                return r;

        return cg_get_path(c ?: SYSTEMD_CGROUP_CONTROLLER, p ?: "/", NULL, result);
}

int cg_get_root_path(char **path) {
        char *p, *e;
        int r;

        assert(path);

        r = cg_pid_get_path(SYSTEMD_CGROUP_CONTROLLER, 1, &p);
        if (r < 0)
                return r;

        e = endswith(p, "/" SPECIAL_INIT_SCOPE);
        if (!e)
                e = endswith(p, "/" SPECIAL_SYSTEM_SLICE); /* legacy */
        if (!e)
                e = endswith(p, "/system"); /* even more legacy */
        if (e)
                *e = 0;

        *path = p;
        return 0;
}

int cg_shift_path(const char *cgroup, const char *root, const char **shifted) {
        _cleanup_free_ char *rt = NULL;
        char *p;
        int r;

        assert(cgroup);
        assert(shifted);

        if (!root) {
                /* If the root was specified let's use that, otherwise
                 * let's determine it from PID 1 */

                r = cg_get_root_path(&rt);
                if (r < 0)
                        return r;

                root = rt;
        }

        p = path_startswith(cgroup, root);
        if (p && p > cgroup)
                *shifted = p - 1;
        else
                *shifted = cgroup;

        return 0;
}

int cg_pid_get_path_shifted(pid_t pid, const char *root, char **cgroup) {
        _cleanup_free_ char *raw = NULL;
        const char *c;
        int r;

        assert(pid >= 0);
        assert(cgroup);

        r = cg_pid_get_path(SYSTEMD_CGROUP_CONTROLLER, pid, &raw);
        if (r < 0)
                return r;

        r = cg_shift_path(raw, root, &c);
        if (r < 0)
                return r;

        if (c == raw)
                *cgroup = TAKE_PTR(raw);
        else {
                char *n;

                n = strdup(c);
                if (!n)
                        return -ENOMEM;

                *cgroup = n;
        }

        return 0;
}

int cg_path_decode_unit(const char *cgroup, char **unit) {
        char *c, *s;
        size_t n;

        assert(cgroup);
        assert(unit);

        n = strcspn(cgroup, "/");
        if (n < 3)
                return -ENXIO;

        c = strndupa(cgroup, n);
        c = cg_unescape(c);

        if (!unit_name_is_valid(c, UNIT_NAME_PLAIN|UNIT_NAME_INSTANCE))
                return -ENXIO;

        s = strdup(c);
        if (!s)
                return -ENOMEM;

        *unit = s;
        return 0;
}

static bool valid_slice_name(const char *p, size_t n) {

        if (!p)
                return false;

        if (n < STRLEN("x.slice"))
                return false;

        if (memcmp(p + n - 6, ".slice", 6) == 0) {
                char buf[n+1], *c;

                memcpy(buf, p, n);
                buf[n] = 0;

                c = cg_unescape(buf);

                return unit_name_is_valid(c, UNIT_NAME_PLAIN);
        }

        return false;
}

static const char *skip_slices(const char *p) {
        assert(p);

        /* Skips over all slice assignments */

        for (;;) {
                size_t n;

                p += strspn(p, "/");

                n = strcspn(p, "/");
                if (!valid_slice_name(p, n))
                        return p;

                p += n;
        }
}

int cg_path_get_unit(const char *path, char **ret) {
        const char *e;
        char *unit;
        int r;

        assert(path);
        assert(ret);

        e = skip_slices(path);

        r = cg_path_decode_unit(e, &unit);
        if (r < 0)
                return r;

        /* We skipped over the slices, don't accept any now */
        if (endswith(unit, ".slice")) {
                free(unit);
                return -ENXIO;
        }

        *ret = unit;
        return 0;
}

int cg_pid_get_unit(pid_t pid, char **unit) {
        _cleanup_free_ char *cgroup = NULL;
        int r;

        assert(unit);

        r = cg_pid_get_path_shifted(pid, NULL, &cgroup);
        if (r < 0)
                return r;

        return cg_path_get_unit(cgroup, unit);
}

/**
 * Skip session-*.scope, but require it to be there.
 */
static const char *skip_session(const char *p) {
        size_t n;

        if (isempty(p))
                return NULL;

        p += strspn(p, "/");

        n = strcspn(p, "/");
        if (n < STRLEN("session-x.scope"))
                return NULL;

        if (memcmp(p, "session-", 8) == 0 && memcmp(p + n - 6, ".scope", 6) == 0) {
                char buf[n - 8 - 6 + 1];

                memcpy(buf, p + 8, n - 8 - 6);
                buf[n - 8 - 6] = 0;

                /* Note that session scopes never need unescaping,
                 * since they cannot conflict with the kernel's own
                 * names, hence we don't need to call cg_unescape()
                 * here. */

                if (!session_id_valid(buf))
                        return false;

                p += n;
                p += strspn(p, "/");
                return p;
        }

        return NULL;
}

/**
 * Skip user@*.service, but require it to be there.
 */
static const char *skip_user_manager(const char *p) {
        size_t n;

        if (isempty(p))
                return NULL;

        p += strspn(p, "/");

        n = strcspn(p, "/");
        if (n < STRLEN("user@x.service"))
                return NULL;

        if (memcmp(p, "user@", 5) == 0 && memcmp(p + n - 8, ".service", 8) == 0) {
                char buf[n - 5 - 8 + 1];

                memcpy(buf, p + 5, n - 5 - 8);
                buf[n - 5 - 8] = 0;

