Adding upstream version 252.22.upstream/252.22

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 2211 insertions, 0 deletions

diff --git a/src/home/homed-manager.c b/src/home/homed-manager.c
new file mode 100644
index 0000000..9075e3e
--- /dev/null
+++ b/src/home/homed-manager.c
@@ -0,0 +1,2211 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include <grp.h>
+#include <linux/fs.h>
+#include <linux/magic.h>
+#include <math.h>
+#include <openssl/pem.h>
+#include <pwd.h>
+#include <sys/ioctl.h>
+#include <sys/quota.h>
+#include <sys/stat.h>
+
+#include "sd-id128.h"
+
+#include "btrfs-util.h"
+#include "bus-common-errors.h"
+#include "bus-error.h"
+#include "bus-log-control-api.h"
+#include "bus-polkit.h"
+#include "clean-ipc.h"
+#include "conf-files.h"
+#include "device-util.h"
+#include "dirent-util.h"
+#include "fd-util.h"
+#include "fileio.h"
+#include "format-util.h"
+#include "fs-util.h"
+#include "glyph-util.h"
+#include "gpt.h"
+#include "home-util.h"
+#include "homed-conf.h"
+#include "homed-home-bus.h"
+#include "homed-home.h"
+#include "homed-manager-bus.h"
+#include "homed-manager.h"
+#include "homed-varlink.h"
+#include "io-util.h"
+#include "mkdir.h"
+#include "process-util.h"
+#include "quota-util.h"
+#include "random-util.h"
+#include "resize-fs.h"
+#include "socket-util.h"
+#include "sort-util.h"
+#include "stat-util.h"
+#include "strv.h"
+#include "sync-util.h"
+#include "tmpfile-util.h"
+#include "udev-util.h"
+#include "user-record-sign.h"
+#include "user-record-util.h"
+#include "user-record.h"
+#include "user-util.h"
+
+/* Where to look for private/public keys that are used to sign the user records. We are not using
+ * CONF_PATHS_NULSTR() here since we want to insert /var/lib/systemd/home/ in the middle. And we insert that
+ * since we want to auto-generate a persistent private/public key pair if we need to. */
+#define KEY_PATHS_NULSTR                        \
+        "/etc/systemd/home/\0"                  \
+        "/run/systemd/home/\0"                  \
+        "/var/lib/systemd/home/\0"              \
+        "/usr/local/lib/systemd/home/\0"        \
+        "/usr/lib/systemd/home/\0"
+
+static bool uid_is_home(uid_t uid) {
+        return uid >= HOME_UID_MIN && uid <= HOME_UID_MAX;
+}
+/* Takes a value generated randomly or by hashing and turns it into a UID in the right range */
+
+#define UID_CLAMP_INTO_HOME_RANGE(rnd) (((uid_t) (rnd) % (HOME_UID_MAX - HOME_UID_MIN + 1)) + HOME_UID_MIN)
+
+DEFINE_PRIVATE_HASH_OPS_WITH_VALUE_DESTRUCTOR(homes_by_uid_hash_ops, void, trivial_hash_func, trivial_compare_func, Home, home_free);
+DEFINE_PRIVATE_HASH_OPS_WITH_VALUE_DESTRUCTOR(homes_by_name_hash_ops, char, string_hash_func, string_compare_func, Home, home_free);
+DEFINE_PRIVATE_HASH_OPS_WITH_VALUE_DESTRUCTOR(homes_by_worker_pid_hash_ops, void, trivial_hash_func, trivial_compare_func, Home, home_free);
+DEFINE_PRIVATE_HASH_OPS_WITH_VALUE_DESTRUCTOR(homes_by_sysfs_hash_ops, char, path_hash_func, path_compare, Home, home_free);
+
+static int on_home_inotify(sd_event_source *s, const struct inotify_event *event, void *userdata);
+static int manager_gc_images(Manager *m);
+static int manager_enumerate_images(Manager *m);
+static int manager_assess_image(Manager *m, int dir_fd, const char *dir_path, const char *dentry_name);
+static void manager_revalidate_image(Manager *m, Home *h);
+
+static void manager_watch_home(Manager *m) {
+        struct statfs sfs;
+        int r;
+
+        assert(m);
+
+        m->inotify_event_source = sd_event_source_disable_unref(m->inotify_event_source);
+        m->scan_slash_home = false;
+
+        if (statfs(get_home_root(), &sfs) < 0) {
+                log_full_errno(errno == ENOENT ? LOG_DEBUG : LOG_WARNING, errno,
+                               "Failed to statfs() %s directory, disabling automatic scanning.", get_home_root());
+                return;
+        }
+
+        if (is_network_fs(&sfs)) {
+                log_info("%s is a network file system, disabling automatic scanning.", get_home_root());
+                return;
+        }
+
+        if (is_fs_type(&sfs, AUTOFS_SUPER_MAGIC)) {
+                log_info("%s is on autofs, disabling automatic scanning.", get_home_root());
+                return;
+        }
+
+        m->scan_slash_home = true;
+
+        r = sd_event_add_inotify(m->event, &m->inotify_event_source, get_home_root(),
+                                 IN_CREATE|IN_CLOSE_WRITE|IN_DELETE_SELF|IN_MOVE_SELF|IN_ONLYDIR|IN_MOVED_TO|IN_MOVED_FROM|IN_DELETE,
+                                 on_home_inotify, m);
+        if (r < 0)
+                log_full_errno(r == -ENOENT ? LOG_DEBUG : LOG_WARNING, r,
+                               "Failed to create inotify watch on %s, ignoring.", get_home_root());
+
+        (void) sd_event_source_set_description(m->inotify_event_source, "home-inotify");
+
+        log_info("Watching %s.", get_home_root());
+}
+
+static int on_home_inotify(sd_event_source *s, const struct inotify_event *event, void *userdata) {
+        _cleanup_free_ char *j = NULL;
+        Manager *m = ASSERT_PTR(userdata);
+        const char *e, *n;
+
+        assert(event);
+
+        if ((event->mask & (IN_Q_OVERFLOW|IN_MOVE_SELF|IN_DELETE_SELF|IN_IGNORED|IN_UNMOUNT)) != 0) {
+
+                if (FLAGS_SET(event->mask, IN_Q_OVERFLOW))
+                        log_debug("%s inotify queue overflow, rescanning.", get_home_root());
+                else if (FLAGS_SET(event->mask, IN_MOVE_SELF))
+                        log_info("%s moved or renamed, recreating watch and rescanning.", get_home_root());
+                else if (FLAGS_SET(event->mask, IN_DELETE_SELF))
+                        log_info("%s deleted, recreating watch and rescanning.", get_home_root());
+                else if (FLAGS_SET(event->mask, IN_UNMOUNT))
+                        log_info("%s unmounted, recreating watch and rescanning.", get_home_root());
+                else if (FLAGS_SET(event->mask, IN_IGNORED))
+                        log_info("%s watch invalidated, recreating watch and rescanning.", get_home_root());
+
+                manager_watch_home(m);
+                (void) manager_gc_images(m);
+                (void) manager_enumerate_images(m);
+                (void) bus_manager_emit_auto_login_changed(m);
+                return 0;
+        }
+
+        /* For the other inotify events, let's ignore all events for file names that don't match our
+         * expectations */
+        if (isempty(event->name))
+                return 0;
+        e = endswith(event->name, FLAGS_SET(event->mask, IN_ISDIR) ? ".homedir" : ".home");
+        if (!e)
+                return 0;
+
+        n = strndupa_safe(event->name, e - event->name);
+        if (!suitable_user_name(n))
+                return 0;
+
+        j = path_join(get_home_root(), event->name);
+        if (!j)
+                return log_oom();
+
+        if ((event->mask & (IN_CREATE|IN_CLOSE_WRITE|IN_MOVED_TO)) != 0) {
+                if (FLAGS_SET(event->mask, IN_CREATE))
+                        log_debug("%s has been created, having a look.", j);
+                else if (FLAGS_SET(event->mask, IN_CLOSE_WRITE))
+                        log_debug("%s has been modified, having a look.", j);
+                else if (FLAGS_SET(event->mask, IN_MOVED_TO))
+                        log_debug("%s has been moved in, having a look.", j);
+
+                (void) manager_assess_image(m, -1, get_home_root(), event->name);
+                (void) bus_manager_emit_auto_login_changed(m);
+        }
+
+        if ((event->mask & (IN_DELETE | IN_CLOSE_WRITE | IN_MOVED_FROM)) != 0) {
+                Home *h;
+
+                if (FLAGS_SET(event->mask, IN_DELETE))
+                        log_debug("%s has been deleted, revalidating.", j);
+                else if (FLAGS_SET(event->mask, IN_CLOSE_WRITE))
+                        log_debug("%s has been closed after writing, revalidating.", j);
+                else if (FLAGS_SET(event->mask, IN_MOVED_FROM))
+                        log_debug("%s has been moved away, revalidating.", j);
+
+                h = hashmap_get(m->homes_by_name, n);
+                if (h) {
+                        manager_revalidate_image(m, h);
+                        (void) bus_manager_emit_auto_login_changed(m);
+                }
+        }
+
+        return 0;
+}
+
+int manager_new(Manager **ret) {
+        _cleanup_(manager_freep) Manager *m = NULL;
+        int r;
+
+        assert(ret);
+
+        m = new(Manager, 1);
+        if (!m)
+                return -ENOMEM;
+
+        *m = (Manager) {
+                .default_storage = _USER_STORAGE_INVALID,
+                .rebalance_interval_usec = 2 * USEC_PER_MINUTE, /* initially, rebalance every 2min */
+        };
+
+        r = manager_parse_config_file(m);
+        if (r < 0)
+                return r;
+
+        r = sd_event_default(&m->event);
+        if (r < 0)
+                return r;
+
+        r = sd_event_add_signal(m->event, NULL, SIGINT, NULL, NULL);
+        if (r < 0)
+                return r;
+
+        r = sd_event_add_signal(m->event, NULL, SIGTERM, NULL, NULL);
+        if (r < 0)
+                return r;
+
+        (void) sd_event_set_watchdog(m->event, true);
+
+        m->homes_by_uid = hashmap_new(&homes_by_uid_hash_ops);
+        if (!m->homes_by_uid)
+                return -ENOMEM;
+
+        m->homes_by_name = hashmap_new(&homes_by_name_hash_ops);
+        if (!m->homes_by_name)
+                return -ENOMEM;
+
+        m->homes_by_worker_pid = hashmap_new(&homes_by_worker_pid_hash_ops);
+        if (!m->homes_by_worker_pid)
+                return -ENOMEM;
+
+        m->homes_by_sysfs = hashmap_new(&homes_by_sysfs_hash_ops);
+        if (!m->homes_by_sysfs)
+                return -ENOMEM;
+
+        *ret = TAKE_PTR(m);
+        return 0;
+}
+
+Manager* manager_free(Manager *m) {
+        Home *h;
+
+        assert(m);
+
+        HASHMAP_FOREACH(h, m->homes_by_worker_pid)
+                (void) home_wait_for_worker(h);
+
+        m->bus = sd_bus_flush_close_unref(m->bus);
+        m->polkit_registry = bus_verify_polkit_async_registry_free(m->polkit_registry);
+
+        m->device_monitor = sd_device_monitor_unref(m->device_monitor);
+
+        m->inotify_event_source = sd_event_source_unref(m->inotify_event_source);
+        m->notify_socket_event_source = sd_event_source_unref(m->notify_socket_event_source);
+        m->deferred_rescan_event_source = sd_event_source_unref(m->deferred_rescan_event_source);
+        m->deferred_gc_event_source = sd_event_source_unref(m->deferred_gc_event_source);
+        m->deferred_auto_login_event_source = sd_event_source_unref(m->deferred_auto_login_event_source);
