Adding upstream version 255.4.upstream/255.4

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

1 files changed, 2394 insertions, 0 deletions

diff --git a/src/sysusers/sysusers.c b/src/sysusers/sysusers.c
new file mode 100644
index 0000000..514f3c7
--- /dev/null
+++ b/src/sysusers/sysusers.c
@@ -0,0 +1,2394 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include <getopt.h>
+#include <utmp.h>
+
+#include "alloc-util.h"
+#include "build.h"
+#include "chase.h"
+#include "conf-files.h"
+#include "constants.h"
+#include "copy.h"
+#include "creds-util.h"
+#include "dissect-image.h"
+#include "env-util.h"
+#include "fd-util.h"
+#include "fileio.h"
+#include "format-util.h"
+#include "fs-util.h"
+#include "hashmap.h"
+#include "libcrypt-util.h"
+#include "main-func.h"
+#include "memory-util.h"
+#include "mount-util.h"
+#include "nscd-flush.h"
+#include "pager.h"
+#include "parse-argument.h"
+#include "path-util.h"
+#include "pretty-print.h"
+#include "selinux-util.h"
+#include "set.h"
+#include "smack-util.h"
+#include "specifier.h"
+#include "stat-util.h"
+#include "string-util.h"
+#include "strv.h"
+#include "sync-util.h"
+#include "tmpfile-util-label.h"
+#include "uid-alloc-range.h"
+#include "uid-range.h"
+#include "user-util.h"
+#include "utf8.h"
+
+typedef enum ItemType {
+        ADD_USER =   'u',
+        ADD_GROUP =  'g',
+        ADD_MEMBER = 'm',
+        ADD_RANGE =  'r',
+} ItemType;
+
+static const char* item_type_to_string(ItemType t) {
+        switch (t) {
+        case ADD_USER:
+                return "user";
+        case ADD_GROUP:
+                return "group";
+        case ADD_MEMBER:
+                return "member";
+        case ADD_RANGE:
+                return "range";
+        default:
+                assert_not_reached();
+        }
+}
+
+typedef struct Item {
+        ItemType type;
+
+        char *name;
+        char *group_name;
+        char *uid_path;
+        char *gid_path;
+        char *description;
+        char *home;
+        char *shell;
+
+        gid_t gid;
+        uid_t uid;
+
+        char *filename;
+        unsigned line;
+
+        bool gid_set;
+
+        /* When set the group with the specified GID must exist
+         * and the check if a UID clashes with the GID is skipped.
+         */
+        bool id_set_strict;
+
+        bool uid_set;
+
+        bool todo_user;
+        bool todo_group;
+} Item;
+
+static char *arg_root = NULL;
+static char *arg_image = NULL;
+static CatFlags arg_cat_flags = CAT_CONFIG_OFF;
+static const char *arg_replace = NULL;
+static bool arg_dry_run = false;
+static bool arg_inline = false;
+static PagerFlags arg_pager_flags = 0;
+static ImagePolicy *arg_image_policy = NULL;
+
+STATIC_DESTRUCTOR_REGISTER(arg_root, freep);
+STATIC_DESTRUCTOR_REGISTER(arg_image, freep);
+STATIC_DESTRUCTOR_REGISTER(arg_image_policy, image_policy_freep);
+
+typedef struct Context {
+        OrderedHashmap *users, *groups;
+        OrderedHashmap *todo_uids, *todo_gids;
+        OrderedHashmap *members;
+
+        Hashmap *database_by_uid, *database_by_username;
+        Hashmap *database_by_gid, *database_by_groupname;
+
+        /* A helper set to hold names that are used by database_by_{uid,gid,username,groupname} above. */
+        Set *names;
+
+        uid_t search_uid;
+        UidRange *uid_range;
+
+        UGIDAllocationRange login_defs;
+        bool login_defs_need_warning;
+} Context;
+
+static void context_done(Context *c) {
+        assert(c);
+
+        ordered_hashmap_free(c->groups);
+        ordered_hashmap_free(c->users);
+        ordered_hashmap_free(c->members);
+        ordered_hashmap_free(c->todo_uids);
+        ordered_hashmap_free(c->todo_gids);
+
+        hashmap_free(c->database_by_uid);
+        hashmap_free(c->database_by_username);
+        hashmap_free(c->database_by_gid);
+        hashmap_free(c->database_by_groupname);
+
+        set_free_free(c->names);
+        uid_range_free(c->uid_range);
+}
+
+static int errno_is_not_exists(int code) {
+        /* See getpwnam(3) and getgrnam(3): those codes and others can be returned if the user or group are
+         * not found. */
+        return IN_SET(code, 0, ENOENT, ESRCH, EBADF, EPERM);
+}
+
+/* Note: the lifetime of the compound literal is the immediately surrounding block,
