path: root/src/nspawn/nspawn-cgroup.c

/***
  This file is part of systemd.

  Copyright 2015 Lennart Poettering

  systemd is free software; you can redistribute it and/or modify it
  under the terms of the GNU Lesser General Public License as published by
  the Free Software Foundation; either version 2.1 of the License, or
  (at your option) any later version.

  systemd is distributed in the hope that it will be useful, but
  WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  Lesser General Public License for more details.

  You should have received a copy of the GNU Lesser General Public License
  along with systemd; If not, see <http://www.gnu.org/licenses/>.
***/

#include <sys/mount.h>
#include <libmount.h>

#include "alloc-util.h"
#include "dirent-util.h"
#include "escape.h"
#include "fd-util.h"
#include "fileio.h"
#include "fs-util.h"
#include "mkdir.h"
#include "mount-util.h"
#include "parse-util.h"
#include "path-util.h"
#include "process-util.h"
#include "rm-rf.h"
#include "string-util.h"
#include "strv.h"
#include "user-util.h"
#include "util.h"

#include "nspawn-cgroup.h"
#include "nspawn-mount.h"

/* Code for managing the list of CGMounts ***************************/

typedef enum CGMountType {
        CGMOUNT_SYMLINK,
        CGMOUNT_TMPFS,
        CGMOUNT_CGROUP1,
        CGMOUNT_CGROUP2,
        _CGMOUNT_MAX
} CGMountType;

struct CGMount {
        CGMountType type;
        char *src;
        char *dst;
};

static CGMount *cgmount_add(CGMounts *mounts, CGMountType type, const char *src, const char *dst) {
        char *hsrc = NULL, *hdst = NULL;
        CGMount *c, *ret;

        assert(mounts);
        assert(type >= 0 && type < _CGMOUNT_MAX);
        assert(src);
        assert(dst);

        hsrc = strdup(src);
        hdst = strdup(dst);
        if (!hsrc || !hdst) {
                free(hsrc);
                free(hdst);
                return NULL;
        }

        c = realloc_multiply(mounts->mounts, sizeof(CGMount), mounts->n + 1);
        if (!c)
                return NULL;

        mounts->mounts = c;
        ret = &(mounts->mounts)[mounts->n];
        (mounts->n)++;

        *ret = (CGMount) {
                .type = type,
                .src = hsrc,
                .dst = hdst,
        };
        return ret;
}

void cgroup_free_mounts(CGMounts *mounts) {
        for (size_t i = 0; i < mounts->n; i++) {
                free(mounts->mounts[i].src);
                free(mounts->mounts[i].dst);
        }
        mounts->mounts = mfree(mounts->mounts);
        mounts->n = 0;
}

/********************************************************************/

static int chown_cgroup_path(const char *path, uid_t uid_shift) {
        _cleanup_close_ int fd = -1;
        const char *fn;

        fd = open(path, O_RDONLY|O_CLOEXEC|O_DIRECTORY);
        if (fd < 0)
                return -errno;

        FOREACH_STRING(fn,
                       ".",
                       "tasks",
                       "notify_on_release",
                       "cgroup.procs",
                       "cgroup.events",
                       "cgroup.clone_children",
                       "cgroup.controllers",
                       "cgroup.subtree_control")
                if (fchownat(fd, fn, uid_shift, uid_shift, 0) < 0)
                        log_full_errno(errno == ENOENT ? LOG_DEBUG :  LOG_WARNING, errno,
                                       "Failed to chown() cgroup file %s, ignoring: %m", fn);

        return 0;
}

static int sync_cgroup(pid_t pid, CGroupUnified outer_cgver, CGroupUnified inner_cgver, uid_t uid_shift) {
        _cleanup_free_ char *cgroup = NULL;
        char mountpoint[] = "/tmp/containerXXXXXX", pid_string[DECIMAL_STR_MAX(pid) + 1];
        bool undo_mount = false;
        const char *fn, *inner_hier;
        int r;

#define LOG_PFIX "PID " PID_FMT ": sync host cgroup -> container cgroup"