                /* Note that user manager services never need unescaping,
                 * since they cannot conflict with the kernel's own
                 * names, hence we don't need to call cg_unescape()
                 * here. */

                if (parse_uid(buf, NULL) < 0)
                        return NULL;

                p += n;
                p += strspn(p, "/");

                return p;
        }

        return NULL;
}

static const char *skip_user_prefix(const char *path) {
        const char *e, *t;

        assert(path);

        /* Skip slices, if there are any */
        e = skip_slices(path);

        /* Skip the user manager, if it's in the path now... */
        t = skip_user_manager(e);
        if (t)
                return t;

        /* Alternatively skip the user session if it is in the path... */
        return skip_session(e);
}

int cg_path_get_user_unit(const char *path, char **ret) {
        const char *t;

        assert(path);
        assert(ret);

        t = skip_user_prefix(path);
        if (!t)
                return -ENXIO;

        /* And from here on it looks pretty much the same as for a system unit, hence let's use the same
         * parser. */
        return cg_path_get_unit(t, ret);
}

int cg_pid_get_user_unit(pid_t pid, char **unit) {
        _cleanup_free_ char *cgroup = NULL;
        int r;

        assert(unit);

        r = cg_pid_get_path_shifted(pid, NULL, &cgroup);
        if (r < 0)
                return r;

        return cg_path_get_user_unit(cgroup, unit);
}

int cg_path_get_machine_name(const char *path, char **machine) {
        _cleanup_free_ char *u = NULL;
        const char *sl;
        int r;

        r = cg_path_get_unit(path, &u);
        if (r < 0)
                return r;

        sl = strjoina("/run/systemd/machines/unit:", u);
        return readlink_malloc(sl, machine);
}

int cg_pid_get_machine_name(pid_t pid, char **machine) {
        _cleanup_free_ char *cgroup = NULL;
        int r;

        assert(machine);

        r = cg_pid_get_path_shifted(pid, NULL, &cgroup);
        if (r < 0)
                return r;

        return cg_path_get_machine_name(cgroup, machine);
}

int cg_path_get_session(const char *path, char **session) {
        _cleanup_free_ char *unit = NULL;
        char *start, *end;
        int r;

        assert(path);

        r = cg_path_get_unit(path, &unit);
        if (r < 0)
                return r;

        start = startswith(unit, "session-");
        if (!start)
                return -ENXIO;
        end = endswith(start, ".scope");
        if (!end)
                return -ENXIO;

        *end = 0;
        if (!session_id_valid(start))
                return -ENXIO;

        if (session) {
                char *rr;

                rr = strdup(start);
                if (!rr)
                        return -ENOMEM;

                *session = rr;
        }

        return 0;
}

int cg_pid_get_session(pid_t pid, char **session) {
        _cleanup_free_ char *cgroup = NULL;
        int r;

        r = cg_pid_get_path_shifted(pid, NULL, &cgroup);
        if (r < 0)
                return r;

        return cg_path_get_session(cgroup, session);
}

int cg_path_get_owner_uid(const char *path, uid_t *uid) {
        _cleanup_free_ char *slice = NULL;
        char *start, *end;
        int r;

        assert(path);

        r = cg_path_get_slice(path, &slice);
        if (r < 0)
                return r;

        start = startswith(slice, "user-");
        if (!start)
                return -ENXIO;
        end = endswith(start, ".slice");
        if (!end)
                return -ENXIO;

        *end = 0;
        if (parse_uid(start, uid) < 0)
                return -ENXIO;

        return 0;
}

int cg_pid_get_owner_uid(pid_t pid, uid_t *uid) {
        _cleanup_free_ char *cgroup = NULL;
        int r;

        r = cg_pid_get_path_shifted(pid, NULL, &cgroup);
        if (r < 0)
                return r;

        return cg_path_get_owner_uid(cgroup, uid);
}

int cg_path_get_slice(const char *p, char **slice) {
        const char *e = NULL;

        assert(p);
        assert(slice);

        /* Finds the right-most slice unit from the beginning, but
         * stops before we come to the first non-slice unit. */

        for (;;) {
                size_t n;

                p += strspn(p, "/");

                n = strcspn(p, "/");
                if (!valid_slice_name(p, n)) {

                        if (!e) {
                                char *s;

                                s = strdup(SPECIAL_ROOT_SLICE);
                                if (!s)
                                        return -ENOMEM;

                                *slice = s;
                                return 0;
                        }

                        return cg_path_decode_unit(e, slice);
                }

                e = p;
                p += n;
        }
}

int cg_pid_get_slice(pid_t pid, char **slice) {
        _cleanup_free_ char *cgroup = NULL;
        int r;

        assert(slice);

        r = cg_pid_get_path_shifted(pid, NULL, &cgroup);
        if (r < 0)
                return r;

        return cg_path_get_slice(cgroup, slice);
}

int cg_path_get_user_slice(const char *p, char **slice) {
        const char *t;
        assert(p);
        assert(slice);

        t = skip_user_prefix(p);
        if (!t)
                return -ENXIO;

        /* And now it looks pretty much the same as for a system
         * slice, so let's just use the same parser from here on. */
        return cg_path_get_slice(t, slice);
}

int cg_pid_get_user_slice(pid_t pid, char **slice) {
        _cleanup_free_ char *cgroup = NULL;
        int r;

        assert(slice);

        r = cg_pid_get_path_shifted(pid, NULL, &cgroup);
        if (r < 0)
                return r;

        return cg_path_get_user_slice(cgroup, slice);
}

char *cg_escape(const char *p) {
        bool need_prefix = false;