+ * see C11 §6.5.2.5, and
+ * https://stackoverflow.com/questions/34880638/compound-literal-lifetime-and-if-blocks */
+#define FORMAT_UID(is_set, uid) \
+        ((is_set) ? snprintf_ok((char[DECIMAL_STR_MAX(uid_t)]){}, DECIMAL_STR_MAX(uid_t), UID_FMT, uid) : "(unset)")
+#define FORMAT_GID(is_set, gid) \
+        ((is_set) ? snprintf_ok((char[DECIMAL_STR_MAX(gid_t)]){}, DECIMAL_STR_MAX(gid_t), GID_FMT, gid) : "(unset)")
+
+static void maybe_emit_login_defs_warning(Context *c) {
+        assert(c);
+
+        if (!c->login_defs_need_warning)
+                return;
+
+        if (c->login_defs.system_alloc_uid_min != SYSTEM_ALLOC_UID_MIN ||
+            c->login_defs.system_uid_max != SYSTEM_UID_MAX)
+                log_warning("login.defs specifies UID allocation range "UID_FMT"–"UID_FMT
+                            " that is different than the built-in defaults ("UID_FMT"–"UID_FMT")",
+                            c->login_defs.system_alloc_uid_min, c->login_defs.system_uid_max,
+                            (uid_t) SYSTEM_ALLOC_UID_MIN, (uid_t) SYSTEM_UID_MAX);
+        if (c->login_defs.system_alloc_gid_min != SYSTEM_ALLOC_GID_MIN ||
+            c->login_defs.system_gid_max != SYSTEM_GID_MAX)
+                log_warning("login.defs specifies GID allocation range "GID_FMT"–"GID_FMT
+                            " that is different than the built-in defaults ("GID_FMT"–"GID_FMT")",
+                            c->login_defs.system_alloc_gid_min, c->login_defs.system_gid_max,
+                            (gid_t) SYSTEM_ALLOC_GID_MIN, (gid_t) SYSTEM_GID_MAX);
+
+        c->login_defs_need_warning = false;
+}
+
+static int load_user_database(Context *c) {
+        _cleanup_fclose_ FILE *f = NULL;
+        const char *passwd_path;
+        struct passwd *pw;
+        int r;
+
+        assert(c);
+
+        passwd_path = prefix_roota(arg_root, "/etc/passwd");
+        f = fopen(passwd_path, "re");
+        if (!f)
+                return errno == ENOENT ? 0 : -errno;
+
+        r = hashmap_ensure_allocated(&c->database_by_username, &string_hash_ops);
+        if (r < 0)
+                return r;
+
+        r = hashmap_ensure_allocated(&c->database_by_uid, NULL);
+        if (r < 0)
+                return r;
+
+        /* Note that we use NULL, i.e. trivial_hash_ops here, so identical strings can exist in the set. */
+        r = set_ensure_allocated(&c->names, NULL);
+        if (r < 0)
+                return r;
+
+        while ((r = fgetpwent_sane(f, &pw)) > 0) {
+
+                char *n = strdup(pw->pw_name);
+                if (!n)
+                        return -ENOMEM;
+
+                r = set_consume(c->names, n);
+                if (r < 0)
+                        return r;
+                assert(r > 0);  /* The set uses pointer comparisons, so n must not be in the set. */
+
+                r = hashmap_put(c->database_by_username, n, UID_TO_PTR(pw->pw_uid));
+                if (r == -EEXIST)
+                        log_debug_errno(r, "%s: user '%s' is listed twice, ignoring duplicate uid.",
+                                        passwd_path, n);
+                else if (r < 0)
+                        return r;
+
+                r = hashmap_put(c->database_by_uid, UID_TO_PTR(pw->pw_uid), n);
+                if (r == -EEXIST)
+                        log_debug_errno(r, "%s: uid "UID_FMT" is listed twice, ignoring duplicate name.",
+                                        passwd_path, pw->pw_uid);
+                else if (r < 0)
+                        return r;
+        }
+        return r;
+}
+
+static int load_group_database(Context *c) {
+        _cleanup_fclose_ FILE *f = NULL;
+        const char *group_path;
+        struct group *gr;
+        int r;
+
+        assert(c);
+
+        group_path = prefix_roota(arg_root, "/etc/group");
+        f = fopen(group_path, "re");
+        if (!f)
+                return errno == ENOENT ? 0 : -errno;
+
+        r = hashmap_ensure_allocated(&c->database_by_groupname, &string_hash_ops);
+        if (r < 0)
+                return r;
+
+        r = hashmap_ensure_allocated(&c->database_by_gid, NULL);
+        if (r < 0)
+                return r;
+