        /* When the host uses the legacy cgroup setup, but the
         * container shall use the unified hierarchy, let's make sure
         * we copy the path from the name=systemd hierarchy into the
         * unified hierarchy. Similar for the reverse situation. */

        r = cg_pid_get_path(SYSTEMD_CGROUP_CONTROLLER, pid, &cgroup);
        if (r < 0)
                return log_error_errno(r, LOG_PFIX ": failed to determine host cgroup: %m", pid);

        /* In order to access the container's hierarchy we need to mount it */
        if (!mkdtemp(mountpoint))
                return log_error_errno(errno, LOG_PFIX ": failed to create temporary mount point for container cgroup hierarchy: %m", pid);

        if (outer_cgver >= CGROUP_UNIFIED_SYSTEMD) {
                /* host: v2 ; container: v1 */
                inner_hier = "?:name=systemd";
                r = mount_verbose(LOG_ERR, "cgroup", mountpoint, "cgroup",
                                  MS_NOSUID|MS_NOEXEC|MS_NODEV, "none,name=systemd,xattr");
        } else {
                /* host: v1 ; container: v2 */
                inner_hier = "0:";
                r = mount_verbose(LOG_ERR, "cgroup", mountpoint, "cgroup2",
                                  MS_NOSUID|MS_NOEXEC|MS_NODEV, NULL);
        }
        if (r < 0) {
                log_error(LOG_PFIX ": failed to mount container cgroup hierarchy", pid);
                goto finish;
        }

        undo_mount = true;

        /* If nspawn dies abruptly the cgroup hierarchy created below
         * its unit isn't cleaned up. So, let's remove it
         * https://github.com/systemd/systemd/pull/4223#issuecomment-252519810 */
        fn = strjoina(mountpoint, cgroup);
        (void) rm_rf(fn, REMOVE_ROOT|REMOVE_ONLY_DIRECTORIES);

        fn = strjoina(mountpoint, cgroup, "/cgroup.procs");
        (void) mkdir_parents(fn, 0755);

        sprintf(pid_string, PID_FMT, pid);
        r = write_string_file(fn, pid_string, 0);
        if (r < 0) {
                log_error_errno(r, LOG_PFIX ": failed to move process to `%s:%s': %m", pid, inner_hier, cgroup);
                goto finish;
        }

        fn = strjoina(mountpoint, cgroup);
        r = chown_cgroup_path(fn, uid_shift);
        if (r < 0)
                log_error_errno(r, LOG_PFIX ": chown(path=\"%s\", uid=" UID_FMT "): %m", pid, fn, uid_shift);
finish:
        if (undo_mount) {
                r = umount_verbose(mountpoint);
                if (r < 0)
                        log_error_errno(r, LOG_PFIX ": umount(\"%s\"): %m", pid, mountpoint);
        }

        (void) rmdir(mountpoint);
        return r;
}

static int create_subcgroup(pid_t pid, CGroupUnified outer_cgver, CGroupUnified inner_cgver) {
        _cleanup_free_ char *cgroup = NULL;
        const char *child;
        int r;
        CGroupMask supported;

        /* In the unified hierarchy inner nodes may only contain
         * subgroups, but not processes. Hence, if we running in the
         * unified hierarchy and the container does the same, and we
         * did not create a scope unit for the container move us and
         * the container into two separate subcgroups. */

        r = cg_mask_supported(&supported);
        if (r < 0)
                return log_error_errno(r, "Failed to create host subcgroup: Failed to determine supported controllers: %m");

        r = cg_pid_get_path(SYSTEMD_CGROUP_CONTROLLER, 0, &cgroup);
        if (r < 0)
                return log_error_errno(r, "Failed to create host subcgroup: Failed to get our control group: %m");

        child = strjoina(cgroup, "/payload");
        r = cg_create_and_attach(SYSTEMD_CGROUP_CONTROLLER, child, pid);
        if (r < 0)
                return log_error_errno(r, "Failed to create host subcgroup: Failed to create %s subcgroup: %m", child);

        child = strjoina(cgroup, "/supervisor");
        r = cg_create_and_attach(SYSTEMD_CGROUP_CONTROLLER, child, 0);
        if (r < 0)
                return log_error_errno(r, "Failed to create host subcgroup: Failed to create %s subcgroup: %m", child);