+        m->rebalance_event_source = sd_event_source_unref(m->rebalance_event_source);
+
+        m->event = sd_event_unref(m->event);
+
+        m->homes_by_uid = hashmap_free(m->homes_by_uid);
+        m->homes_by_name = hashmap_free(m->homes_by_name);
+        m->homes_by_worker_pid = hashmap_free(m->homes_by_worker_pid);
+        m->homes_by_sysfs = hashmap_free(m->homes_by_sysfs);
+
+        if (m->private_key)
+                EVP_PKEY_free(m->private_key);
+
+        hashmap_free(m->public_keys);
+
+        varlink_server_unref(m->varlink_server);
+        free(m->userdb_service);
+
+        free(m->default_file_system_type);
+
+        return mfree(m);
+}
+
+int manager_verify_user_record(Manager *m, UserRecord *hr) {
+        EVP_PKEY *pkey;
+        int r;
+
+        assert(m);
+        assert(hr);
+
+        if (!m->private_key && hashmap_isempty(m->public_keys)) {
+                r = user_record_has_signature(hr);
+                if (r < 0)
+                        return r;
+
+                return r ? -ENOKEY : USER_RECORD_UNSIGNED;
+        }
+
+        /* Is it our own? */
+        if (m->private_key) {
+                r = user_record_verify(hr, m->private_key);
+                switch (r) {
+
+                case USER_RECORD_FOREIGN:
+                        /* This record is not signed by this key, but let's see below */
+                        break;
+
+                case USER_RECORD_SIGNED:               /* Signed by us, but also by others, let's propagate that */
+                case USER_RECORD_SIGNED_EXCLUSIVE:     /* Signed by us, and nothing else, ditto */
+                case USER_RECORD_UNSIGNED:             /* Not signed at all, ditto  */
+                default:
+                        return r;
+                }
+        }
+
+        HASHMAP_FOREACH(pkey, m->public_keys) {
+                r = user_record_verify(hr, pkey);
+                switch (r) {
+
+                case USER_RECORD_FOREIGN:
+                        /* This record is not signed by this key, but let's see our other keys */
+                        break;
+
+                case USER_RECORD_SIGNED:            /* It's signed by this key we are happy with, but which is not our own. */
+                case USER_RECORD_SIGNED_EXCLUSIVE:
+                        return USER_RECORD_FOREIGN;
+
+                case USER_RECORD_UNSIGNED: /* It's not signed at all */
+                default:
+                        return r;
+                }
+        }
+
+        return -ENOKEY;
+}
+
+static int manager_add_home_by_record(
+                Manager *m,
+                const char *name,
+                int dir_fd,
+                const char *fname) {
+
+        _cleanup_(json_variant_unrefp) JsonVariant *v = NULL;
+        _cleanup_(user_record_unrefp) UserRecord *hr = NULL;
+        unsigned line, column;
+        int r, is_signed;
+        struct stat st;
+        Home *h;
+
+        assert(m);
+        assert(name);
+        assert(fname);
+
+        if (fstatat(dir_fd, fname, &st, 0) < 0)
+                return log_error_errno(errno, "Failed to stat identity record %s: %m", fname);
+
+        if (!S_ISREG(st.st_mode)) {
+                log_debug("Identity record file %s is not a regular file, ignoring.", fname);
+                return 0;
+        }
+
+        if (st.st_size == 0)
+                goto unlink_this_file;
+
+        r = json_parse_file_at(NULL, dir_fd, fname, JSON_PARSE_SENSITIVE, &v, &line, &column);
+        if (r < 0)
+                return log_error_errno(r, "Failed to parse identity record at %s:%u%u: %m", fname, line, column);
+
+        if (json_variant_is_blank_object(v))
+                goto unlink_this_file;
+
+        hr = user_record_new();
+        if (!hr)
+                return log_oom();
+
+        r = user_record_load(hr, v, USER_RECORD_LOAD_REFUSE_SECRET|USER_RECORD_LOG|USER_RECORD_PERMISSIVE);
+        if (r < 0)
+                return r;
+
+        if (!streq_ptr(hr->user_name, name))
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
+                                       "Identity's user name %s does not match file name %s, refusing.",
+                                       hr->user_name, name);
+
+        is_signed = manager_verify_user_record(m, hr);
+        switch (is_signed) {
+
+        case -ENOKEY:
+                return log_warning_errno(is_signed, "User record %s is not signed by any accepted key, ignoring.", fname);
+        case USER_RECORD_UNSIGNED:
+                return log_warning_errno(SYNTHETIC_ERRNO(EPERM), "User record %s is not signed at all, ignoring.", fname);
+        case USER_RECORD_SIGNED:
+                log_info("User record %s is signed by us (and others), accepting.", fname);
+                break;
+        case USER_RECORD_SIGNED_EXCLUSIVE:
+                log_info("User record %s is signed only by us, accepting.", fname);
+                break;
+        case USER_RECORD_FOREIGN:
+                log_info("User record %s is signed by registered key from others, accepting.", fname);
+                break;
+        default:
+                assert(is_signed < 0);
+                return log_error_errno(is_signed, "Failed to verify signature of user record in %s: %m", fname);
+        }
+
+        h = hashmap_get(m->homes_by_name, name);
+        if (h) {
+                r = home_set_record(h, hr);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to update home record for %s: %m", name);
+
+                /* If we acquired a record now for a previously unallocated entry, then reset the state. This
+                 * makes sure home_get_state() will check for the availability of the image file dynamically
+                 * in order to detect to distinguish HOME_INACTIVE and HOME_ABSENT. */
+                if (h->state == HOME_UNFIXATED)
+                        h->state = _HOME_STATE_INVALID;
+        } else {
+                r = home_new(m, hr, NULL, &h);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to allocate new home object: %m");
+
+                log_info("Added registered home for user %s.", hr->user_name);
+        }
+
+        /* Only entries we exclusively signed are writable to us, hence remember the result */
+        h->signed_locally = is_signed == USER_RECORD_SIGNED_EXCLUSIVE;
+
+        return 1;
+
+unlink_this_file:
+        /* If this is an empty file, then let's just remove it. An empty file is not useful in any case, and
+         * apparently xfs likes to leave empty files around when not unmounted cleanly (see
+         * https://github.com/systemd/systemd/issues/15178 for example). Note that we don't delete non-empty
+         * files even if they are invalid, because that's just too risky, we might delete data the user still
+         * needs. But empty files are never useful, hence let's just remove them. */
+
+        if (unlinkat(dir_fd, fname, 0) < 0)
+                return log_error_errno(errno, "Failed to remove empty user record file %s: %m", fname);
+
+        log_notice("Discovered empty user record file %s/%s, removed automatically.", home_record_dir(), fname);
+        return 0;
+}
+
+static int manager_enumerate_records(Manager *m) {
+        _cleanup_closedir_ DIR *d = NULL;
+
+        assert(m);
+
+        d = opendir(home_record_dir());
+        if (!d)
+                return log_full_errno(errno == ENOENT ? LOG_DEBUG : LOG_ERR, errno,
+                                      "Failed to open %s: %m", home_record_dir());
+
+        FOREACH_DIRENT(de, d, return log_error_errno(errno, "Failed to read record directory: %m")) {
+                _cleanup_free_ char *n = NULL;
+                const char *e;
+
+                if (!dirent_is_file(de))
+                        continue;
+
+                e = endswith(de->d_name, ".identity");
+                if (!e)
+                        continue;
+
+                n = strndup(de->d_name, e - de->d_name);
+                if (!n)
+                        return log_oom();
+
+                if (!suitable_user_name(n))
+                        continue;
+
+                (void) manager_add_home_by_record(m, n, dirfd(d), de->d_name);
+        }
+
+        return 0;
+}
+
+static int search_quota(uid_t uid, const char *exclude_quota_path) {
+        struct stat exclude_st = {};
+        dev_t previous_devno = 0;
+        int r;
+
+        /* Checks whether the specified UID owns any files on the files system, but ignore any file system
+         * backing the specified file. The file is used when operating on home directories, where it's OK if
+         * the UID of them already owns files. */
+
+        if (exclude_quota_path && stat(exclude_quota_path, &exclude_st) < 0) {
+                if (errno != ENOENT)
+                        return log_warning_errno(errno, "Failed to stat %s, ignoring: %m", exclude_quota_path);
+        }
+
+        /* Check a few usual suspects where regular users might own files. Note that this is by no means
+         * comprehensive, but should cover most cases. Note that in an ideal world every user would be
+         * registered in NSS and avoid our own UID range, but for all other cases, it's a good idea to be
+         * paranoid and check quota if we can. */
+        FOREACH_STRING(where, get_home_root(), "/tmp/", "/var/", "/var/mail/", "/var/tmp/", "/var/spool/") {
+                struct dqblk req;
+                struct stat st;
+
+                if (stat(where, &st) < 0) {
+                        log_full_errno(errno == ENOENT ? LOG_DEBUG : LOG_ERR, errno,
+                                       "Failed to stat %s, ignoring: %m", where);
+                        continue;
+                }
+
+                if (major(st.st_dev) == 0) {
+                        log_debug("Directory %s is not on a real block device, not checking quota for UID use.", where);
+                        continue;
+                }
+
+                if (st.st_dev == exclude_st.st_dev) { /* If an exclude path is specified, then ignore quota
+                                                       * reported on the same block device as that path. */
+                        log_debug("Directory %s is where the home directory is located, not checking quota for UID use.", where);
+                        continue;
+                }
+
+                if (st.st_dev == previous_devno) { /* Does this directory have the same devno as the previous
+                                                    * one we tested? If so, there's no point in testing this
+                                                    * again. */