+        /* Note that we use NULL, i.e. trivial_hash_ops here, so identical strings can exist in the set. */
+        r = set_ensure_allocated(&c->names, NULL);
+        if (r < 0)
+                return r;
+
+        while ((r = fgetgrent_sane(f, &gr)) > 0) {
+
+                char *n = strdup(gr->gr_name);
+                if (!n)
+                        return -ENOMEM;
+
+                r = set_consume(c->names, n);
+                if (r < 0)
+                        return r;
+                assert(r > 0);  /* The set uses pointer comparisons, so n must not be in the set. */
+
+                r = hashmap_put(c->database_by_groupname, n, GID_TO_PTR(gr->gr_gid));
+                if (r == -EEXIST)
+                        log_debug_errno(r, "%s: group '%s' is listed twice, ignoring duplicate gid.",
+                                        group_path, n);
+                else if (r < 0)
+                        return r;
+
+                r = hashmap_put(c->database_by_gid, GID_TO_PTR(gr->gr_gid), n);
+                if (r == -EEXIST)
+                        log_debug_errno(r, "%s: gid "GID_FMT" is listed twice, ignoring duplicate name.",
+                                        group_path, gr->gr_gid);
+                else if (r < 0)
+                        return r;
+        }
+        return r;
+}
+
+static int make_backup(const char *target, const char *x) {
+        _cleanup_(unlink_and_freep) char *dst_tmp = NULL;
+        _cleanup_fclose_ FILE *dst = NULL;
+        _cleanup_close_ int src = -EBADF;
+        const char *backup;
+        struct stat st;
+        int r;
+
+        assert(target);
+        assert(x);
+
+        src = open(x, O_RDONLY|O_CLOEXEC|O_NOCTTY);
+        if (src < 0) {
+                if (errno == ENOENT) /* No backup necessary... */
+                        return 0;
+
+                return -errno;
+        }
+
+        if (fstat(src, &st) < 0)
+                return -errno;
+
+        r = fopen_temporary_label(
+                        target,   /* The path for which to the look up the label */
+                        x,        /* Where we want the file actually to end up */
+                        &dst,     /* The temporary file we write to */
+                        &dst_tmp);
+        if (r < 0)
+                return r;
+
+        r = copy_bytes(src, fileno(dst), UINT64_MAX, COPY_REFLINK);
+        if (r < 0)
+                return r;
+
+        backup = strjoina(x, "-");
+
+        /* Copy over the access mask. Don't fail on chmod() or chown(). If it stays owned by us and/or
+         * unreadable by others, then it isn't too bad... */
+        r = fchmod_and_chown_with_fallback(fileno(dst), dst_tmp, st.st_mode & 07777, st.st_uid, st.st_gid);
+        if (r < 0)
+                log_warning_errno(r, "Failed to change access mode or ownership of %s: %m", backup);
+
+        if (futimens(fileno(dst), (const struct timespec[2]) { st.st_atim, st.st_mtim }) < 0)
+                log_warning_errno(errno, "Failed to fix access and modification time of %s: %m", backup);
+
+        r = fsync_full(fileno(dst));
+        if (r < 0)
+                return r;
+
+        if (rename(dst_tmp, backup) < 0)
+                return errno;
+
+        dst_tmp = mfree(dst_tmp); /* disable the unlink_and_freep() hook now that the file has been renamed */
+        return 0;
+}
+
+static int putgrent_with_members(
+                Context *c,
+                const struct group *gr,
+                FILE *group) {
+
+        char **a;
+
+        assert(c);
+        assert(gr);
+        assert(group);
+
+        a = ordered_hashmap_get(c->members, gr->gr_name);
+        if (a) {
+                _cleanup_strv_free_ char **l = NULL;
+                bool added = false;
+
+                l = strv_copy(gr->gr_mem);
+                if (!l)
+                        return -ENOMEM;
+
+                STRV_FOREACH(i, a) {
+                        if (strv_contains(l, *i))
+                                continue;
+
+                        if (strv_extend(&l, *i) < 0)
+                                return -ENOMEM;
+
+                        added = true;
+                }
+
+                if (added) {
+                        struct group t;
+                        int r;
+