        /* Try to enable as many controllers as possible for the new payload. */
        (void) cg_enable_everywhere(supported, supported, cgroup);
        return 0;
}

static int cgpath_count_procs(const char *cgpath, Set **ret_pids) {
        char line[LINE_MAX];
        _cleanup_set_free_ Set *pid_set = NULL;
        _cleanup_fclose_ FILE *procs = NULL;

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

        procs = fopen(prefix_roota(cgpath, "cgroup.procs"), "re");
        if (!procs)
                return -errno;

        FOREACH_LINE(line, procs, return -errno) {
                int r;
                pid_t pid;

                truncate_nl(line);
                r = parse_pid(line, &pid);
                if (r < 0)
                        return r;
                set_put(pid_set, PID_TO_PTR(pid));
        }

        if (ret_pids) {
                *ret_pids = pid_set;
                pid_set = NULL;
        }
        return 0;
}

int cgroup_setup(pid_t pid, CGroupUnified outer_cgver, CGroupUnified inner_cgver, uid_t uid_shift) {
        _cleanup_free_ char *cgpath = NULL, *cgroup = NULL;
        _cleanup_set_free_ Set *peers = NULL;
        int r;

        if (outer_cgver == CGROUP_UNIFIED_UNKNOWN && inner_cgver != CGROUP_UNIFIED_INHERIT) {
                log_error("Specifying a cgroup version is only supported when running on a systemd host");
                return -EINVAL;
        }

        if ((inner_cgver != CGROUP_UNIFIED_INHERIT) &&
            ((outer_cgver >= CGROUP_UNIFIED_SYSTEMD) != (inner_cgver >= CGROUP_UNIFIED_SYSTEMD))) {
                /* sync the name=systemd hierarchy with the unified hierarchy */
                r = sync_cgroup(pid, outer_cgver, inner_cgver, uid_shift);
                if (r < 0)
                        return r;
        }

        /* The rest applies only to the unified hierarchy (which if is in use, the setup is known), or the systemd
         * hierarchy. */
        if (outer_cgver == CGROUP_UNIFIED_UNKNOWN)
                return 0;

        r = cg_pid_get_path(SYSTEMD_CGROUP_CONTROLLER, pid, &cgroup);
        if (r < 0)
                return log_error_errno(r, "Failed to get host cgroup of the container: %m");

        r = cg_get_path(SYSTEMD_CGROUP_CONTROLLER, cgroup, NULL, &cgpath);
        if (r < 0)
                return log_error_errno(r, "Failed to get host file system path for container cgroup: %m");

        r = cgpath_count_procs(cgpath, &peers);
        if (r < 0)
                return log_error_errno(r, "Unable to count the processes in the container's cgroup: %m");

        /* This applies only to the unified hierarchy */
        if (outer_cgver >= CGROUP_UNIFIED_SYSTEMD && inner_cgver >= CGROUP_UNIFIED_SYSTEMD) {
                if (set_size(peers) == 2 && set_contains(peers, PID_TO_PTR(getpid()))) {
                        char *tmp;

                        r = create_subcgroup(pid, outer_cgver, inner_cgver);
                        if (r < 0)
                                return r;

                        set_remove(peers, PID_TO_PTR(getpid()));
                        tmp = strappend(cgpath, "/payload");
                        if (!tmp)
                                return log_oom();
                        cgpath = tmp;
                }
        }