+                        log_debug("Directory %s is on same device as previous tested directory, not checking quota for UID use a second time.", where);
+                        continue;
+                }
+
+                previous_devno = st.st_dev;
+
+                r = quotactl_devnum(QCMD_FIXED(Q_GETQUOTA, USRQUOTA), st.st_dev, uid, &req);
+                if (r < 0) {
+                        if (ERRNO_IS_NOT_SUPPORTED(r))
+                                log_debug_errno(r, "No UID quota support on %s, ignoring.", where);
+                        else if (ERRNO_IS_PRIVILEGE(r))
+                                log_debug_errno(r, "UID quota support for %s prohibited, ignoring.", where);
+                        else
+                                log_warning_errno(r, "Failed to query quota on %s, ignoring: %m", where);
+
+                        continue;
+                }
+
+                if ((FLAGS_SET(req.dqb_valid, QIF_SPACE) && req.dqb_curspace > 0) ||
+                    (FLAGS_SET(req.dqb_valid, QIF_INODES) && req.dqb_curinodes > 0)) {
+                        log_debug_errno(errno, "Quota reports UID " UID_FMT " occupies disk space on %s.", uid, where);
+                        return 1;
+                }
+        }
+
+        return 0;
+}
+
+static int manager_acquire_uid(
+                Manager *m,
+                uid_t start_uid,
+                const char *user_name,
+                const char *exclude_quota_path,
+                uid_t *ret) {
+
+        static const uint8_t hash_key[] = {
+                0xa3, 0xb8, 0x82, 0x69, 0x9a, 0x71, 0xf7, 0xa9,
+                0xe0, 0x7c, 0xf6, 0xf1, 0x21, 0x69, 0xd2, 0x1e
+        };
+
+        enum {
+                PHASE_SUGGESTED,
+                PHASE_HASHED,
+                PHASE_RANDOM
+        } phase = PHASE_SUGGESTED;
+
+        unsigned n_tries = 100;
+        int r;
+
+        assert(m);
+        assert(ret);
+
+        for (;;) {
+                struct passwd *pw;
+                struct group *gr;
+                uid_t candidate;
+                Home *other;
+
+                if (--n_tries <= 0)
+                        return -EBUSY;
+
+                switch (phase) {
+
+                case PHASE_SUGGESTED:
+                        phase = PHASE_HASHED;
+
+                        if (!uid_is_home(start_uid))
+                                continue;
+
+                        candidate = start_uid;
+                        break;
+
+                case PHASE_HASHED:
+                        phase = PHASE_RANDOM;
+
+                        if (!user_name)
+                                continue;
+
+                        candidate = UID_CLAMP_INTO_HOME_RANGE(siphash24(user_name, strlen(user_name), hash_key));
+                        break;
+
+                case PHASE_RANDOM:
+                        random_bytes(&candidate, sizeof(candidate));
+                        candidate = UID_CLAMP_INTO_HOME_RANGE(candidate);
+                        break;
+
+                default:
+                        assert_not_reached();
+                }
+
+                other = hashmap_get(m->homes_by_uid, UID_TO_PTR(candidate));
+                if (other) {
+                        log_debug("Candidate UID " UID_FMT " already used by another home directory (%s), let's try another.",
+                                  candidate, other->user_name);
+                        continue;
+                }
+
+                pw = getpwuid(candidate);
+                if (pw) {
+                        log_debug("Candidate UID " UID_FMT " already registered by another user in NSS (%s), let's try another.",
+                                  candidate, pw->pw_name);
+                        continue;
+                }
+
+                gr = getgrgid((gid_t) candidate);
+                if (gr) {
+                        log_debug("Candidate UID " UID_FMT " already registered by another group in NSS (%s), let's try another.",
+                                  candidate, gr->gr_name);
+                        continue;
+                }
+
+                r = search_ipc(candidate, (gid_t) candidate);
+                if (r < 0)
+                        continue;
+                if (r > 0) {
+                        log_debug_errno(r, "Candidate UID " UID_FMT " already owns IPC objects, let's try another: %m",
+                                        candidate);
+                        continue;
+                }
+
+                r = search_quota(candidate, exclude_quota_path);
+                if (r != 0)
+                        continue;
+
+                *ret = candidate;
+                return 0;
+        }
+}
+
+static int manager_add_home_by_image(
+                Manager *m,
+                const char *user_name,
+                const char *realm,
+                const char *image_path,
+                const char *sysfs,
+                UserStorage storage,
+                uid_t start_uid) {
+
+        _cleanup_(user_record_unrefp) UserRecord *hr = NULL;
+        uid_t uid;
+        Home *h;
+        int r;
+
+        assert(m);
+
+        assert(m);
+        assert(user_name);
+        assert(image_path);
+        assert(storage >= 0);
+        assert(storage < _USER_STORAGE_MAX);
+
+        h = hashmap_get(m->homes_by_name, user_name);
+        if (h) {
+                bool same;
+
+                if (h->state != HOME_UNFIXATED) {
+                        log_debug("Found an image for user %s which already has a record, skipping.", user_name);
+                        return 0; /* ignore images that synthesize a user we already have a record for */
+                }
+
+                same = user_record_storage(h->record) == storage;
+                if (same) {
+                        if (h->sysfs && sysfs)
+                                same = path_equal(h->sysfs, sysfs);
+                        else if (!!h->sysfs != !!sysfs)
+                                same = false;
+                        else {
+                                const char *p;
+
+                                p = user_record_image_path(h->record);
+                                same = p && path_equal(p, image_path);
+                        }
+                }
+
+                if (!same) {
+                        log_debug("Found multiple images for user '%s', ignoring image '%s'.", user_name, image_path);
+                        return 0;
+                }
+        } else {
+                /* Check NSS, in case there's another user or group by this name */
+                if (getpwnam(user_name) || getgrnam(user_name)) {
+                        log_debug("Found an existing user or group by name '%s', ignoring image '%s'.", user_name, image_path);
+                        return 0;
+                }
+        }
+
+        if (h && uid_is_valid(h->uid))
+                uid = h->uid;
+        else {
+                r = manager_acquire_uid(m, start_uid, user_name,
+                                        IN_SET(storage, USER_SUBVOLUME, USER_DIRECTORY, USER_FSCRYPT) ? image_path : NULL,
+                                        &uid);
+                if (r < 0)
+                        return log_warning_errno(r, "Failed to acquire unused UID for %s: %m", user_name);
+        }
+
+        hr = user_record_new();
+        if (!hr)
+                return log_oom();
+
+        r = user_record_synthesize(hr, user_name, realm, image_path, storage, uid, (gid_t) uid);
+        if (r < 0)
+                return log_error_errno(r, "Failed to synthesize home record for %s (image %s): %m", user_name, image_path);
+
+        if (h) {
+                r = home_set_record(h, hr);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to update home record for %s: %m", user_name);
+        } else {
+                r = home_new(m, hr, sysfs, &h);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to allocate new home object: %m");
+
+                h->state = HOME_UNFIXATED;
+
+                log_info("Discovered new home for user %s through image %s.", user_name, image_path);
+        }
+
+        return 1;
+}
+
+int manager_augment_record_with_uid(
+                Manager *m,
+                UserRecord *hr) {
+
+        const char *exclude_quota_path = NULL;
+        uid_t start_uid = UID_INVALID, uid;
+        int r;
+
+        assert(m);
+        assert(hr);
+
+        if (uid_is_valid(hr->uid))
+                return 0;
+
+        if (IN_SET(hr->storage, USER_CLASSIC, USER_SUBVOLUME, USER_DIRECTORY, USER_FSCRYPT)) {
+                const char * ip;
+
+                ip = user_record_image_path(hr);
+                if (ip) {
+                        struct stat st;
+
+                        if (stat(ip, &st) < 0) {
+                                if (errno != ENOENT)
+                                        log_warning_errno(errno, "Failed to stat(%s): %m", ip);
+                        }  else if (uid_is_home(st.st_uid)) {
+                                start_uid = st.st_uid;
+                                exclude_quota_path = ip;
+                        }
+                }
+        }
+
+        r = manager_acquire_uid(m, start_uid, hr->user_name, exclude_quota_path, &uid);
+        if (r < 0)
+                return r;
+
+        log_debug("Acquired new UID " UID_FMT " for %s.", uid, hr->user_name);
+
+        r = user_record_add_binding(
+                        hr,
+                        _USER_STORAGE_INVALID,
+                        NULL,
+                        SD_ID128_NULL,
+                        SD_ID128_NULL,
+                        SD_ID128_NULL,
+                        NULL,
+                        NULL,
+                        UINT64_MAX,
+                        NULL,
+                        NULL,
+                        uid,
+                        (gid_t) uid);
+        if (r < 0)
+                return r;
+
+        return 1;
+}
+
+static int manager_assess_image(
+                Manager *m,
+                int dir_fd,
+                const char *dir_path,
+                const char *dentry_name) {
+
+        char *luks_suffix, *directory_suffix;
+        _cleanup_free_ char *path = NULL;
+        struct stat st;
+        int r;
+
+        assert(m);
+        assert(dir_path);
+        assert(dentry_name);
+
+        luks_suffix = endswith(dentry_name, ".home");
+        if (luks_suffix)
+                directory_suffix = NULL;
+        else
+                directory_suffix = endswith(dentry_name, ".homedir");
+
+        /* Early filter out: by name */
+        if (!luks_suffix && !directory_suffix)
+                return 0;
+
+        path = path_join(dir_path, dentry_name);
+        if (!path)
+                return log_oom();
+