+                        strv_uniq(l);
+                        strv_sort(l);
+
+                        t = *gr;
+                        t.gr_mem = l;
+
+                        r = putgrent_sane(&t, group);
+                        return r < 0 ? r : 1;
+                }
+        }
+
+        return putgrent_sane(gr, group);
+}
+
+#if ENABLE_GSHADOW
+static int putsgent_with_members(
+                Context *c,
+                const struct sgrp *sg,
+                FILE *gshadow) {
+
+        char **a;
+
+        assert(sg);
+        assert(gshadow);
+
+        a = ordered_hashmap_get(c->members, sg->sg_namp);
+        if (a) {
+                _cleanup_strv_free_ char **l = NULL;
+                bool added = false;
+
+                l = strv_copy(sg->sg_mem);
+                if (!l)
+                        return -ENOMEM;
+
+                STRV_FOREACH(i, a) {
+                        if (strv_contains(l, *i))
+                                continue;
+
+                        if (strv_extend(&l, *i) < 0)
+                                return -ENOMEM;
+
+                        added = true;
+                }
+
+                if (added) {
+                        struct sgrp t;
+                        int r;
+
+                        strv_uniq(l);
+                        strv_sort(l);
+
+                        t = *sg;
+                        t.sg_mem = l;
+
+                        r = putsgent_sane(&t, gshadow);
+                        return r < 0 ? r : 1;
+                }
+        }
+
+        return putsgent_sane(sg, gshadow);
+}
+#endif
+
+static const char* pick_shell(const Item *i) {
+        if (i->type != ADD_USER)
+                return NULL;
+        if (i->shell)
+                return i->shell;
+        if (i->uid_set && i->uid == 0)
+                return default_root_shell(arg_root);
+        return NOLOGIN;
+}
+
+static int write_temporary_passwd(
+                Context *c,
+                const char *passwd_path,
+                FILE **ret_tmpfile,
+                char **ret_tmpfile_path) {
+
+        _cleanup_fclose_ FILE *original = NULL, *passwd = NULL;
+        _cleanup_(unlink_and_freep) char *passwd_tmp = NULL;
+        struct passwd *pw = NULL;
+        Item *i;
+        int r;
+
+        assert(c);
+
+        if (ordered_hashmap_isempty(c->todo_uids))
+                return 0;
+
+        if (arg_dry_run) {
+                log_info("Would write /etc/passwd%s", special_glyph(SPECIAL_GLYPH_ELLIPSIS));
+                return 0;
+        }
+
+        r = fopen_temporary_label("/etc/passwd", passwd_path, &passwd, &passwd_tmp);
+        if (r < 0)
+                return log_debug_errno(r, "Failed to open temporary copy of %s: %m", passwd_path);
+
+        original = fopen(passwd_path, "re");
+        if (original) {
+
+                /* Allow fallback path for when /proc is not mounted. On any normal system /proc will be
+                 * mounted, but e.g. when 'dnf --installroot' is used, it might not be. There is no security
+                 * relevance here, since the environment is ultimately trusted, and not requiring /proc makes
+                 * it easier to depend on sysusers in packaging scripts and suchlike. */
+                r = copy_rights_with_fallback(fileno(original), fileno(passwd), passwd_tmp);
+                if (r < 0)
+                        return log_debug_errno(r, "Failed to copy permissions from %s to %s: %m",
+                                               passwd_path, passwd_tmp);
+
+                while ((r = fgetpwent_sane(original, &pw)) > 0) {
+                        i = ordered_hashmap_get(c->users, pw->pw_name);
+                        if (i && i->todo_user)
+                                return log_error_errno(SYNTHETIC_ERRNO(EEXIST),
+                                                       "%s: User \"%s\" already exists.",
+                                                       passwd_path, pw->pw_name);
+
+                        if (ordered_hashmap_contains(c->todo_uids, UID_TO_PTR(pw->pw_uid)))
+                                return log_error_errno(SYNTHETIC_ERRNO(EEXIST),
+                                                       "%s: Detected collision for UID " UID_FMT ".",