        /* This implements very minimal escaping for names to be used
         * as file names in the cgroup tree: any name which might
         * conflict with a kernel name or is prefixed with '_' is
         * prefixed with a '_'. That way, when reading cgroup names it
         * is sufficient to remove a single prefixing underscore if
         * there is one. */

        /* The return value of this function (unlike cg_unescape())
         * needs free()! */

        if (IN_SET(p[0], 0, '_', '.') ||
            STR_IN_SET(p, "notify_on_release", "release_agent", "tasks") ||
            startswith(p, "cgroup."))
                need_prefix = true;
        else {
                const char *dot;

                dot = strrchr(p, '.');
                if (dot) {
                        CGroupController c;
                        size_t l = dot - p;

                        for (c = 0; c < _CGROUP_CONTROLLER_MAX; c++) {
                                const char *n;

                                n = cgroup_controller_to_string(c);

                                if (l != strlen(n))
                                        continue;

                                if (memcmp(p, n, l) != 0)
                                        continue;

                                need_prefix = true;
                                break;
                        }
                }
        }

        if (need_prefix)
                return strjoin("_", p);

        return strdup(p);
}

char *cg_unescape(const char *p) {
        assert(p);

        /* The return value of this function (unlike cg_escape())
         * doesn't need free()! */

        if (p[0] == '_')
                return (char*) p+1;

        return (char*) p;
}

#define CONTROLLER_VALID                        \
        DIGITS LETTERS                          \
        "_"

bool cg_controller_is_valid(const char *p) {
        const char *t, *s;

        if (!p)
                return false;

        if (streq(p, SYSTEMD_CGROUP_CONTROLLER))
                return true;

        s = startswith(p, "name=");
        if (s)
                p = s;

        if (IN_SET(*p, 0, '_'))
                return false;

        for (t = p; *t; t++)
                if (!strchr(CONTROLLER_VALID, *t))
                        return false;

        if (t - p > FILENAME_MAX)
                return false;

        return true;
}

int cg_slice_to_path(const char *unit, char **ret) {
        _cleanup_free_ char *p = NULL, *s = NULL, *e = NULL;
        const char *dash;
        int r;

        assert(unit);
        assert(ret);

        if (streq(unit, SPECIAL_ROOT_SLICE)) {
                char *x;

                x = strdup("");
                if (!x)
                        return -ENOMEM;
                *ret = x;
                return 0;
        }

        if (!unit_name_is_valid(unit, UNIT_NAME_PLAIN))
                return -EINVAL;

        if (!endswith(unit, ".slice"))
                return -EINVAL;

        r = unit_name_to_prefix(unit, &p);
        if (r < 0)
                return r;

        dash = strchr(p, '-');

        /* Don't allow initial dashes */
        if (dash == p)
                return -EINVAL;

        while (dash) {
                _cleanup_free_ char *escaped = NULL;
                char n[dash - p + sizeof(".slice")];

#if HAS_FEATURE_MEMORY_SANITIZER
                /* msan doesn't instrument stpncpy, so it thinks
                 * n is later used uninitialized:
                 * https://github.com/google/sanitizers/issues/926
                 */
                zero(n);
#endif

                /* Don't allow trailing or double dashes */
                if (IN_SET(dash[1], 0, '-'))
                        return -EINVAL;

                strcpy(stpncpy(n, p, dash - p), ".slice");
                if (!unit_name_is_valid(n, UNIT_NAME_PLAIN))
                        return -EINVAL;

                escaped = cg_escape(n);
                if (!escaped)
                        return -ENOMEM;

                if (!strextend(&s, escaped, "/", NULL))
                        return -ENOMEM;

                dash = strchr(dash+1, '-');
        }

        e = cg_escape(unit);
        if (!e)
                return -ENOMEM;

        if (!strextend(&s, e, NULL))
                return -ENOMEM;

        *ret = TAKE_PTR(s);

        return 0;
}

int cg_set_attribute(const char *controller, const char *path, const char *attribute, const char *value) {
        _cleanup_free_ char *p = NULL;
        int r;

        r = cg_get_path(controller, path, attribute, &p);
        if (r < 0)
                return r;

        return write_string_file(p, value, WRITE_STRING_FILE_DISABLE_BUFFER);
}

int cg_get_attribute(const char *controller, const char *path, const char *attribute, char **ret) {
        _cleanup_free_ char *p = NULL;
        int r;

        r = cg_get_path(controller, path, attribute, &p);
        if (r < 0)
                return r;

        return read_one_line_file(p, ret);
}

int cg_get_keyed_attribute(
                const char *controller,
                const char *path,
                const char *attribute,
                char **keys,
                char **ret_values) {

        _cleanup_free_ char *filename = NULL, *contents = NULL;
        const char *p;
        size_t n, i, n_done = 0;
        char **v;
        int r;

        /* Reads one or more fields of a cgroup v2 keyed attribute file. The 'keys' parameter should be an strv with
         * all keys to retrieve. The 'ret_values' parameter should be passed as string size with the same number of
         * entries as 'keys'. On success each entry will be set to the value of the matching key.
         *
         * If the attribute file doesn't exist at all returns ENOENT, if any key is not found returns ENXIO. */

        r = cg_get_path(controller, path, attribute, &filename);
        if (r < 0)
                return r;

        r = read_full_file(filename, &contents, NULL);
        if (r < 0)
                return r;

        n = strv_length(keys);
        if (n == 0) /* No keys to retrieve? That's easy, we are done then */
                return 0;

        /* Let's build this up in a temporary array for now in order not to clobber the return parameter on failure */
        v = newa0(char*, n);

        for (p = contents; *p;) {
                const char *w = NULL;

                for (i = 0; i < n; i++)
                        if (!v[i]) {
                                w = first_word(p, keys[i]);
                                if (w)
                                        break;
                        }

                if (w) {
                        size_t l;