        /* This applies only to the name=systemd/unified hierarchy, but IMO it should apply to all
         * hierarchies. */
        if (uid_shift != UID_INVALID && set_size(peers) == 1) {
                r = chown_cgroup_path(cgpath, uid_shift);
                if (r < 0)
                        return log_error_errno(r, "Failed to chown() cgroup %s: %m", cgpath);
        }

        return 0;
}

/********************************************************************/

static int cgroup_decide_mounts_inherit(CGMounts *ret_mounts) {
        _cleanup_fclose_ FILE *proc_self_mountinfo = NULL;
        _cleanup_(cgroup_free_mounts) CGMounts mounts = {};
        int r;

        proc_self_mountinfo = fopen("/proc/self/mountinfo", "re");
        if (!proc_self_mountinfo)
                return -errno;

        for (;;) {
                _cleanup_free_ char *escmountpoint = NULL, *mountpoint = NULL, *fstype = NULL, *superopts = NULL, *fsopts = NULL;
                char *name;
                CGMountType type;
                int k;

                k = fscanf(proc_self_mountinfo,
                           "%*s "       /* (1) mount id */
                           "%*s "       /* (2) parent id */
                           "%*s "       /* (3) major:minor */
                           "%*s "       /* (4) root */
                           "%ms "       /* (5) mount point */
                           "%*s"        /* (6) per-mount options */
                           "%*[^-]"     /* (7) optional fields */
                           "- "         /* (8) separator */
                           "%ms "       /* (9) file system type */
                           "%*s"        /* (10) mount source */
                           "%ms"        /* (11) per-superblock options */
                           "%*[^\n]",   /* some rubbish at the end */
                           &escmountpoint,
                           &fstype,
                           &superopts
                );
                if (k != 3) {
                        if (k == EOF)
                                break;

                        continue;
                }

                r = cunescape(escmountpoint, UNESCAPE_RELAX, &mountpoint);
                if (r < 0)
                        return r;

                name = path_startswith(mountpoint, "/sys/fs/cgroup");
                if (!name)
                        continue;

                if (!filename_is_valid(name) && !isempty(name))
                        continue;

                if (streq(fstype, "tmpfs"))
                        type = CGMOUNT_TMPFS;
                else if(streq(fstype, "cgroup"))
                        type = CGMOUNT_CGROUP1;
                else if (streq(fstype, "cgroup2"))
                        type = CGMOUNT_CGROUP2;
                else
                        continue;

                r = mnt_split_optstr(superopts, NULL, NULL, &fsopts, 0, 0);
                if (r < 0)
                        return r;

                if (!cgmount_add(&mounts, type, fsopts, name)) {
                        return -ENOMEM;
                }

                if (type == CGMOUNT_TMPFS) {
                        _cleanup_closedir_ DIR *dir;
                        struct dirent *entry;

                        dir = opendir(mountpoint);
                        if (!dir)
                                return log_error_errno(errno, "Failed to open directory %s: %m", mountpoint);

                        FOREACH_DIRENT(entry, dir, break) {
                                _cleanup_free_ char *target = NULL;
                                r = dirent_ensure_type(dir, entry);
                                if (r < 0)
                                        return r;
                                if (entry->d_type != DT_LNK)
                                        continue;
                                r = readlinkat_malloc(dirfd(dir), entry->d_name, &target);
                                if (r < 0)
                                        return r;
                                if (!cgmount_add(&mounts, CGMOUNT_SYMLINK, target, entry->d_name))
                                        return -ENOMEM;
                        }
                }
        }

        *ret_mounts = mounts;
        mounts.mounts = NULL;
        mounts.n = 0;

        return 0;
}

/* Retrieve a list of cgroup v1 hierarchies. */
static int get_v1_hierarchies(Set *subsystems) {
        _cleanup_fclose_ FILE *f = NULL;
        char line[LINE_MAX];

        assert(subsystems);

        f = fopen("/proc/self/cgroup", "re");
        if (!f)
                return errno == ENOENT ? -ESRCH : -errno;

        FOREACH_LINE(line, f, return -errno) {
                int r;
                char *e, *l, *p;

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

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

                *e = 0;

                if (streq(l, ""))
                        continue;

                p = strdup(l);
                if (!p)
                        return -ENOMEM;

                r = set_consume(subsystems, p);
                if (r < 0)
                        return r;
        }

        return 0;
}

/* Decide the legacy cgroup mounts when cgroup namespaces are used. */
static int cgroup_decide_mounts_sd_y_cgns(
                CGMounts *ret_mounts,
                CGroupUnified outer_cgver, CGroupUnified inner_cgver) {
        _cleanup_(cgroup_free_mounts) CGMounts mounts = {};
        _cleanup_set_free_free_ Set *hierarchies = NULL;
        const char *c;
        int r;