+        /* Follow symlinks here, to allow people to link in stuff to make them available locally. */
+        if (dir_fd >= 0)
+                r = fstatat(dir_fd, dentry_name, &st, 0);
+        else
+                r = stat(path, &st);
+        if (r < 0)
+                return log_full_errno(errno == ENOENT ? LOG_DEBUG : LOG_WARNING, errno,
+                                      "Failed to stat() directory entry '%s', ignoring: %m", dentry_name);
+
+        if (S_ISREG(st.st_mode)) {
+                _cleanup_free_ char *n = NULL, *user_name = NULL, *realm = NULL;
+
+                if (!luks_suffix)
+                        return 0;
+
+                n = strndup(dentry_name, luks_suffix - dentry_name);
+                if (!n)
+                        return log_oom();
+
+                r = split_user_name_realm(n, &user_name, &realm);
+                if (r == -EINVAL) /* Not the right format: ignore */
+                        return 0;
+                if (r < 0)
+                        return log_error_errno(r, "Failed to split image name into user name/realm: %m");
+
+                return manager_add_home_by_image(m, user_name, realm, path, NULL, USER_LUKS, UID_INVALID);
+        }
+
+        if (S_ISDIR(st.st_mode)) {
+                _cleanup_free_ char *n = NULL, *user_name = NULL, *realm = NULL;
+                _cleanup_close_ int fd = -1;
+                UserStorage storage;
+
+                if (!directory_suffix)
+                        return 0;
+
+                n = strndup(dentry_name, directory_suffix - dentry_name);
+                if (!n)
+                        return log_oom();
+
+                r = split_user_name_realm(n, &user_name, &realm);
+                if (r == -EINVAL) /* Not the right format: ignore */
+                        return 0;
+                if (r < 0)
+                        return log_error_errno(r, "Failed to split image name into user name/realm: %m");
+
+                if (dir_fd >= 0)
+                        fd = openat(dir_fd, dentry_name, O_DIRECTORY|O_RDONLY|O_CLOEXEC);
+                else
+                        fd = open(path, O_DIRECTORY|O_RDONLY|O_CLOEXEC);
+                if (fd < 0)
+                        return log_full_errno(errno == ENOENT ? LOG_DEBUG : LOG_WARNING, errno,
+                                              "Failed to open directory '%s', ignoring: %m", path);
+
+                if (fstat(fd, &st) < 0)
+                        return log_warning_errno(errno, "Failed to fstat() %s, ignoring: %m", path);
+
+                assert(S_ISDIR(st.st_mode)); /* Must hold, we used O_DIRECTORY above */
+
+                r = btrfs_is_subvol_fd(fd);
+                if (r < 0)
+                        return log_warning_errno(errno, "Failed to determine whether %s is a btrfs subvolume: %m", path);
+                if (r > 0)
+                        storage = USER_SUBVOLUME;
+                else {
+                        struct fscrypt_policy policy;
+
+                        if (ioctl(fd, FS_IOC_GET_ENCRYPTION_POLICY, &policy) < 0) {
+
+                                if (errno == ENODATA)
+                                        log_debug_errno(errno, "Determined %s is not fscrypt encrypted.", path);
+                                else if (ERRNO_IS_NOT_SUPPORTED(errno))
+                                        log_debug_errno(errno, "Determined %s is not fscrypt encrypted because kernel or file system doesn't support it.", path);
+                                else
+                                        log_debug_errno(errno, "FS_IOC_GET_ENCRYPTION_POLICY failed with unexpected error code on %s, ignoring: %m", path);
+
+                                storage = USER_DIRECTORY;
+                        } else
+                                storage = USER_FSCRYPT;
+                }
+
+                return manager_add_home_by_image(m, user_name, realm, path, NULL, storage, st.st_uid);
+        }
+
+        return 0;
+}
+
+int manager_enumerate_images(Manager *m) {
+        _cleanup_closedir_ DIR *d = NULL;
+
+        assert(m);
+
+        if (!m->scan_slash_home)
+                return 0;
+
+        d = opendir(get_home_root());
+        if (!d)
+                return log_full_errno(errno == ENOENT ? LOG_DEBUG : LOG_ERR, errno,
+                                      "Failed to open %s: %m", get_home_root());
+
+        FOREACH_DIRENT(de, d, return log_error_errno(errno, "Failed to read %s directory: %m", get_home_root()))
+                (void) manager_assess_image(m, dirfd(d), get_home_root(), de->d_name);
+
+        return 0;
+}
+
+static int manager_connect_bus(Manager *m) {
+        _cleanup_free_ char *b = NULL;
+        const char *suffix, *busname;
+        int r;
+
+        assert(m);
+        assert(!m->bus);
+
+        r = sd_bus_default_system(&m->bus);
+        if (r < 0)
+                return log_error_errno(r, "Failed to connect to system bus: %m");
+
+        r = bus_add_implementation(m->bus, &manager_object, m);
+        if (r < 0)
+                return r;
+
+        r = bus_log_control_api_register(m->bus);
+        if (r < 0)
+                return r;
+
+        suffix = getenv("SYSTEMD_HOME_DEBUG_SUFFIX");
+        if (suffix) {
+                b = strjoin("org.freedesktop.home1.", suffix);
+                if (!b)
+                        return log_oom();
+                busname = b;
+        } else
+                busname = "org.freedesktop.home1";
+
+        r = sd_bus_request_name_async(m->bus, NULL, busname, 0, NULL, NULL);
+        if (r < 0)
+                return log_error_errno(r, "Failed to request name: %m");
+
+        r = sd_bus_attach_event(m->bus, m->event, 0);
+        if (r < 0)
+                return log_error_errno(r, "Failed to attach bus to event loop: %m");
+
+        (void) sd_bus_set_exit_on_disconnect(m->bus, true);
+
+        return 0;
+}
+
+static int manager_bind_varlink(Manager *m) {
+        _cleanup_free_ char *p = NULL;
+        const char *suffix, *socket_path;
+        int r;
+
+        assert(m);
+        assert(!m->varlink_server);
+
+        r = varlink_server_new(&m->varlink_server, VARLINK_SERVER_ACCOUNT_UID|VARLINK_SERVER_INHERIT_USERDATA);
+        if (r < 0)
+                return log_error_errno(r, "Failed to allocate varlink server object: %m");
+
+        varlink_server_set_userdata(m->varlink_server, m);
+
+        r = varlink_server_bind_method_many(
+                        m->varlink_server,
+                        "io.systemd.UserDatabase.GetUserRecord",  vl_method_get_user_record,
+                        "io.systemd.UserDatabase.GetGroupRecord", vl_method_get_group_record,
+                        "io.systemd.UserDatabase.GetMemberships", vl_method_get_memberships);
+        if (r < 0)
+                return log_error_errno(r, "Failed to register varlink methods: %m");
+
+        (void) mkdir_p("/run/systemd/userdb", 0755);
+
+        /* To make things easier to debug, when working from a homed managed home directory, let's optionally
+         * use a different varlink socket name */
+        suffix = getenv("SYSTEMD_HOME_DEBUG_SUFFIX");
+        if (suffix) {
+                p = strjoin("/run/systemd/userdb/io.systemd.Home.", suffix);
+                if (!p)
+                        return log_oom();
+                socket_path = p;
+        } else
+                socket_path = "/run/systemd/userdb/io.systemd.Home";
+
+        r = varlink_server_listen_address(m->varlink_server, socket_path, 0666);
+        if (r < 0)
+                return log_error_errno(r, "Failed to bind to varlink socket: %m");
+
+        r = varlink_server_attach_event(m->varlink_server, m->event, SD_EVENT_PRIORITY_NORMAL);
+        if (r < 0)
+                return log_error_errno(r, "Failed to attach varlink connection to event loop: %m");
+
+        assert(!m->userdb_service);
+        m->userdb_service = strdup(basename(socket_path));
+        if (!m->userdb_service)
+                return log_oom();
+
+        /* Avoid recursion */
+        if (setenv("SYSTEMD_BYPASS_USERDB", m->userdb_service, 1) < 0)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Failed to set $SYSTEMD_BYPASS_USERDB: %m");
+
+        return 0;
+}
+
+static ssize_t read_datagram(
+                int fd,
+                struct ucred *ret_sender,
+                void **ret,
+                int *ret_passed_fd) {
+
+        CMSG_BUFFER_TYPE(CMSG_SPACE(sizeof(struct ucred)) + CMSG_SPACE(sizeof(int))) control;
+        _cleanup_free_ void *buffer = NULL;
+        _cleanup_close_ int passed_fd = -1;
+        struct ucred *sender = NULL;
+        struct cmsghdr *cmsg;
+        struct msghdr mh;
+        struct iovec iov;
+        ssize_t n, m;
+
+        assert(fd >= 0);
+        assert(ret_sender);
+        assert(ret);
+        assert(ret_passed_fd);
+
+        n = next_datagram_size_fd(fd);
+        if (n < 0)
+                return n;
+
+        buffer = malloc(n + 2);
+        if (!buffer)
+                return -ENOMEM;
+
+        /* Pass one extra byte, as a size check */
+        iov = IOVEC_MAKE(buffer, n + 1);
+
+        mh = (struct msghdr) {
+                .msg_iov = &iov,
+                .msg_iovlen = 1,
+                .msg_control = &control,
+                .msg_controllen = sizeof(control),
+        };
+
+        m = recvmsg_safe(fd, &mh, MSG_DONTWAIT|MSG_CMSG_CLOEXEC);
+        if (m < 0)
+                return m;
+
+        /* Ensure the size matches what we determined before */
+        if (m != n) {
+                cmsg_close_all(&mh);
+                return -EMSGSIZE;
+        }
+
+        CMSG_FOREACH(cmsg, &mh) {
+                if (cmsg->cmsg_level == SOL_SOCKET &&
+                    cmsg->cmsg_type == SCM_CREDENTIALS &&
+                    cmsg->cmsg_len == CMSG_LEN(sizeof(struct ucred))) {
+                        assert(!sender);
+                        sender = (struct ucred*) CMSG_DATA(cmsg);
+                }
+
+                if (cmsg->cmsg_level == SOL_SOCKET &&
+                    cmsg->cmsg_type == SCM_RIGHTS) {
+
+                        if (cmsg->cmsg_len != CMSG_LEN(sizeof(int))) {
+                                cmsg_close_all(&mh);
+                                return -EMSGSIZE;
+                        }
+
+                        assert(passed_fd < 0);
+                        passed_fd = *(int*) CMSG_DATA(cmsg);
+                }
+        }
+
+        if (sender)
+                *ret_sender = *sender;
+        else
+                *ret_sender = (struct ucred) UCRED_INVALID;
+
+        *ret_passed_fd = TAKE_FD(passed_fd);
+
+        /* For safety reasons: let's always NUL terminate.  */
+        ((char*) buffer)[n] = 0;