                        l = strcspn(w, NEWLINE);
                        v[i] = strndup(w, l);
                        if (!v[i]) {
                                r = -ENOMEM;
                                goto fail;
                        }

                        n_done++;
                        if (n_done >= n)
                                goto done;

                        p = w + l;
                } else
                        p += strcspn(p, NEWLINE);

                p += strspn(p, NEWLINE);
        }

        r = -ENXIO;

fail:
        for (i = 0; i < n; i++)
                free(v[i]);

        return r;

done:
        memcpy(ret_values, v, sizeof(char*) * n);
        return 0;

}

int cg_create_everywhere(CGroupMask supported, CGroupMask mask, const char *path) {
        CGroupController c;
        CGroupMask done;
        bool created;
        int r;

        /* This one will create a cgroup in our private tree, but also
         * duplicate it in the trees specified in mask, and remove it
         * in all others.
         *
         * Returns 0 if the group already existed in the systemd hierarchy,
         * 1 on success, negative otherwise.
         */

        /* First create the cgroup in our own hierarchy. */
        r = cg_create(SYSTEMD_CGROUP_CONTROLLER, path);
        if (r < 0)
                return r;
        created = r;

        /* If we are in the unified hierarchy, we are done now */
        r = cg_all_unified();
        if (r < 0)
                return r;
        if (r > 0)
                return created;

        supported &= CGROUP_MASK_V1;
        mask = CGROUP_MASK_EXTEND_JOINED(mask);
        done = 0;

        /* Otherwise, do the same in the other hierarchies */
        for (c = 0; c < _CGROUP_CONTROLLER_MAX; c++) {
                CGroupMask bit = CGROUP_CONTROLLER_TO_MASK(c);
                const char *n;

                if (!FLAGS_SET(supported, bit))
                        continue;

                if (FLAGS_SET(done, bit))
                        continue;

                n = cgroup_controller_to_string(c);
                if (FLAGS_SET(mask, bit))
                        (void) cg_create(n, path);
                else
                        (void) cg_trim(n, path, true);

                done |= CGROUP_MASK_EXTEND_JOINED(bit);
        }

        return created;
}

int cg_attach_everywhere(CGroupMask supported, const char *path, pid_t pid, cg_migrate_callback_t path_callback, void *userdata) {
        CGroupController c;
        CGroupMask done;
        int r;

        r = cg_attach(SYSTEMD_CGROUP_CONTROLLER, path, pid);
        if (r < 0)
                return r;

        r = cg_all_unified();
        if (r < 0)
                return r;
        if (r > 0)
                return 0;

        supported &= CGROUP_MASK_V1;
        done = 0;

        for (c = 0; c < _CGROUP_CONTROLLER_MAX; c++) {
                CGroupMask bit = CGROUP_CONTROLLER_TO_MASK(c);
                const char *p = NULL;

                if (!FLAGS_SET(supported, bit))
                        continue;

                if (FLAGS_SET(done, bit))
                        continue;

                if (path_callback)
                        p = path_callback(bit, userdata);
                if (!p)
                        p = path;

                (void) cg_attach_fallback(cgroup_controller_to_string(c), p, pid);
                done |= CGROUP_MASK_EXTEND_JOINED(bit);
        }

        return 0;
}

int cg_attach_many_everywhere(CGroupMask supported, const char *path, Set* pids, cg_migrate_callback_t path_callback, void *userdata) {
        Iterator i;
        void *pidp;
        int r = 0;

        SET_FOREACH(pidp, pids, i) {
                pid_t pid = PTR_TO_PID(pidp);
                int q;

                q = cg_attach_everywhere(supported, path, pid, path_callback, userdata);
                if (q < 0 && r >= 0)
                        r = q;
        }

        return r;
}

int cg_migrate_everywhere(CGroupMask supported, const char *from, const char *to, cg_migrate_callback_t to_callback, void *userdata) {
        CGroupController c;
        CGroupMask done;
        int r = 0, q;

        if (!path_equal(from, to))  {
                r = cg_migrate_recursive(SYSTEMD_CGROUP_CONTROLLER, from, SYSTEMD_CGROUP_CONTROLLER, to, CGROUP_REMOVE);
                if (r < 0)
                        return r;
        }

        q = cg_all_unified();
        if (q < 0)
                return q;
        if (q > 0)
                return r;

        supported &= CGROUP_MASK_V1;
        done = 0;

        for (c = 0; c < _CGROUP_CONTROLLER_MAX; c++) {
                CGroupMask bit = CGROUP_CONTROLLER_TO_MASK(c);
                const char *p = NULL;

                if (!FLAGS_SET(supported, bit))
                        continue;

                if (FLAGS_SET(done, bit))
                        continue;

                if (to_callback)
                        p = to_callback(bit, userdata);
                if (!p)
                        p = to;

                (void) cg_migrate_recursive_fallback(SYSTEMD_CGROUP_CONTROLLER, to, cgroup_controller_to_string(c), p, 0);
                done |= CGROUP_MASK_EXTEND_JOINED(bit);
        }

        return r;
}

int cg_trim_everywhere(CGroupMask supported, const char *path, bool delete_root) {
        CGroupController c;
        CGroupMask done;
        int r, q;

        r = cg_trim(SYSTEMD_CGROUP_CONTROLLER, path, delete_root);
        if (r < 0)
                return r;

        q = cg_all_unified();
        if (q < 0)
                return q;
        if (q > 0)
                return r;

        supported &= CGROUP_MASK_V1;
        done = 0;

        for (c = 0; c < _CGROUP_CONTROLLER_MAX; c++) {
                CGroupMask bit = CGROUP_CONTROLLER_TO_MASK(c);

                if (!FLAGS_SET(supported, bit))
                        continue;

                if (FLAGS_SET(done, bit))
                        continue;