        /* Mount a tmpfs to /sys/fs/cgroup if it's not mounted there yet. */
        if (!cgmount_add(&mounts, CGMOUNT_TMPFS, "mode=755", ""))
                return log_oom();

        if (outer_cgver >= CGROUP_UNIFIED_ALL)
                goto skip_controllers;

        hierarchies = set_new(&string_hash_ops);
        if (!hierarchies)
                return log_oom();

        r = get_v1_hierarchies(hierarchies);
        if (r < 0)
                return log_error_errno(r, "Failed to determine cgroup hierarchies: %m");

        for (;;) {
                _cleanup_free_ const char *hierarchy = NULL;

                hierarchy = set_steal_first(hierarchies);
                if (!hierarchy)
                        break;

                if (streq(hierarchy, "name=systemd"))
                        continue;

                if (!cgmount_add(&mounts, CGMOUNT_CGROUP1, hierarchy, hierarchy))
                        return log_oom();

                /* When multiple hierarchies are co-mounted, make their
                 * constituting individual hierarchies a symlink to the
                 * co-mount.
                 */
                c = hierarchy;
                for (;;) {
                        _cleanup_free_ char *target = NULL, *controller = NULL;

                        r = extract_first_word(&c, &controller, ",", 0);
                        if (r < 0)
                                return log_error_errno(r, "Failed to extract co-mounted cgroup controller: %m");
                        if (r == 0)
                                break;

                        if (!cgmount_add(&mounts, CGMOUNT_SYMLINK, hierarchy, controller))
                                return log_oom();
                }
        }

skip_controllers:
        switch (inner_cgver) {
        case CGROUP_UNIFIED_NONE:
                if (!cgmount_add(&mounts, CGMOUNT_CGROUP1, "name=systmed", "systemd"))
                        return log_oom();
                break;
        case CGROUP_UNIFIED_ALL:
                if (!cgmount_add(&mounts, CGMOUNT_CGROUP2, "", "systemd"))
                        return log_oom();
                break;
        default:
                assert_not_reached("non-legacy cgroup version desired in legacy setup function");
                return -EINVAL;
        }

        *ret_mounts = mounts;
        mounts = (CGMounts){};

        return 0;
}

/* Decide the legacy cgroup mounts when cgroup namespaces are not used. */
static int cgroup_decide_mounts_sd_n_cgns(
                CGMounts *ret_mounts,
                CGroupUnified outer_cgver, CGroupUnified inner_cgver) {
        _cleanup_(cgroup_free_mounts) CGMounts mounts = {};
        _cleanup_set_free_free_ Set *controllers = NULL;
        int r;

        /* Mount a tmpfs to /sys/fs/cgroup if it's not mounted there yet. */
        if (!cgmount_add(&mounts, CGMOUNT_TMPFS, "mode=755", ""))
                return log_oom();

        if (outer_cgver >= CGROUP_UNIFIED_ALL)
                goto skip_controllers;

        controllers = set_new(&string_hash_ops);
        if (!controllers)
                return log_oom();

        r = cg_kernel_controllers(controllers);
        if (r < 0)
                return log_error_errno(r, "Failed to determine cgroup controllers: %m");

        for (;;) {
                _cleanup_free_ char *controller = NULL, *origin = NULL, *combined = NULL;

                controller = set_steal_first(controllers);
                if (!controller)
                        break;

                origin = prefix_root("/sys/fs/cgroup/", controller);
                if (!origin)
                        return log_oom();

                r = readlink_malloc(origin, &combined);
                if (r == -EINVAL) {
                        /* Not a symbolic link, but directly a single cgroup hierarchy */

                        if (!cgmount_add(&mounts, CGMOUNT_CGROUP1, controller, controller))
                                return log_oom();

                } else if (r < 0)
                        return log_error_errno(r, "Failed to read link %s: %m", origin);
                else {
                        /* A symbolic link, a combination of controllers in one hierarchy */

                        if (!filename_is_valid(combined)) {
                                log_warning("Ignoring invalid combined hierarchy %s.", combined);
                                continue;
                        }