+        *ret = TAKE_PTR(buffer);
+
+        return 0;
+}
+
+static int on_notify_socket(sd_event_source *s, int fd, uint32_t revents, void *userdata) {
+        _cleanup_strv_free_ char **l = NULL;
+        _cleanup_free_ void *datagram = NULL;
+        _cleanup_close_ int passed_fd = -1;
+        struct ucred sender = UCRED_INVALID;
+        Manager *m = ASSERT_PTR(userdata);
+        ssize_t n;
+        Home *h;
+
+        assert(s);
+
+        n = read_datagram(fd, &sender, &datagram, &passed_fd);
+        if (n < 0) {
+                if (ERRNO_IS_TRANSIENT(n))
+                        return 0;
+                return log_error_errno(n, "Failed to read notify datagram: %m");
+        }
+
+        if (sender.pid <= 0) {
+                log_warning("Received notify datagram without valid sender PID, ignoring.");
+                return 0;
+        }
+
+        h = hashmap_get(m->homes_by_worker_pid, PID_TO_PTR(sender.pid));
+        if (!h) {
+                log_warning("Received notify datagram of unknown process, ignoring.");
+                return 0;
+        }
+
+        l = strv_split(datagram, "\n");
+        if (!l)
+                return log_oom();
+
+        home_process_notify(h, l, TAKE_FD(passed_fd));
+        return 0;
+}
+
+static int manager_listen_notify(Manager *m) {
+        _cleanup_close_ int fd = -1;
+        union sockaddr_union sa = {
+                .un.sun_family = AF_UNIX,
+                .un.sun_path = "/run/systemd/home/notify",
+        };
+        const char *suffix;
+        int r;
+
+        assert(m);
+        assert(!m->notify_socket_event_source);
+
+        suffix = getenv("SYSTEMD_HOME_DEBUG_SUFFIX");
+        if (suffix) {
+                _cleanup_free_ char *unix_path = NULL;
+
+                unix_path = strjoin("/run/systemd/home/notify.", suffix);
+                if (!unix_path)
+                        return log_oom();
+                r = sockaddr_un_set_path(&sa.un, unix_path);
+                if (r < 0)
+                        return log_error_errno(r, "Socket path %s does not fit in sockaddr_un: %m", unix_path);
+        }
+
+        fd = socket(AF_UNIX, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, 0);
+        if (fd < 0)
+                return log_error_errno(errno, "Failed to create listening socket: %m");
+
+        (void) mkdir_parents(sa.un.sun_path, 0755);
+        (void) sockaddr_un_unlink(&sa.un);
+
+        if (bind(fd, &sa.sa, SOCKADDR_UN_LEN(sa.un)) < 0)
+                return log_error_errno(errno, "Failed to bind to socket: %m");
+
+        r = setsockopt_int(fd, SOL_SOCKET, SO_PASSCRED, true);
+        if (r < 0)
+                return r;
+
+        r = sd_event_add_io(m->event, &m->notify_socket_event_source, fd, EPOLLIN, on_notify_socket, m);
+        if (r < 0)
+                return log_error_errno(r, "Failed to allocate event source for notify socket: %m");
+
+        (void) sd_event_source_set_description(m->notify_socket_event_source, "notify-socket");
+
+        /* Make sure we process sd_notify() before SIGCHLD for any worker, so that we always know the error
+         * number of a client before it exits. */
+        r = sd_event_source_set_priority(m->notify_socket_event_source, SD_EVENT_PRIORITY_NORMAL - 5);
+        if (r < 0)
+                return log_error_errno(r, "Failed to alter priority of NOTIFY_SOCKET event source: %m");
+
+        r = sd_event_source_set_io_fd_own(m->notify_socket_event_source, true);
+        if (r < 0)
+                return log_error_errno(r, "Failed to pass ownership of notify socket: %m");
+
+        return TAKE_FD(fd);
+}
+
+static int manager_add_device(Manager *m, sd_device *d) {
+        _cleanup_free_ char *user_name = NULL, *realm = NULL, *node = NULL;
+        const char *tabletype, *parttype, *partname, *partuuid, *sysfs;
+        sd_id128_t id;
+        int r;
+
+        assert(m);
+        assert(d);
+
+        r = sd_device_get_syspath(d, &sysfs);
+        if (r < 0)
+                return log_error_errno(r, "Failed to acquire sysfs path of device: %m");
+
+        r = sd_device_get_property_value(d, "ID_PART_TABLE_TYPE", &tabletype);
+        if (r == -ENOENT)
+                return 0;
+        if (r < 0)
+                return log_error_errno(r, "Failed to acquire ID_PART_TABLE_TYPE device property, ignoring: %m");
+
+        if (!streq(tabletype, "gpt")) {
+                log_debug("Found partition (%s) on non-GPT table, ignoring.", sysfs);
+                return 0;
+        }
+
+        r = sd_device_get_property_value(d, "ID_PART_ENTRY_TYPE", &parttype);
+        if (r == -ENOENT)
+                return 0;
+        if (r < 0)
+                return log_error_errno(r, "Failed to acquire ID_PART_ENTRY_TYPE device property, ignoring: %m");
+        if (sd_id128_string_equal(parttype, SD_GPT_USER_HOME) <= 0) {
+                log_debug("Found partition (%s) we don't care about, ignoring.", sysfs);
+                return 0;
+        }
+
+        r = sd_device_get_property_value(d, "ID_PART_ENTRY_NAME", &partname);
+        if (r < 0)
+                return log_warning_errno(r, "Failed to acquire ID_PART_ENTRY_NAME device property, ignoring: %m");
+
+        r = split_user_name_realm(partname, &user_name, &realm);
+        if (r == -EINVAL)
+                return log_warning_errno(r, "Found partition with correct partition type but a non-parsable partition name '%s', ignoring.", partname);
+        if (r < 0)
+                return log_error_errno(r, "Failed to validate partition name '%s': %m", partname);
+
+        r = sd_device_get_property_value(d, "ID_FS_UUID", &partuuid);
+        if (r < 0)
+                return log_warning_errno(r, "Failed to acquire ID_FS_UUID device property, ignoring: %m");
+
+        r = sd_id128_from_string(partuuid, &id);
+        if (r < 0)
+                return log_warning_errno(r, "Failed to parse ID_FS_UUID field '%s', ignoring: %m", partuuid);
+
+        if (asprintf(&node, "/dev/disk/by-uuid/" SD_ID128_UUID_FORMAT_STR, SD_ID128_FORMAT_VAL(id)) < 0)
+                return log_oom();
+
+        return manager_add_home_by_image(m, user_name, realm, node, sysfs, USER_LUKS, UID_INVALID);
+}
+
+static int manager_on_device(sd_device_monitor *monitor, sd_device *d, void *userdata) {
+        Manager *m = ASSERT_PTR(userdata);
+        int r;
+
+        assert(d);
+
+        if (device_for_action(d, SD_DEVICE_REMOVE)) {
+                const char *sysfs;
+                Home *h;
+
+                r = sd_device_get_syspath(d, &sysfs);
+                if (r < 0) {
+                        log_warning_errno(r, "Failed to acquire sysfs path from device: %m");
+                        return 0;
+                }
+
+                log_info("block device %s has been removed.", sysfs);
+
+                /* Let's see if we previously synthesized a home record from this device, if so, let's just
+                 * revalidate that. Otherwise let's revalidate them all, but asynchronously. */
+                h = hashmap_get(m->homes_by_sysfs, sysfs);
+                if (h)
+                        manager_revalidate_image(m, h);
+                else
+                        manager_enqueue_gc(m, NULL);
+        } else
+                (void) manager_add_device(m, d);
+
+        (void) bus_manager_emit_auto_login_changed(m);
+        return 0;
+}
+
+static int manager_watch_devices(Manager *m) {
+        int r;
+
+        assert(m);
+        assert(!m->device_monitor);
+
+        r = sd_device_monitor_new(&m->device_monitor);
+        if (r < 0)
+                return log_error_errno(r, "Failed to allocate device monitor: %m");
+
+        r = sd_device_monitor_filter_add_match_subsystem_devtype(m->device_monitor, "block", NULL);
+        if (r < 0)
+                return log_error_errno(r, "Failed to configure device monitor match: %m");
+
+        r = sd_device_monitor_attach_event(m->device_monitor, m->event);
+        if (r < 0)
+                return log_error_errno(r, "Failed to attach device monitor to event loop: %m");
+
+        r = sd_device_monitor_start(m->device_monitor, manager_on_device, m);
+        if (r < 0)
+                return log_error_errno(r, "Failed to start device monitor: %m");
+
+        return 0;
+}
+
+static int manager_enumerate_devices(Manager *m) {
+        _cleanup_(sd_device_enumerator_unrefp) sd_device_enumerator *e = NULL;
+        sd_device *d;
+        int r;
+
+        assert(m);
+
+        r = sd_device_enumerator_new(&e);
+        if (r < 0)
+                return r;
+
+        r = sd_device_enumerator_add_match_subsystem(e, "block", true);
+        if (r < 0)
+                return r;
+
+        FOREACH_DEVICE(e, d)
+                (void) manager_add_device(m, d);
+
+        return 0;
+}
+
+static int manager_load_key_pair(Manager *m) {
+        _cleanup_(fclosep) FILE *f = NULL;
+        struct stat st;
+        int r;
+
+        assert(m);
+
+        if (m->private_key) {
+                EVP_PKEY_free(m->private_key);
+                m->private_key = NULL;
+        }
+
+        r = search_and_fopen_nulstr("local.private", "re", NULL, KEY_PATHS_NULSTR, &f, NULL);
+        if (r == -ENOENT)
+                return 0;
+        if (r < 0)
+                return log_error_errno(r, "Failed to read private key file: %m");
+
+        if (fstat(fileno(f), &st) < 0)
+                return log_error_errno(errno, "Failed to stat private key file: %m");
+
+        r = stat_verify_regular(&st);
+        if (r < 0)
+                return log_error_errno(r, "Private key file is not regular: %m");
+
+        if (st.st_uid != 0 || (st.st_mode & 0077) != 0)
+                return log_error_errno(SYNTHETIC_ERRNO(EPERM), "Private key file is readable by more than the root user");
+
+        m->private_key = PEM_read_PrivateKey(f, NULL, NULL, NULL);
+        if (!m->private_key)
+                return log_error_errno(SYNTHETIC_ERRNO(EIO), "Failed to load private key pair");
+
+        log_info("Successfully loaded private key pair.");
+
+        return 1;
+}
+
+DEFINE_TRIVIAL_CLEANUP_FUNC_FULL(EVP_PKEY_CTX*, EVP_PKEY_CTX_free, NULL);
+
+static int manager_generate_key_pair(Manager *m) {
+        _cleanup_(EVP_PKEY_CTX_freep) EVP_PKEY_CTX *ctx = NULL;
+        _cleanup_(unlink_and_freep) char *temp_public = NULL, *temp_private = NULL;
+        _cleanup_fclose_ FILE *fpublic = NULL, *fprivate = NULL;
+        int r;
+
+        if (m->private_key) {
+                EVP_PKEY_free(m->private_key);
+                m->private_key = NULL;
+        }
+
+        ctx = EVP_PKEY_CTX_new_id(EVP_PKEY_ED25519, NULL);
+        if (!ctx)
+                return log_error_errno(SYNTHETIC_ERRNO(EIO), "Failed to allocate Ed25519 key generation context.");