                (void) cg_trim(cgroup_controller_to_string(c), path, delete_root);
                done |= CGROUP_MASK_EXTEND_JOINED(bit);
        }

        return r;
}

int cg_mask_to_string(CGroupMask mask, char **ret) {
        _cleanup_free_ char *s = NULL;
        size_t n = 0, allocated = 0;
        bool space = false;
        CGroupController c;

        assert(ret);

        if (mask == 0) {
                *ret = NULL;
                return 0;
        }

        for (c = 0; c < _CGROUP_CONTROLLER_MAX; c++) {
                const char *k;
                size_t l;

                if (!FLAGS_SET(mask, CGROUP_CONTROLLER_TO_MASK(c)))
                        continue;

                k = cgroup_controller_to_string(c);
                l = strlen(k);

                if (!GREEDY_REALLOC(s, allocated, n + space + l + 1))
                        return -ENOMEM;

                if (space)
                        s[n] = ' ';
                memcpy(s + n + space, k, l);
                n += space + l;

                space = true;
        }

        assert(s);

        s[n] = 0;
        *ret = TAKE_PTR(s);

        return 0;
}

int cg_mask_from_string(const char *value, CGroupMask *ret) {
        CGroupMask m = 0;

        assert(ret);
        assert(value);

        for (;;) {
                _cleanup_free_ char *n = NULL;
                CGroupController v;
                int r;

                r = extract_first_word(&value, &n, NULL, 0);
                if (r < 0)
                        return r;
                if (r == 0)
                        break;

                v = cgroup_controller_from_string(n);
                if (v < 0)
                        continue;

                m |= CGROUP_CONTROLLER_TO_MASK(v);
        }

        *ret = m;
        return 0;
}

int cg_mask_supported(CGroupMask *ret) {
        CGroupMask mask;
        int r;

        /* Determines the mask of supported cgroup controllers. Only includes controllers we can make sense of and that
         * are actually accessible. Only covers real controllers, i.e. not the CGROUP_CONTROLLER_BPF_xyz
         * pseudo-controllers. */

        r = cg_all_unified();
        if (r < 0)
                return r;
        if (r > 0) {
                _cleanup_free_ char *root = NULL, *controllers = NULL, *path = NULL;

                /* In the unified hierarchy we can read the supported
                 * and accessible controllers from a the top-level
                 * cgroup attribute */

                r = cg_get_root_path(&root);
                if (r < 0)
                        return r;

                r = cg_get_path(SYSTEMD_CGROUP_CONTROLLER, root, "cgroup.controllers", &path);
                if (r < 0)
                        return r;

                r = read_one_line_file(path, &controllers);
                if (r < 0)
                        return r;

                r = cg_mask_from_string(controllers, &mask);
                if (r < 0)
                        return r;

                /* Currently, we support the cpu, memory, io and pids controller in the unified hierarchy, mask
                 * everything else off. */
                mask &= CGROUP_MASK_V2;

        } else {
                CGroupController c;

                /* In the legacy hierarchy, we check which hierarchies are mounted. */

                mask = 0;
                for (c = 0; c < _CGROUP_CONTROLLER_MAX; c++) {
                        CGroupMask bit = CGROUP_CONTROLLER_TO_MASK(c);
                        const char *n;

                        if (!FLAGS_SET(CGROUP_MASK_V1, bit))
                                continue;

                        n = cgroup_controller_to_string(c);
                        if (controller_is_accessible(n) >= 0)
                                mask |= bit;
                }
        }

        *ret = mask;
        return 0;
}

int cg_kernel_controllers(Set **ret) {
        _cleanup_set_free_free_ Set *controllers = NULL;
        _cleanup_fclose_ FILE *f = NULL;
        int r;

        assert(ret);

        /* Determines the full list of kernel-known controllers. Might include controllers we don't actually support
         * and controllers that aren't currently accessible (because not mounted). This does not include "name="
         * pseudo-controllers. */

        controllers = set_new(&string_hash_ops);
        if (!controllers)
                return -ENOMEM;

        r = fopen_unlocked("/proc/cgroups", "re", &f);
        if (r == -ENOENT) {
                *ret = NULL;
                return 0;
        }
        if (r < 0)
                return r;

        /* Ignore the header line */
        (void) read_line(f, (size_t) -1, NULL);

        for (;;) {
                char *controller;
                int enabled = 0;

                errno = 0;
                if (fscanf(f, "%ms %*i %*i %i", &controller, &enabled) != 2) {

                        if (feof(f))
                                break;

                        if (ferror(f))
                                return errno_or_else(EIO);

                        return -EBADMSG;
                }

                if (!enabled) {
                        free(controller);
                        continue;
                }

                if (!cg_controller_is_valid(controller)) {
                        free(controller);
                        return -EBADMSG;
                }

                r = set_consume(controllers, controller);
                if (r < 0)
                        return r;
        }

        *ret = TAKE_PTR(controllers);

        return 0;
}

static thread_local CGroupUnified unified_cache = CGROUP_UNIFIED_UNKNOWN;