+                                                       passwd_path, pw->pw_uid);
+
+                        /* Make sure we keep the NIS entries (if any) at the end. */
+                        if (IN_SET(pw->pw_name[0], '+', '-'))
+                                break;
+
+                        r = putpwent_sane(pw, passwd);
+                        if (r < 0)
+                                return log_debug_errno(r, "Failed to add existing user \"%s\" to temporary passwd file: %m",
+                                                       pw->pw_name);
+                }
+                if (r < 0)
+                        return log_debug_errno(r, "Failed to read %s: %m", passwd_path);
+
+        } else {
+                if (errno != ENOENT)
+                        return log_debug_errno(errno, "Failed to open %s: %m", passwd_path);
+                if (fchmod(fileno(passwd), 0644) < 0)
+                        return log_debug_errno(errno, "Failed to fchmod %s: %m", passwd_tmp);
+        }
+
+        ORDERED_HASHMAP_FOREACH(i, c->todo_uids) {
+                _cleanup_free_ char *creds_shell = NULL, *cn = NULL;
+
+                struct passwd n = {
+                        .pw_name = i->name,
+                        .pw_uid = i->uid,
+                        .pw_gid = i->gid,
+                        .pw_gecos = (char*) strempty(i->description),
+
+                        /* "x" means the password is stored in the shadow file */
+                        .pw_passwd = (char*) PASSWORD_SEE_SHADOW,
+
+                        /* We default to the root directory as home */
+                        .pw_dir = i->home ?: (char*) "/",
+
+                        /* Initialize the shell to nologin, with one exception:
+                         * for root we patch in something special */
+                        .pw_shell = (char*) pick_shell(i),
+                };
+
+                /* Try to pick up the shell for this account via the credentials logic */
+                cn = strjoin("passwd.shell.", i->name);
+                if (!cn)
+                        return -ENOMEM;
+
+                r = read_credential(cn, (void**) &creds_shell, NULL);
+                if (r < 0)
+                        log_debug_errno(r, "Couldn't read credential '%s', ignoring: %m", cn);
+                else
+                        n.pw_shell = creds_shell;
+
+                r = putpwent_sane(&n, passwd);
+                if (r < 0)
+                        return log_debug_errno(r, "Failed to add new user \"%s\" to temporary passwd file: %m",
+                                               i->name);
+        }
+
+        /* Append the remaining NIS entries if any */
+        while (pw) {
+                r = putpwent_sane(pw, passwd);
+                if (r < 0)
+                        return log_debug_errno(r, "Failed to add existing user \"%s\" to temporary passwd file: %m",
+                                               pw->pw_name);
+
+                r = fgetpwent_sane(original, &pw);
+                if (r < 0)
+                        return log_debug_errno(r, "Failed to read %s: %m", passwd_path);
+                if (r == 0)
+                        break;
+        }
+
+        r = fflush_sync_and_check(passwd);
+        if (r < 0)
+                return log_debug_errno(r, "Failed to flush %s: %m", passwd_tmp);
+
+        *ret_tmpfile = TAKE_PTR(passwd);
+        *ret_tmpfile_path = TAKE_PTR(passwd_tmp);
+
+        return 0;
+}
+
+static usec_t epoch_or_now(void) {
+        uint64_t epoch;
+
+        if (getenv_uint64_secure("SOURCE_DATE_EPOCH", &epoch) >= 0) {
+                if (epoch > UINT64_MAX/USEC_PER_SEC) /* Overflow check */
+                        return USEC_INFINITY;
+                return (usec_t) epoch * USEC_PER_SEC;
+        }
+
+        return now(CLOCK_REALTIME);
+}
+
+static int write_temporary_shadow(
+                Context *c,
+                const char *shadow_path,
+                FILE **ret_tmpfile,
+                char **ret_tmpfile_path) {
+
+        _cleanup_fclose_ FILE *original = NULL, *shadow = NULL;
+        _cleanup_(unlink_and_freep) char *shadow_tmp = NULL;
+        struct spwd *sp = NULL;
+        long lstchg;
+        Item *i;
+        int r;
+
+        assert(c);