+
+        if (EVP_PKEY_keygen_init(ctx) <= 0)
+                return log_error_errno(SYNTHETIC_ERRNO(EIO), "Failed to initialize Ed25519 key generation context.");
+
+        log_info("Generating key pair for signing local user identity records.");
+
+        if (EVP_PKEY_keygen(ctx, &m->private_key) <= 0)
+                return log_error_errno(SYNTHETIC_ERRNO(EIO), "Failed to generate Ed25519 key pair");
+
+        log_info("Successfully created Ed25519 key pair.");
+
+        (void) mkdir_p("/var/lib/systemd/home", 0755);
+
+        /* Write out public key (note that we only do that as a help to the user, we don't make use of this ever */
+        r = fopen_temporary("/var/lib/systemd/home/local.public", &fpublic, &temp_public);
+        if (r < 0)
+                return log_error_errno(errno, "Failed to open key file for writing: %m");
+
+        if (PEM_write_PUBKEY(fpublic, m->private_key) <= 0)
+                return log_error_errno(SYNTHETIC_ERRNO(EIO), "Failed to write public key.");
+
+        r = fflush_sync_and_check(fpublic);
+        if (r < 0)
+                return log_error_errno(r, "Failed to write private key: %m");
+
+        fpublic = safe_fclose(fpublic);
+
+        /* Write out the private key (this actually writes out both private and public, OpenSSL is confusing) */
+        r = fopen_temporary("/var/lib/systemd/home/local.private", &fprivate, &temp_private);
+        if (r < 0)
+                return log_error_errno(errno, "Failed to open key file for writing: %m");
+
+        if (PEM_write_PrivateKey(fprivate, m->private_key, NULL, NULL, 0, NULL, 0) <= 0)
+                return log_error_errno(SYNTHETIC_ERRNO(EIO), "Failed to write private key pair.");
+
+        r = fflush_sync_and_check(fprivate);
+        if (r < 0)
+                return log_error_errno(r, "Failed to write private key: %m");
+
+        fprivate = safe_fclose(fprivate);
+
+        /* Both are written now, move them into place */
+
+        if (rename(temp_public, "/var/lib/systemd/home/local.public") < 0)
+                return log_error_errno(errno, "Failed to move public key file into place: %m");
+        temp_public = mfree(temp_public);
+
+        if (rename(temp_private, "/var/lib/systemd/home/local.private") < 0) {
+                (void) unlink_noerrno("/var/lib/systemd/home/local.public"); /* try to remove the file we already created */
+                return log_error_errno(errno, "Failed to move private key file into place: %m");
+        }
+        temp_private = mfree(temp_private);
+
+        r = fsync_path_at(AT_FDCWD, "/var/lib/systemd/home/");
+        if (r < 0)
+                log_warning_errno(r, "Failed to sync /var/lib/systemd/home/, ignoring: %m");
+
+        return 1;
+}
+
+int manager_acquire_key_pair(Manager *m) {
+        int r;
+
+        assert(m);
+
+        /* Already there? */
+        if (m->private_key)
+                return 1;
+
+        /* First try to load key off disk */
+        r = manager_load_key_pair(m);
+        if (r != 0)
+                return r;
+
+        /* Didn't work, generate a new one */
+        return manager_generate_key_pair(m);
+}
+
+int manager_sign_user_record(Manager *m, UserRecord *u, UserRecord **ret, sd_bus_error *error) {
+        int r;
+
+        assert(m);
+        assert(u);
+        assert(ret);
+
+        r = manager_acquire_key_pair(m);
+        if (r < 0)
+                return r;
+        if (r == 0)
+                return sd_bus_error_set(error, BUS_ERROR_NO_PRIVATE_KEY, "Can't sign without local key.");
+
+        return user_record_sign(u, m->private_key, ret);
+}
+
+DEFINE_PRIVATE_HASH_OPS_FULL(public_key_hash_ops, char, string_hash_func, string_compare_func, free, EVP_PKEY, EVP_PKEY_free);
+DEFINE_TRIVIAL_CLEANUP_FUNC_FULL(EVP_PKEY*, EVP_PKEY_free, NULL);
+
+static int manager_load_public_key_one(Manager *m, const char *path) {
+        _cleanup_(EVP_PKEY_freep) EVP_PKEY *pkey = NULL;
+        _cleanup_fclose_ FILE *f = NULL;
+        _cleanup_free_ char *fn = NULL;
+        struct stat st;
+        int r;
+
+        assert(m);
+
+        if (streq(basename(path), "local.public")) /* we already loaded the private key, which includes the public one */
+                return 0;
+
+        f = fopen(path, "re");
+        if (!f) {
+                if (errno == ENOENT)
+                        return 0;
+
+                return log_error_errno(errno, "Failed to open public key %s: %m", path);
+        }
+
+        if (fstat(fileno(f), &st) < 0)
+                return log_error_errno(errno, "Failed to stat public key %s: %m", path);
+
+        r = stat_verify_regular(&st);
+        if (r < 0)
+                return log_error_errno(r, "Public key file %s is not a regular file: %m", path);
+
+        if (st.st_uid != 0 || (st.st_mode & 0022) != 0)
+                return log_error_errno(SYNTHETIC_ERRNO(EPERM), "Public key file %s is writable by more than the root user, refusing.", path);
+
+        r = hashmap_ensure_allocated(&m->public_keys, &public_key_hash_ops);
+        if (r < 0)
+                return log_oom();
+
+        pkey = PEM_read_PUBKEY(f, &pkey, NULL, NULL);
+        if (!pkey)
+                return log_error_errno(SYNTHETIC_ERRNO(EIO), "Failed to parse public key file %s.", path);
+
+        fn = strdup(basename(path));
+        if (!fn)
+                return log_oom();
+
+        r = hashmap_put(m->public_keys, fn, pkey);
+        if (r < 0)
+                return log_error_errno(r, "Failed to add public key to set: %m");
+
+        TAKE_PTR(fn);
+        TAKE_PTR(pkey);
+
+        return 0;
+}
+
+static int manager_load_public_keys(Manager *m) {
+        _cleanup_strv_free_ char **files = NULL;
+        int r;
+
+        assert(m);
+
+        m->public_keys = hashmap_free(m->public_keys);
+
+        r = conf_files_list_nulstr(
+                        &files,
+                        ".public",
+                        NULL,
+                        CONF_FILES_REGULAR|CONF_FILES_FILTER_MASKED,
+                        KEY_PATHS_NULSTR);
+        if (r < 0)
+                return log_error_errno(r, "Failed to assemble list of public key directories: %m");
+
+        STRV_FOREACH(i, files)
+                (void) manager_load_public_key_one(m, *i);
+
+        return 0;
+}
+
+int manager_startup(Manager *m) {
+        int r;
+
+        assert(m);
+
+        r = manager_listen_notify(m);
+        if (r < 0)
+                return r;
+
+        r = manager_connect_bus(m);
+        if (r < 0)
+                return r;
+
+        r = manager_bind_varlink(m);
+        if (r < 0)
+                return r;
+
+        r = manager_load_key_pair(m); /* only try to load it, don't generate any */
+        if (r < 0)
+                return r;
+
+        r = manager_load_public_keys(m);
+        if (r < 0)
+                return r;
+
+        manager_watch_home(m);
+        (void) manager_watch_devices(m);
+
+        (void) manager_enumerate_records(m);
+        (void) manager_enumerate_images(m);
+        (void) manager_enumerate_devices(m);
+
+        /* Let's clean up home directories whose devices got removed while we were not running */
+        (void) manager_enqueue_gc(m, NULL);
+
+        return 0;
+}
+
+void manager_revalidate_image(Manager *m, Home *h) {
+        int r;
+
+        assert(m);
+        assert(h);
+
+        /* Frees an automatically discovered image, if it's synthetic and its image disappeared. Unmounts any
+         * image if it's mounted but its image vanished. */
+
+        if (h->current_operation || !ordered_set_isempty(h->pending_operations))
+                return;
+
+        if (h->state == HOME_UNFIXATED) {
+                r = user_record_test_image_path(h->record);
+                if (r < 0)
+                        log_warning_errno(r, "Can't determine if image of %s exists, freeing unfixated user: %m", h->user_name);
+                else if (r == USER_TEST_ABSENT)
+                        log_info("Image for %s disappeared, freeing unfixated user.", h->user_name);
+                else
+                        return;
+
+                home_free(h);
+
+        } else if (h->state < 0) {
+
+                r = user_record_test_home_directory(h->record);
+                if (r < 0) {
+                        log_warning_errno(r, "Unable to determine state of home directory, ignoring: %m");
+                        return;
+                }
+
+                if (r == USER_TEST_MOUNTED) {
+                        r = user_record_test_image_path(h->record);
+                        if (r < 0) {
+                                log_warning_errno(r, "Unable to determine state of image path, ignoring: %m");
+                                return;
+                        }
+
+                        if (r == USER_TEST_ABSENT) {
+                                _cleanup_(operation_unrefp) Operation *o = NULL;
+
+                                log_notice("Backing image disappeared while home directory %s was mounted, unmounting it forcibly.", h->user_name);
+                                /* Wowza, the thing is mounted, but the device is gone? Act on it. */
+
+                                r = home_killall(h);
+                                if (r < 0)
+                                        log_warning_errno(r, "Failed to kill processes of user %s, ignoring: %m", h->user_name);
+
+                                /* We enqueue the operation here, after all the home directory might
+                                 * currently already run some operation, and we can deactivate it only after
+                                 * that's complete. */
+                                o = operation_new(OPERATION_DEACTIVATE_FORCE, NULL);
+                                if (!o) {
+                                        log_oom();
+                                        return;
+                                }
+
+                                r = home_schedule_operation(h, o, NULL);
+                                if (r < 0)
+                                        log_warning_errno(r, "Failed to enqueue forced home directory %s deactivation, ignoring: %m", h->user_name);
+                        }
+                }
+        }
+}
+
+int manager_gc_images(Manager *m) {
+        Home *h;
+
+        assert_se(m);
+
+        if (m->gc_focus) {
+                /* Focus on a specific home */
+
+                h = TAKE_PTR(m->gc_focus);
+                manager_revalidate_image(m, h);
+        } else {
+                /* Gc all */
+
+                HASHMAP_FOREACH(h, m->homes_by_name)
+                        manager_revalidate_image(m, h);