/* The hybrid mode was initially implemented in v232 and simply mounted cgroup2 on /sys/fs/cgroup/systemd.  This
 * unfortunately broke other tools (such as docker) which expected the v1 "name=systemd" hierarchy on
 * /sys/fs/cgroup/systemd.  From v233 and on, the hybrid mode mountnbs v2 on /sys/fs/cgroup/unified and maintains
 * "name=systemd" hierarchy on /sys/fs/cgroup/systemd for compatibility with other tools.
 *
 * To keep live upgrade working, we detect and support v232 layout.  When v232 layout is detected, to keep cgroup v2
 * process management but disable the compat dual layout, we return %true on
 * cg_unified_controller(SYSTEMD_CGROUP_CONTROLLER) and %false on cg_hybrid_unified().
 */
static thread_local bool unified_systemd_v232;

static int cg_unified_update(void) {

        struct statfs fs;

        /* Checks if we support the unified hierarchy. Returns an
         * error when the cgroup hierarchies aren't mounted yet or we
         * have any other trouble determining if the unified hierarchy
         * is supported. */

        if (unified_cache >= CGROUP_UNIFIED_NONE)
                return 0;

        if (statfs("/sys/fs/cgroup/", &fs) < 0)
                return log_debug_errno(errno, "statfs(\"/sys/fs/cgroup/\") failed: %m");

        if (F_TYPE_EQUAL(fs.f_type, CGROUP2_SUPER_MAGIC)) {
                log_debug("Found cgroup2 on /sys/fs/cgroup/, full unified hierarchy");
                unified_cache = CGROUP_UNIFIED_ALL;
        } else if (F_TYPE_EQUAL(fs.f_type, TMPFS_MAGIC)) {
                if (statfs("/sys/fs/cgroup/unified/", &fs) == 0 &&
                    F_TYPE_EQUAL(fs.f_type, CGROUP2_SUPER_MAGIC)) {
                        log_debug("Found cgroup2 on /sys/fs/cgroup/unified, unified hierarchy for systemd controller");
                        unified_cache = CGROUP_UNIFIED_SYSTEMD;
                        unified_systemd_v232 = false;
                } else {
                        if (statfs("/sys/fs/cgroup/systemd/", &fs) < 0)
                                return log_debug_errno(errno, "statfs(\"/sys/fs/cgroup/systemd\" failed: %m");

                        if (F_TYPE_EQUAL(fs.f_type, CGROUP2_SUPER_MAGIC)) {
                                log_debug("Found cgroup2 on /sys/fs/cgroup/systemd, unified hierarchy for systemd controller (v232 variant)");
                                unified_cache = CGROUP_UNIFIED_SYSTEMD;
                                unified_systemd_v232 = true;
                        } else if (F_TYPE_EQUAL(fs.f_type, CGROUP_SUPER_MAGIC)) {
                                log_debug("Found cgroup on /sys/fs/cgroup/systemd, legacy hierarchy");
                                unified_cache = CGROUP_UNIFIED_NONE;
                        } else {
                                log_debug("Unexpected filesystem type %llx mounted on /sys/fs/cgroup/systemd, assuming legacy hierarchy",
                                          (unsigned long long) fs.f_type);
                                unified_cache = CGROUP_UNIFIED_NONE;
                        }
                }
        } else
                return log_debug_errno(SYNTHETIC_ERRNO(ENOMEDIUM),
                                       "Unknown filesystem type %llx mounted on /sys/fs/cgroup.",
                                       (unsigned long long)fs.f_type);

        return 0;
}

int cg_unified_controller(const char *controller) {
        int r;

        r = cg_unified_update();
        if (r < 0)
                return r;

        if (unified_cache == CGROUP_UNIFIED_NONE)
                return false;

        if (unified_cache >= CGROUP_UNIFIED_ALL)
                return true;

        return streq_ptr(controller, SYSTEMD_CGROUP_CONTROLLER);
}

int cg_all_unified(void) {
        int r;

        r = cg_unified_update();
        if (r < 0)
                return r;

        return unified_cache >= CGROUP_UNIFIED_ALL;
}

int cg_hybrid_unified(void) {
        int r;

        r = cg_unified_update();
        if (r < 0)
                return r;

        return unified_cache == CGROUP_UNIFIED_SYSTEMD && !unified_systemd_v232;
}

int cg_unified_flush(void) {
        unified_cache = CGROUP_UNIFIED_UNKNOWN;

        return cg_unified_update();
}

int cg_enable_everywhere(
                CGroupMask supported,
                CGroupMask mask,
                const char *p,
                CGroupMask *ret_result_mask) {

        _cleanup_fclose_ FILE *f = NULL;
        _cleanup_free_ char *fs = NULL;
        CGroupController c;
        CGroupMask ret = 0;
        int r;

        assert(p);

        if (supported == 0) {
                if (ret_result_mask)
                        *ret_result_mask = 0;
                return 0;
        }

        r = cg_all_unified();
        if (r < 0)
                return r;
        if (r == 0) {
                /* On the legacy hierarchy there's no concept of "enabling" controllers in cgroups defined. Let's claim
                 * complete success right away. (If you wonder why we return the full mask here, rather than zero: the
                 * caller tends to use the returned mask later on to compare if all controllers where properly joined,
                 * and if not requeues realization. This use is the primary purpose of the return value, hence let's
                 * minimize surprises here and reduce triggers for re-realization by always saying we fully
                 * succeeded.) */
                if (ret_result_mask)
                        *ret_result_mask = mask & supported & CGROUP_MASK_V2; /* If you wonder why we mask this with
                                                                               * CGROUP_MASK_V2: The 'supported' mask
                                                                               * might contain pure-V1 or BPF
                                                                               * controllers, and we never want to
                                                                               * claim that we could enable those with
                                                                               * cgroup.subtree_control */
                return 0;
        }

        r = cg_get_path(SYSTEMD_CGROUP_CONTROLLER, p, "cgroup.subtree_control", &fs);
        if (r < 0)
                return r;

        for (c = 0; c < _CGROUP_CONTROLLER_MAX; c++) {
                CGroupMask bit = CGROUP_CONTROLLER_TO_MASK(c);
                const char *n;

                if (!FLAGS_SET(CGROUP_MASK_V2, bit))
                        continue;

                if (!FLAGS_SET(supported, bit))
                        continue;

                n = cgroup_controller_to_string(c);
                {
                        char s[1 + strlen(n) + 1];