+
+        if (ordered_hashmap_isempty(c->todo_uids))
+                return 0;
+
+        if (arg_dry_run) {
+                log_info("Would write /etc/shadow%s", special_glyph(SPECIAL_GLYPH_ELLIPSIS));
+                return 0;
+        }
+
+        r = fopen_temporary_label("/etc/shadow", shadow_path, &shadow, &shadow_tmp);
+        if (r < 0)
+                return log_debug_errno(r, "Failed to open temporary copy of %s: %m", shadow_path);
+
+        lstchg = (long) (epoch_or_now() / USEC_PER_DAY);
+
+        original = fopen(shadow_path, "re");
+        if (original) {
+
+                r = copy_rights_with_fallback(fileno(original), fileno(shadow), shadow_tmp);
+                if (r < 0)
+                        return log_debug_errno(r, "Failed to copy permissions from %s to %s: %m",
+                                               shadow_path, shadow_tmp);
+
+                while ((r = fgetspent_sane(original, &sp)) > 0) {
+                        i = ordered_hashmap_get(c->users, sp->sp_namp);
+                        if (i && i->todo_user) {
+                                /* we will update the existing entry */
+                                sp->sp_lstchg = lstchg;
+
+                                /* only the /etc/shadow stage is left, so we can
+                                 * safely remove the item from the todo set */
+                                i->todo_user = false;
+                                ordered_hashmap_remove(c->todo_uids, UID_TO_PTR(i->uid));
+                        }
+
+                        /* Make sure we keep the NIS entries (if any) at the end. */
+                        if (IN_SET(sp->sp_namp[0], '+', '-'))
+                                break;
+
+                        r = putspent_sane(sp, shadow);
+                        if (r < 0)
+                                return log_debug_errno(r, "Failed to add existing user \"%s\" to temporary shadow file: %m",
+                                                       sp->sp_namp);
+
+                }
+                if (r < 0)
+                        return log_debug_errno(r, "Failed to read %s: %m", shadow_path);
+
+        } else {
+                if (errno != ENOENT)
+                        return log_debug_errno(errno, "Failed to open %s: %m", shadow_path);
+                if (fchmod(fileno(shadow), 0000) < 0)
+                        return log_debug_errno(errno, "Failed to fchmod %s: %m", shadow_tmp);
+        }
+
+        ORDERED_HASHMAP_FOREACH(i, c->todo_uids) {
+                _cleanup_(erase_and_freep) char *creds_password = NULL;
+                bool is_hashed;
+
+                struct spwd n = {
+                        .sp_namp = i->name,
+                        .sp_lstchg = lstchg,
+                        .sp_min = -1,
+                        .sp_max = -1,
+                        .sp_warn = -1,
+                        .sp_inact = -1,
+                        .sp_expire = -1,
+                        .sp_flag = ULONG_MAX, /* this appears to be what everybody does ... */
+                };
+
+                r = get_credential_user_password(i->name, &creds_password, &is_hashed);
+                if (r < 0)
+                        log_debug_errno(r, "Couldn't read password credential for user '%s', ignoring: %m", i->name);
+
+                if (creds_password && !is_hashed) {
+                        _cleanup_(erase_and_freep) char* plaintext_password = TAKE_PTR(creds_password);
+                        r = hash_password(plaintext_password, &creds_password);
+                        if (r < 0)
+                                return log_debug_errno(r, "Failed to hash password: %m");
+                }
+
+                if (creds_password)
+                        n.sp_pwdp = creds_password;
+                else if (streq(i->name, "root"))
+                        /* Let firstboot set the password later */
+                        n.sp_pwdp = (char*) PASSWORD_UNPROVISIONED;
+                else
+                        n.sp_pwdp = (char*) PASSWORD_LOCKED_AND_INVALID;
+
+                r = putspent_sane(&n, shadow);
+                if (r < 0)
+                        return log_debug_errno(r, "Failed to add new user \"%s\" to temporary shadow file: %m",