+        }
+
+        return 0;
+}
+
+static int on_deferred_rescan(sd_event_source *s, void *userdata) {
+        Manager *m = ASSERT_PTR(userdata);
+
+        m->deferred_rescan_event_source = sd_event_source_disable_unref(m->deferred_rescan_event_source);
+
+        manager_enumerate_devices(m);
+        manager_enumerate_images(m);
+        return 0;
+}
+
+int manager_enqueue_rescan(Manager *m) {
+        int r;
+
+        assert(m);
+
+        if (m->deferred_rescan_event_source)
+                return 0;
+
+        if (!m->event)
+                return 0;
+
+        if (IN_SET(sd_event_get_state(m->event), SD_EVENT_FINISHED, SD_EVENT_EXITING))
+                return 0;
+
+        r = sd_event_add_defer(m->event, &m->deferred_rescan_event_source, on_deferred_rescan, m);
+        if (r < 0)
+                return log_error_errno(r, "Failed to allocate rescan event source: %m");
+
+        r = sd_event_source_set_priority(m->deferred_rescan_event_source, SD_EVENT_PRIORITY_IDLE+1);
+        if (r < 0)
+                log_warning_errno(r, "Failed to tweak priority of event source, ignoring: %m");
+
+        (void) sd_event_source_set_description(m->deferred_rescan_event_source, "deferred-rescan");
+        return 1;
+}
+
+static int on_deferred_gc(sd_event_source *s, void *userdata) {
+        Manager *m = ASSERT_PTR(userdata);
+
+        m->deferred_gc_event_source = sd_event_source_disable_unref(m->deferred_gc_event_source);
+
+        manager_gc_images(m);
+        return 0;
+}
+
+int manager_enqueue_gc(Manager *m, Home *focus) {
+        int r;
+
+        assert(m);
+
+        /* This enqueues a request to GC dead homes. It may be called with focus=NULL in which case all homes
+         * will be scanned, or with the parameter set, in which case only that home is checked. */
+
+        if (!m->event)
+                return 0;
+
+        if (IN_SET(sd_event_get_state(m->event), SD_EVENT_FINISHED, SD_EVENT_EXITING))
+                return 0;
+
+        /* If a focus home is specified, then remember to focus just on this home. Otherwise invalidate any
+         * focus that might be set to look at all homes. */
+
+        if (m->deferred_gc_event_source) {
+                if (m->gc_focus != focus) /* not the same focus, then look at everything */
+                        m->gc_focus = NULL;
+
+                return 0;
+        } else
+                m->gc_focus = focus; /* start focused */
+
+        r = sd_event_add_defer(m->event, &m->deferred_gc_event_source, on_deferred_gc, m);
+        if (r < 0)
+                return log_error_errno(r, "Failed to allocate GC event source: %m");
+
+        r = sd_event_source_set_priority(m->deferred_gc_event_source, SD_EVENT_PRIORITY_IDLE);
+        if (r < 0)
+                log_warning_errno(r, "Failed to tweak priority of event source, ignoring: %m");
+
+        (void) sd_event_source_set_description(m->deferred_gc_event_source, "deferred-gc");
+        return 1;
+}
+
+static bool manager_shall_rebalance(Manager *m) {
+        Home *h;
+
+        assert(m);
+
+        if (IN_SET(m->rebalance_state, REBALANCE_PENDING, REBALANCE_SHRINKING, REBALANCE_GROWING))
+                return true;
+
+        HASHMAP_FOREACH(h, m->homes_by_name)
+                if (home_shall_rebalance(h))
+                        return true;
+
+        return false;
+}
+
+static int home_cmp(Home *const*a, Home *const*b) {
+        int r;
+
+        assert(a);
+        assert(*a);
+        assert(b);
+        assert(*b);
+
+        /* Order user records by their weight (and by their name, to make things stable). We put the records
+         * with the highest weight last, since we distribute space from the beginning and round down, hence
+         * later entries tend to get slightly more than earlier entries. */
+
+        r = CMP(user_record_rebalance_weight((*a)->record), user_record_rebalance_weight((*b)->record));
+        if (r != 0)
+                return r;
+
+        return strcmp((*a)->user_name, (*b)->user_name);
+}
+
+static int manager_rebalance_calculate(Manager *m) {
+        uint64_t weight_sum, free_sum, usage_sum = 0, min_free = UINT64_MAX;
+        _cleanup_free_ Home **array = NULL;
+        bool relevant = false;
+        struct statfs sfs;
+        int c = 0, r;
+        Home *h;
+
+        assert(m);
+
+        if (statfs(get_home_root(), &sfs) < 0)
+                return log_error_errno(errno, "Failed to statfs() /home: %m");
+
+        free_sum = (uint64_t) sfs.f_bsize * sfs.f_bavail; /* This much free space is available on the
+                                                           * underlying pool directory */
+
+        weight_sum = REBALANCE_WEIGHT_BACKING; /* Grant the underlying pool directory a fixed weight of 20
+                                                * (home dirs get 100 by default, i.e. 5x more). This weight
+                                                * is not configurable, the per-home weights are. */
+
+        HASHMAP_FOREACH(h, m->homes_by_name) {
+                statfs_f_type_t fstype;
+                h->rebalance_pending = false; /* First, reset the flag, we only want it to be true for the
+                                               * homes that qualify for rebalancing */
+
+                if (!home_shall_rebalance(h)) /* Only look at actual candidates */
+                        continue;
+
+                if (home_is_busy(h))
+                        return -EBUSY; /* Let's not rebalance if there's a busy home directory. */
+
+                r = home_get_disk_status(
+                                h,
+                                &h->rebalance_size,
+                                &h->rebalance_usage,
+                                &h->rebalance_free,
+                                NULL,
+                                NULL,
+                                &fstype,
+                                NULL);
+                if (r < 0) {
+                        log_warning_errno(r, "Failed to get free space of home '%s', ignoring.", h->user_name);
+                        continue;
+                }
+
+                if (h->rebalance_free > UINT64_MAX - free_sum)
+                        return log_error_errno(SYNTHETIC_ERRNO(EOVERFLOW), "Rebalance free overflow");
+                free_sum += h->rebalance_free;
+
+                if (h->rebalance_usage > UINT64_MAX - usage_sum)
+                        return log_error_errno(SYNTHETIC_ERRNO(EOVERFLOW), "Rebalance usage overflow");
+                usage_sum += h->rebalance_usage;
+
+                h->rebalance_weight = user_record_rebalance_weight(h->record);
+                if (h->rebalance_weight > UINT64_MAX - weight_sum)
+                        return log_error_errno(SYNTHETIC_ERRNO(EOVERFLOW), "Rebalance weight overflow");
+                weight_sum += h->rebalance_weight;
+
+                h->rebalance_min = minimal_size_by_fs_magic(fstype);
+
+                if (!GREEDY_REALLOC(array, c+1))
+                        return log_oom();
+
+                array[c++] = h;
+        }
+
+        if (c == 0) {
+                log_debug("No homes to rebalance.");
+                return 0;
+        }
+
+        assert(weight_sum > 0);
+
+        log_debug("Disk space usage by all home directories to rebalance: %s — available disk space: %s",
+                  FORMAT_BYTES(usage_sum), FORMAT_BYTES(free_sum));
+
+        /* Bring the home directories in a well-defined order, so that we distribute space in a reproducible
+         * way for the same parameters. */
+        typesafe_qsort(array, c, home_cmp);
+
+        for (int i = 0; i < c; i++) {
+                uint64_t new_free;
+                double d;
+
+                h = array[i];
+
+                assert(h->rebalance_free <= free_sum);
+                assert(h->rebalance_usage <= usage_sum);
+                assert(h->rebalance_weight <= weight_sum);
+
+                d = ((double) (free_sum / 4096) * (double) h->rebalance_weight) / (double) weight_sum; /* Calculate new space for this home in units of 4K */
+
+                /* Convert from units of 4K back to bytes */
+                if (d >= (double) (UINT64_MAX/4096))
+                        new_free = UINT64_MAX;
+                else
+                        new_free = (uint64_t) d * 4096;
+
+                /* Subtract the weight and assigned space from the sums now, to distribute the rounding noise
+                 * to the remaining home dirs */
+                free_sum = LESS_BY(free_sum, new_free);
+                weight_sum = LESS_BY(weight_sum, h->rebalance_weight);
+
+                /* Keep track of home directory with the least amount of space left: we want to schedule the
+                 * next rebalance more quickly if this is low */
+                if (new_free < min_free)
+                        min_free = h->rebalance_size;
+
+                if (new_free > UINT64_MAX - h->rebalance_usage)
+                        h->rebalance_goal = UINT64_MAX-1; /* maximum size */
+                else {
+                        h->rebalance_goal = h->rebalance_usage + new_free;
+
+                        if (h->rebalance_min != UINT64_MAX && h->rebalance_goal < h->rebalance_min)
+                                h->rebalance_goal = h->rebalance_min;
+                }
+
+                /* Skip over this home if the state doesn't match the operation */
+                if ((m->rebalance_state == REBALANCE_SHRINKING && h->rebalance_goal > h->rebalance_size) ||
+                    (m->rebalance_state == REBALANCE_GROWING && h->rebalance_goal < h->rebalance_size))
+                        h->rebalance_pending = false;
+                else {
+                        log_debug("Rebalancing home directory '%s' %s %s %s.", h->user_name,
+                                  FORMAT_BYTES(h->rebalance_size),
+                                  special_glyph(SPECIAL_GLYPH_ARROW_RIGHT),
+                                  FORMAT_BYTES(h->rebalance_goal));
+                        h->rebalance_pending = true;
+                }
+
+                if ((fabs((double) h->rebalance_size - (double) h->rebalance_goal) * 100 / (double) h->rebalance_size) >= 5.0)
+                        relevant = true;
+        }
+
+        /* Scale next rebalancing interval based on the least amount of space of any of the home
+         * directories. We pick a time in the range 1min … 15min, scaled by log2(min_free), so that:
+         * 10M → ~0.7min, 100M → ~2.7min, 1G → ~4.6min, 10G → ~6.5min, 100G ~8.4 */
+        m->rebalance_interval_usec = (usec_t) CLAMP((LESS_BY(log2(min_free), 22)*15*USEC_PER_MINUTE)/26,
+                                                    1 * USEC_PER_MINUTE,