                        if (!cgmount_add(&mounts, CGMOUNT_CGROUP1, combined, combined))
                                return log_oom();

                        if (!cgmount_add(&mounts, CGMOUNT_SYMLINK, combined, controller))
                                return log_oom();
                }
        }

skip_controllers:
        switch (inner_cgver) {
        case CGROUP_UNIFIED_NONE:
                if (!cgmount_add(&mounts, CGMOUNT_CGROUP1, "name=systmed", "systemd"))
                        return log_oom();
                break;
        case CGROUP_UNIFIED_ALL:
                if (!cgmount_add(&mounts, CGMOUNT_CGROUP2, "", "systemd"))
                        return log_oom();
                break;
        default:
                assert_not_reached("non-legacy cgroup version desired in legacy setup function");
                return -EINVAL;
        }

        *ret_mounts = mounts;
        mounts = (CGMounts){};

        return 0;
}

int cgroup_decide_mounts(
                CGMounts *ret_mounts,
                CGroupUnified outer_cgver, CGroupUnified inner_cgver,
                bool use_cgns) {
        switch (inner_cgver) {
        case CGROUP_UNIFIED_INHERIT:
                return cgroup_decide_mounts_inherit(ret_mounts);
        case CGROUP_UNIFIED_NONE:
        case CGROUP_UNIFIED_SYSTEMD:
                if (use_cgns)
                        return cgroup_decide_mounts_sd_y_cgns(ret_mounts, outer_cgver, inner_cgver);
                else
                        return cgroup_decide_mounts_sd_n_cgns(ret_mounts, outer_cgver, inner_cgver);
        case CGROUP_UNIFIED_ALL:
                if (!cgmount_add(ret_mounts, CGMOUNT_CGROUP2, "cgroup", ""))
                        return log_oom();
                return 0;
        default:
                assert_not_reached("Invalid cgroup ver requested");
                return -EINVAL;
        }
}

/********************************************************************/

static int cgroup_mount_cg(
                const char *mountpoint, const char *opts, CGMountType fstype,
                FILE *cgfile, bool use_userns) {
        const bool use_cgns = cgfile == NULL;
        /* If we are using userns and cgns, then we always let it be RW, because we can count on the shifted root user
         * to not have access to the things that would make us want to mount it RO.  Otherwise, we give the container
         * RW access to its unified or name=systemd cgroup. */
        const bool rw = (use_userns && use_cgns) || fstype == CGMOUNT_CGROUP2 || streq(mountpoint, "/sys/fs/cgroup/systemd");
        int r;

        /* First the base mount; this is always RW, as to not change the superblock settings */
        r = mount_verbose(LOG_ERR, "cgroup", mountpoint, fstype == CGMOUNT_CGROUP1 ? "cgroup" : "cgroup2",
                          MS_NOSUID|MS_NOEXEC|MS_NODEV, opts);
        if (r < 0)
                return r;

        /* Now, if nescessary, we remount RO */
        if (rw) {
                if (!use_cgns) {
                        /* emulate cgns by mounting everything but our subcgroup RO */
                        const char *cgpath;
                        char *cgroup = NULL;
                        if (fstype == CGMOUNT_CGROUP2) {
                                rewind(cgfile);
                                r = cg_pid_get_path_internal(NULL, cgfile, &cgroup);
                                if (r < 0)
                                        return log_error_errno(r, "Failed to get child's cgroup v2 path");
                        } else {
                                const char *scontroller, *state;
                                size_t controller_len;
                                FOREACH_WORD_SEPARATOR(scontroller, controller_len, opts, ",", state) {
                                        _cleanup_free_ const char *controller = strndup(scontroller, controller_len);
                                        rewind(cgfile);
                                        if (cg_pid_get_path_internal(controller, cgfile, &cgroup) == 0)
                                                break;
                                }
                                if (!cgroup)
                                        return log_error_errno(EBADMSG, "Failed to associate mounted cgroup hierarchy %s with numbered cgroup hierarchy", mountpoint);
                        }
                        cgpath = prefix_roota(mountpoint, cgroup);