+                                               i->name);
+        }
+
+        /* Append the remaining NIS entries if any */
+        while (sp) {
+                r = putspent_sane(sp, shadow);
+                if (r < 0)
+                        return log_debug_errno(r, "Failed to add existing user \"%s\" to temporary shadow file: %m",
+                                               sp->sp_namp);
+
+                r = fgetspent_sane(original, &sp);
+                if (r < 0)
+                        return log_debug_errno(r, "Failed to read %s: %m", shadow_path);
+                if (r == 0)
+                        break;
+        }
+        if (!IN_SET(errno, 0, ENOENT))
+                return -errno;
+
+        r = fflush_sync_and_check(shadow);
+        if (r < 0)
+                return log_debug_errno(r, "Failed to flush %s: %m", shadow_tmp);
+
+        *ret_tmpfile = TAKE_PTR(shadow);
+        *ret_tmpfile_path = TAKE_PTR(shadow_tmp);
+
+        return 0;
+}
+
+static int write_temporary_group(
+                Context *c,
+                const char *group_path,
+                FILE **ret_tmpfile,
+                char **ret_tmpfile_path) {
+
+        _cleanup_fclose_ FILE *original = NULL, *group = NULL;
+        _cleanup_(unlink_and_freep) char *group_tmp = NULL;
+        bool group_changed = false;
+        struct group *gr = NULL;
+        Item *i;
+        int r;
+
+        assert(c);
+
+        if (ordered_hashmap_isempty(c->todo_gids) && ordered_hashmap_isempty(c->members))
+                return 0;
+
+        if (arg_dry_run) {
+                log_info("Would write /etc/group%s", special_glyph(SPECIAL_GLYPH_ELLIPSIS));
+                return 0;
+        }
+
+        r = fopen_temporary_label("/etc/group", group_path, &group, &group_tmp);
+        if (r < 0)
+                return log_error_errno(r, "Failed to open temporary copy of %s: %m", group_path);
+
+        original = fopen(group_path, "re");
+        if (original) {
+
+                r = copy_rights_with_fallback(fileno(original), fileno(group), group_tmp);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to copy permissions from %s to %s: %m",
+                                               group_path, group_tmp);
+
+                while ((r = fgetgrent_sane(original, &gr)) > 0) {
+                        /* Safety checks against name and GID collisions. Normally,
+                         * this should be unnecessary, but given that we look at the
+                         * entries anyway here, let's make an extra verification
+                         * step that we don't generate duplicate entries. */
+
+                        i = ordered_hashmap_get(c->groups, gr->gr_name);
+                        if (i && i->todo_group)
+                                return log_error_errno(SYNTHETIC_ERRNO(EEXIST),
+                                                       "%s: Group \"%s\" already exists.",
+                                                       group_path, gr->gr_name);
+
+                        if (ordered_hashmap_contains(c->todo_gids, GID_TO_PTR(gr->gr_gid)))
+                                return log_error_errno(SYNTHETIC_ERRNO(EEXIST),
+                                                       "%s: Detected collision for GID " GID_FMT ".",
+                                                       group_path, gr->gr_gid);
+
+                        /* Make sure we keep the NIS entries (if any) at the end. */
+                        if (IN_SET(gr->gr_name[0], '+', '-'))
+                                break;
+
+                        r = putgrent_with_members(c, gr, group);
+                        if (r < 0)
+                                return log_error_errno(r, "Failed to add existing group \"%s\" to temporary group file: %m",
+                                                       gr->gr_name);
+                        if (r > 0)
+                                group_changed = true;
+                }
+                if (r < 0)
+                        return log_error_errno(r, "Failed to read %s: %m", group_path);
+
+        } else {
+                if (errno != ENOENT)
+                        return log_error_errno(errno, "Failed to open %s: %m", group_path);