                        s[0] = FLAGS_SET(mask, bit) ? '+' : '-';
                        strcpy(s + 1, n);

                        if (!f) {
                                f = fopen(fs, "we");
                                if (!f)
                                        return log_debug_errno(errno, "Failed to open cgroup.subtree_control file of %s: %m", p);
                        }

                        r = write_string_stream(f, s, WRITE_STRING_FILE_DISABLE_BUFFER);
                        if (r < 0) {
                                log_debug_errno(r, "Failed to %s controller %s for %s (%s): %m",
                                                FLAGS_SET(mask, bit) ? "enable" : "disable", n, p, fs);
                                clearerr(f);

                                /* If we can't turn off a controller, leave it on in the reported resulting mask. This
                                 * happens for example when we attempt to turn off a controller up in the tree that is
                                 * used down in the tree. */
                                if (!FLAGS_SET(mask, bit) && r == -EBUSY) /* You might wonder why we check for EBUSY
                                                                           * only here, and not follow the same logic
                                                                           * for other errors such as EINVAL or
                                                                           * EOPNOTSUPP or anything else. That's
                                                                           * because EBUSY indicates that the
                                                                           * controllers is currently enabled and
                                                                           * cannot be disabled because something down
                                                                           * the hierarchy is still using it. Any other
                                                                           * error most likely means something like "I
                                                                           * never heard of this controller" or
                                                                           * similar. In the former case it's hence
                                                                           * safe to assume the controller is still on
                                                                           * after the failed operation, while in the
                                                                           * latter case it's safer to assume the
                                                                           * controller is unknown and hence certainly
                                                                           * not enabled. */
                                        ret |= bit;
                        } else {
                                /* Otherwise, if we managed to turn on a controller, set the bit reflecting that. */
                                if (FLAGS_SET(mask, bit))
                                        ret |= bit;
                        }
                }
        }

        /* Let's return the precise set of controllers now enabled for the cgroup. */
        if (ret_result_mask)
                *ret_result_mask = ret;

        return 0;
}

bool cg_is_unified_wanted(void) {
        static thread_local int wanted = -1;
        int r;
        bool b;
        const bool is_default = DEFAULT_HIERARCHY == CGROUP_UNIFIED_ALL;
        _cleanup_free_ char *c = NULL;

        /* If we have a cached value, return that. */
        if (wanted >= 0)
                return wanted;

        /* If the hierarchy is already mounted, then follow whatever
         * was chosen for it. */
        if (cg_unified_flush() >= 0)
                return (wanted = unified_cache >= CGROUP_UNIFIED_ALL);

        /* If we were explicitly passed systemd.unified_cgroup_hierarchy,
         * respect that. */
        r = proc_cmdline_get_bool("systemd.unified_cgroup_hierarchy", &b);
        if (r > 0)
                return (wanted = b);

        /* If we passed cgroup_no_v1=all with no other instructions, it seems
         * highly unlikely that we want to use hybrid or legacy hierarchy. */
        r = proc_cmdline_get_key("cgroup_no_v1", 0, &c);
        if (r > 0 && streq_ptr(c, "all"))
                return (wanted = true);

        return (wanted = is_default);
}

bool cg_is_legacy_wanted(void) {
        static thread_local int wanted = -1;

        /* If we have a cached value, return that. */
        if (wanted >= 0)
                return wanted;

        /* Check if we have cgroup v2 already mounted. */
        if (cg_unified_flush() >= 0 &&
            unified_cache == CGROUP_UNIFIED_ALL)
                return (wanted = false);

        /* Otherwise, assume that at least partial legacy is wanted,
         * since cgroup v2 should already be mounted at this point. */
        return (wanted = true);
}

bool cg_is_hybrid_wanted(void) {
        static thread_local int wanted = -1;
        int r;
        bool b;
        const bool is_default = DEFAULT_HIERARCHY >= CGROUP_UNIFIED_SYSTEMD;
        /* We default to true if the default is "hybrid", obviously,
         * but also when the default is "unified", because if we get
         * called, it means that unified hierarchy was not mounted. */

        /* If we have a cached value, return that. */
        if (wanted >= 0)
                return wanted;

        /* If the hierarchy is already mounted, then follow whatever
         * was chosen for it. */
        if (cg_unified_flush() >= 0 &&
            unified_cache == CGROUP_UNIFIED_ALL)
                return (wanted = false);

        /* Otherwise, let's see what the kernel command line has to say.
         * Since checking is expensive, cache a non-error result. */
        r = proc_cmdline_get_bool("systemd.legacy_systemd_cgroup_controller", &b);

        /* The meaning of the kernel option is reversed wrt. to the return value
         * of this function, hence the negation. */
        return (wanted = r > 0 ? !b : is_default);
}

int cg_weight_parse(const char *s, uint64_t *ret) {
        uint64_t u;
        int r;

        if (isempty(s)) {
                *ret = CGROUP_WEIGHT_INVALID;
                return 0;
        }

        r = safe_atou64(s, &u);
        if (r < 0)
                return r;

        if (u < CGROUP_WEIGHT_MIN || u > CGROUP_WEIGHT_MAX)
                return -ERANGE;