                        r = mount_verbose(LOG_ERR, cgpath, cgpath, NULL, MS_BIND, NULL);
                        if (r < 0)
                                return r;
                        r = mount_verbose(LOG_ERR, NULL, mountpoint, NULL,
                                          MS_BIND|MS_REMOUNT|MS_NOSUID|MS_NOEXEC|MS_NODEV|MS_RDONLY, NULL);
                        if (r < 0)
                                return r;
                }
        } else {
                r = mount_verbose(LOG_ERR, NULL, mountpoint, NULL,
                                  MS_BIND|MS_REMOUNT|MS_NOSUID|MS_NOEXEC|MS_NODEV|MS_RDONLY, NULL);
                if (r < 0)
                        return r;
        }

        return 0;
}

int cgroup_mount_mounts(CGMounts m, FILE *cgfile, uid_t uid_shift, const char *selinux_apifs_context) {
        const bool use_cgns = cgfile == NULL;
        const bool use_userns = uid_shift != UID_INVALID;

        bool used_tmpfs = false;

        for (size_t i = 0; i < m.n; i++) {
                _cleanup_free_ char *options = NULL;
                const char *dst;
                int r;

                dst = prefix_roota("/sys/fs/cgroup", m.mounts[i].dst);

                /* The checks here to see if things are already mounted are kind of primative.  Perhaps they should
                 * actually check the statfs() f_type to verify that the thing mounted is what we want to be mounted
                 * (similar to cgroup-util's detection logic)?  But I don't really understand the use-case for having
                 * any of these already mounted, so I'm not sure if such increased strictness would be unwelcome. */

                switch (m.mounts[i].type) {
                case CGMOUNT_SYMLINK:
                        (void) mkdir_parents(dst, 0755);
                        r = symlink_idempotent(m.mounts[i].src, dst);
                        if (r < 0)
                                return r;
                        break;
                case CGMOUNT_TMPFS:
                        used_tmpfs = true;
                        r = path_is_mount_point(dst, AT_SYMLINK_FOLLOW);
                        if (r < 0)
                                return log_error_errno(r, "Failed to determine if %s is mounted already: %m", dst);
                        if (r > 0)
                                continue;
                        r = tmpfs_patch_options(m.mounts[i].src, uid_shift, selinux_apifs_context, &options);
                        if (r < 0)
                                return log_oom();
                        r = mount_verbose(LOG_ERR, /*name*/"tmpfs", dst, /*fstype*/"tmpfs",
                                          MS_NOSUID|MS_NOEXEC|MS_NODEV|MS_STRICTATIME, options);
                        if (r < 0)
                                return r;
                        break;
                case CGMOUNT_CGROUP1:
                case CGMOUNT_CGROUP2:
                        r = path_is_mount_point(dst, 0);
                        if (r < 0 && r != -ENOENT)
                                return log_error_errno(r, "Failed to determine if %s is mounted already: %m", dst);
                        if (r > 0) {
                                if (access(prefix_roota(dst, "cgroup.procs"), F_OK) >= 0)
                                        continue;
                                if (errno != ENOENT)
                                        return log_error_errno(errno, "Failed to determine if mount point %s is a cgroup hierarchy: %m", dst);
                                return log_error_errno(EINVAL, "%s is already mounted but not a cgroup hierarchy. Refusing.", dst);
                        }
                        (void) mkdir_p(dst, 0755);
                        r = cgroup_mount_cg(dst, m.mounts[i].src, m.mounts[i].type, cgfile, use_userns);
                        if (r < 0)
                                return r;
                        break;
                default:
                        assert_not_reached("Invalid CGMount type");
                        return -EINVAL;
                }
        }

        /* I'm going to be honest: I don't understand why we don't do this if we're using both userns and cgns. */
        if (used_tmpfs && (!use_userns || !use_cgns))
                return mount_verbose(LOG_ERR, NULL, "/sys/fs/cgroup", NULL,
                                     MS_REMOUNT|MS_NOSUID|MS_NOEXEC|MS_NODEV|MS_STRICTATIME|MS_RDONLY, "mode=755");

        return 0;
}