+                                                    15 * USEC_PER_MINUTE);
+
+
+        log_debug("Rebalancing interval set to %s.", FORMAT_TIMESPAN(m->rebalance_interval_usec, USEC_PER_MSEC));
+
+        /* Let's suppress small resizes, growing/shrinking file systems isn't free after all */
+        if (!relevant) {
+                log_debug("Skipping rebalancing, since all calculated size changes are below ±5%%.");
+                return 0;
+        }
+
+        return c;
+}
+
+static int manager_rebalance_apply(Manager *m) {
+        int c = 0, r;
+        Home *h;
+
+        assert(m);
+
+        HASHMAP_FOREACH(h, m->homes_by_name) {
+                _cleanup_(sd_bus_error_free) sd_bus_error error = SD_BUS_ERROR_NULL;
+
+                if (!h->rebalance_pending)
+                        continue;
+
+                h->rebalance_pending = false;
+
+                r = home_resize(h, h->rebalance_goal, /* secret= */ NULL, /* automatic= */ true, &error);
+                if (r < 0)
+                        log_warning_errno(r, "Failed to resize home '%s' for rebalancing, ignoring: %s",
+                                          h->user_name, bus_error_message(&error, r));
+                else
+                        c++;
+        }
+
+        return c;
+}
+
+static void manager_rebalance_reply_messages(Manager *m) {
+        int r;
+
+        assert(m);
+
+        for (;;) {
+                _cleanup_(sd_bus_message_unrefp) sd_bus_message *msg =
+                        set_steal_first(m->rebalance_pending_method_calls);
+
+                if (!msg)
+                        break;
+
+                r = sd_bus_reply_method_return(msg, NULL);
+                if (r < 0)
+                        log_debug_errno(r, "Failed to reply to rebalance method call, ignoring: %m");
+        }
+}
+
+static int manager_rebalance_now(Manager *m) {
+        RebalanceState busy_state; /* the state to revert to when operation fails if busy */
+        int r;
+
+        assert(m);
+
+        log_debug("Rebalancing now...");
+
+        /* We maintain a simple state engine here to keep track of what we are doing. We'll first shrink all
+         * homes that shall be shrunk and then grow all homes that shall be grown, so that they can take up
+         * the space now freed. */
+
+        for (;;) {
+                switch (m->rebalance_state) {
+
+                case REBALANCE_IDLE:
+                case REBALANCE_PENDING:
+                case REBALANCE_WAITING:
+                        /* First shrink large home dirs */
+                        m->rebalance_state = REBALANCE_SHRINKING;
+                        busy_state = REBALANCE_PENDING;
+
+                        /* We are initiating the next rebalancing cycle now, let's make the queued methods
+                         * calls the pending ones, and flush out any pending ones (which shouldn't exist at
+                         * this time anyway) */
+                        set_clear(m->rebalance_pending_method_calls);
+                        SWAP_TWO(m->rebalance_pending_method_calls, m->rebalance_queued_method_calls);
+
+                        log_debug("Shrinking phase..");
+                        break;
+
+                case REBALANCE_SHRINKING:
+                        /* Then grow small home dirs */
+                        m->rebalance_state = REBALANCE_GROWING;
+                        busy_state = REBALANCE_SHRINKING;
+                        log_debug("Growing phase..");
+                        break;
+
+                case REBALANCE_GROWING:
+                        /* Finally, we are done */
+                        log_info("Rebalancing complete.");
+                        m->rebalance_state = REBALANCE_IDLE;
+                        r = 0;
+                        goto finish;
+
+                case REBALANCE_OFF:
+                default:
+                        assert_not_reached();
+                }
+
+                r = manager_rebalance_calculate(m);
+                if (r == -EBUSY) {
+                        /* Calculations failed because one home directory is currently busy. Revert to a state that
+                         * tells us what to do next. */
+                        log_debug("Can't enter phase, busy.");
+                        m->rebalance_state = busy_state;
+                        return r;
+                }
+                if (r < 0)
+                        goto finish;
+                if (r == 0)
+                        continue; /* got to next step immediately, if there's nothing to do */
+
+                r = manager_rebalance_apply(m);
+                if (r < 0)
+                        goto finish;
+                if (r > 0)
+                        break; /* At least one resize operation is now pending, we are done for now */
+
+                /* If there was nothing to apply, go for next state right-away */
+        }
+
+        return 0;
+
+finish:
+        /* Reset state and schedule next rebalance */
+        m->rebalance_state = REBALANCE_IDLE;
+        manager_rebalance_reply_messages(m);
+        (void) manager_schedule_rebalance(m, /* immediately= */ false);
+        return r;
+}
+
+static int on_rebalance_timer(sd_event_source *s, usec_t t, void *userdata) {
+        Manager *m = ASSERT_PTR(userdata);
+
+        assert(s);
+        assert(IN_SET(m->rebalance_state, REBALANCE_WAITING, REBALANCE_PENDING, REBALANCE_SHRINKING, REBALANCE_GROWING));
+
+        (void) manager_rebalance_now(m);
+        return 0;
+}
+
+int manager_schedule_rebalance(Manager *m, bool immediately) {
+        int r;
+
+        assert(m);
+
+        /* Check if there are any records where rebalancing is requested */
+        if (!manager_shall_rebalance(m)) {
+                log_debug("Not scheduling rebalancing, not needed.");
+                r = 0; /* report that we didn't schedule anything because nothing needed it */
+                goto turn_off;
+        }
+
+        if (immediately) {
+                /* If we are told to rebalance immediately, then mark a rebalance as pending (even if we area
+                 * already running one) */
+
+                if (m->rebalance_event_source) {
+                        r = sd_event_source_set_time(m->rebalance_event_source, 0);
+                        if (r < 0) {
+                                log_error_errno(r, "Failed to schedule immediate rebalancing: %m");
+                                goto turn_off;
+                        }
+
+                        r = sd_event_source_set_enabled(m->rebalance_event_source, SD_EVENT_ONESHOT);
+                        if (r < 0) {
+                                log_error_errno(r, "Failed to enable rebalancing event source: %m");
+                                goto turn_off;
+                        }
+                } else {
+                        r = sd_event_add_time(m->event, &m->rebalance_event_source, CLOCK_MONOTONIC, 0, USEC_PER_SEC, on_rebalance_timer, m);
+                        if (r < 0) {
+                                log_error_errno(r, "Failed to allocate rebalance event source: %m");
+                                goto turn_off;
+                        }
+
+                        r = sd_event_source_set_priority(m->rebalance_event_source, SD_EVENT_PRIORITY_IDLE + 10);
+                        if (r < 0) {
+                                log_error_errno(r, "Failed to set rebalance event source priority: %m");
+                                goto turn_off;
+                        }
+
+                        (void) sd_event_source_set_description(m->rebalance_event_source, "rebalance");
+
+                }
+
+                if (!IN_SET(m->rebalance_state, REBALANCE_PENDING, REBALANCE_SHRINKING, REBALANCE_GROWING))
+                        m->rebalance_state = REBALANCE_PENDING;
+
+                log_debug("Scheduled immediate rebalancing...");
+                return 1; /* report that we scheduled something */
+        }
+
+        /* If we are told to schedule a rebalancing eventually, then do so only if we are not executing
+         * anything yet. Also if we have something scheduled already, leave it in place */
+        if (!IN_SET(m->rebalance_state, REBALANCE_OFF, REBALANCE_IDLE))
+                return 1; /* report that there's already something scheduled */
+
+        if (m->rebalance_event_source) {
+                r = sd_event_source_set_time_relative(m->rebalance_event_source, m->rebalance_interval_usec);
+                if (r < 0) {
+                        log_error_errno(r, "Failed to schedule immediate rebalancing: %m");
+                        goto turn_off;
+                }
+
+                r = sd_event_source_set_enabled(m->rebalance_event_source, SD_EVENT_ONESHOT);
+                if (r < 0) {
+                        log_error_errno(r, "Failed to enable rebalancing event source: %m");
+                        goto turn_off;
+                }
+        } else {
+                r = sd_event_add_time_relative(m->event, &m->rebalance_event_source, CLOCK_MONOTONIC, m->rebalance_interval_usec, USEC_PER_SEC, on_rebalance_timer, m);
+                if (r < 0) {
+                        log_error_errno(r, "Failed to allocate rebalance event source: %m");
+                        goto turn_off;
+                }
+
+                r = sd_event_source_set_priority(m->rebalance_event_source, SD_EVENT_PRIORITY_IDLE + 10);
+                if (r < 0) {
+                        log_error_errno(r, "Failed to set rebalance event source priority: %m");
+                        goto turn_off;
+                }
+
+                (void) sd_event_source_set_description(m->rebalance_event_source, "rebalance");
+        }
+
+        m->rebalance_state = REBALANCE_WAITING; /* We managed to enqueue a timer event, we now wait until it fires */
+        log_debug("Scheduled rebalancing in %s...", FORMAT_TIMESPAN(m->rebalance_interval_usec, 0));
+        return 1; /* report that we scheduled something */
+
+turn_off:
+        m->rebalance_event_source = sd_event_source_disable_unref(m->rebalance_event_source);
+        m->rebalance_state = REBALANCE_OFF;
+        manager_rebalance_reply_messages(m);
+        return r;
+}
+
+int manager_reschedule_rebalance(Manager *m) {
+        int r;
+
+        assert(m);
+
+        /* If a rebalance is pending reschedules it so it gets executed immediately */
+
+        if (!IN_SET(m->rebalance_state, REBALANCE_PENDING, REBALANCE_SHRINKING, REBALANCE_GROWING))
+                return 0;
+
+        r = manager_schedule_rebalance(m, /* immediately= */ true);
+        if (r < 0)
+                return r;
+
+        return 1;
+}