        *ret = u;
        return 0;
}

const uint64_t cgroup_io_limit_defaults[_CGROUP_IO_LIMIT_TYPE_MAX] = {
        [CGROUP_IO_RBPS_MAX]    = CGROUP_LIMIT_MAX,
        [CGROUP_IO_WBPS_MAX]    = CGROUP_LIMIT_MAX,
        [CGROUP_IO_RIOPS_MAX]   = CGROUP_LIMIT_MAX,
        [CGROUP_IO_WIOPS_MAX]   = CGROUP_LIMIT_MAX,
};

static const char* const cgroup_io_limit_type_table[_CGROUP_IO_LIMIT_TYPE_MAX] = {
        [CGROUP_IO_RBPS_MAX]    = "IOReadBandwidthMax",
        [CGROUP_IO_WBPS_MAX]    = "IOWriteBandwidthMax",
        [CGROUP_IO_RIOPS_MAX]   = "IOReadIOPSMax",
        [CGROUP_IO_WIOPS_MAX]   = "IOWriteIOPSMax",
};

DEFINE_STRING_TABLE_LOOKUP(cgroup_io_limit_type, CGroupIOLimitType);

int cg_cpu_shares_parse(const char *s, uint64_t *ret) {
        uint64_t u;
        int r;

        if (isempty(s)) {
                *ret = CGROUP_CPU_SHARES_INVALID;
                return 0;
        }

        r = safe_atou64(s, &u);
        if (r < 0)
                return r;

        if (u < CGROUP_CPU_SHARES_MIN || u > CGROUP_CPU_SHARES_MAX)
                return -ERANGE;

        *ret = u;
        return 0;
}

int cg_blkio_weight_parse(const char *s, uint64_t *ret) {
        uint64_t u;
        int r;

        if (isempty(s)) {
                *ret = CGROUP_BLKIO_WEIGHT_INVALID;
                return 0;
        }

        r = safe_atou64(s, &u);
        if (r < 0)
                return r;

        if (u < CGROUP_BLKIO_WEIGHT_MIN || u > CGROUP_BLKIO_WEIGHT_MAX)
                return -ERANGE;

        *ret = u;
        return 0;
}

bool is_cgroup_fs(const struct statfs *s) {
        return is_fs_type(s, CGROUP_SUPER_MAGIC) ||
               is_fs_type(s, CGROUP2_SUPER_MAGIC);
}

bool fd_is_cgroup_fs(int fd) {
        struct statfs s;

        if (fstatfs(fd, &s) < 0)
                return -errno;

        return is_cgroup_fs(&s);
}

static const char *const cgroup_controller_table[_CGROUP_CONTROLLER_MAX] = {
        [CGROUP_CONTROLLER_CPU] = "cpu",
        [CGROUP_CONTROLLER_CPUACCT] = "cpuacct",
        [CGROUP_CONTROLLER_IO] = "io",
        [CGROUP_CONTROLLER_BLKIO] = "blkio",
        [CGROUP_CONTROLLER_MEMORY] = "memory",
        [CGROUP_CONTROLLER_DEVICES] = "devices",
        [CGROUP_CONTROLLER_PIDS] = "pids",
        [CGROUP_CONTROLLER_BPF_FIREWALL] = "bpf-firewall",
        [CGROUP_CONTROLLER_BPF_DEVICES] = "bpf-devices",
};

DEFINE_STRING_TABLE_LOOKUP(cgroup_controller, CGroupController);

CGroupMask get_cpu_accounting_mask(void) {
        static CGroupMask needed_mask = (CGroupMask) -1;

        /* On kernel ≥4.15 with unified hierarchy, cpu.stat's usage_usec is
         * provided externally from the CPU controller, which means we don't
         * need to enable the CPU controller just to get metrics. This is good,
         * because enabling the CPU controller comes at a minor performance
         * hit, especially when it's propagated deep into large hierarchies.
         * There's also no separate CPU accounting controller available within
         * a unified hierarchy.
         *
         * This combination of factors results in the desired cgroup mask to
         * enable for CPU accounting varying as follows:
         *
         *                   ╔═════════════════════╤═════════════════════╗
         *                   ║     Linux ≥4.15     │     Linux <4.15     ║
         *   ╔═══════════════╬═════════════════════╪═════════════════════╣
         *   ║ Unified       ║ nothing             │ CGROUP_MASK_CPU     ║
         *   ╟───────────────╫─────────────────────┼─────────────────────╢
         *   ║ Hybrid/Legacy ║ CGROUP_MASK_CPUACCT │ CGROUP_MASK_CPUACCT ║
         *   ╚═══════════════╩═════════════════════╧═════════════════════╝
         *
         * We check kernel version here instead of manually checking whether
         * cpu.stat is present for every cgroup, as that check in itself would
         * already be fairly expensive.
         *
         * Kernels where this patch has been backported will therefore have the
         * CPU controller enabled unnecessarily. This is more expensive than
         * necessary, but harmless. ☺️
         */

        if (needed_mask == (CGroupMask) -1) {
                if (cg_all_unified()) {
                        struct utsname u;
                        assert_se(uname(&u) >= 0);

                        if (str_verscmp(u.release, "4.15") < 0)
                                needed_mask = CGROUP_MASK_CPU;
                        else
                                needed_mask = 0;
                } else
                        needed_mask = CGROUP_MASK_CPUACCT;
        }

        return needed_mask;
}

bool cpu_accounting_is_cheap(void) {
        return get_cpu_accounting_mask() == 0;
}