+                if (fchmod(fileno(group), 0644) < 0)
+                        return log_error_errno(errno, "Failed to fchmod %s: %m", group_tmp);
+        }
+
+        ORDERED_HASHMAP_FOREACH(i, c->todo_gids) {
+                struct group n = {
+                        .gr_name = i->name,
+                        .gr_gid = i->gid,
+                        .gr_passwd = (char*) PASSWORD_SEE_SHADOW,
+                };
+
+                r = putgrent_with_members(c, &n, group);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to add new group \"%s\" to temporary group file: %m",
+                                               gr->gr_name);
+
+                group_changed = true;
+        }
+
+        /* Append the remaining NIS entries if any */
+        while (gr) {
+                r = putgrent_sane(gr, group);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to add existing group \"%s\" to temporary group file: %m",
+                                               gr->gr_name);
+
+                r = fgetgrent_sane(original, &gr);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to read %s: %m", group_path);
+                if (r == 0)
+                        break;
+        }
+
+        r = fflush_sync_and_check(group);
+        if (r < 0)
+                return log_error_errno(r, "Failed to flush %s: %m", group_tmp);
+
+        if (group_changed) {
+                *ret_tmpfile = TAKE_PTR(group);
+                *ret_tmpfile_path = TAKE_PTR(group_tmp);
+        }
+        return 0;
+}
+
+static int write_temporary_gshadow(
+                Context *c,
+                const char * gshadow_path,
+                FILE **ret_tmpfile,
+                char **ret_tmpfile_path) {
+
+#if ENABLE_GSHADOW
+        _cleanup_fclose_ FILE *original = NULL, *gshadow = NULL;
+        _cleanup_(unlink_and_freep) char *gshadow_tmp = NULL;
+        bool group_changed = false;
+        Item *i;
+        int r;
+
+        assert(c);
+
+        if (ordered_hashmap_isempty(c->todo_gids) && ordered_hashmap_isempty(c->members))
+                return 0;
+
+        if (arg_dry_run) {
+                log_info("Would write /etc/gshadow%s", special_glyph(SPECIAL_GLYPH_ELLIPSIS));
+                return 0;
+        }
+
+        r = fopen_temporary_label("/etc/gshadow", gshadow_path, &gshadow, &gshadow_tmp);
+        if (r < 0)
+                return log_error_errno(r, "Failed to open temporary copy of %s: %m", gshadow_path);
+
+        original = fopen(gshadow_path, "re");
+        if (original) {
+                struct sgrp *sg;
+
+                r = copy_rights_with_fallback(fileno(original), fileno(gshadow), gshadow_tmp);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to copy permissions from %s to %s: %m",
+                                               gshadow_path, gshadow_tmp);
+
+                while ((r = fgetsgent_sane(original, &sg)) > 0) {
+
+                        i = ordered_hashmap_get(c->groups, sg->sg_namp);
+                        if (i && i->todo_group)
+                                return log_error_errno(SYNTHETIC_ERRNO(EEXIST),
+                                                       "%s: Group \"%s\" already exists.",
+                                                       gshadow_path, sg->sg_namp);
+
+                        r = putsgent_with_members(c, sg, gshadow);
+                        if (r < 0)
+                                return log_error_errno(r, "Failed to add existing group \"%s\" to temporary gshadow file: %m",
+                                                       sg->sg_namp);
+                        if (r > 0)
+                                group_changed = true;
+                }
+                if (r < 0)
+                        return r;
+
+        } else {
+                if (errno != ENOENT)
+                        return log_error_errno(errno, "Failed to open %s: %m", gshadow_path);
+                if (fchmod(fileno(gshadow), 0000) < 0)
+                        return log_error_errno(errno, "Failed to fchmod %s: %m", gshadow_tmp);
+        }
+
+        ORDERED_HASHMAP_FOREACH(i, c->todo_gids) {
+                struct sgrp n = {
+                        .sg_namp = i->name,
+                        .sg_passwd = (char*) PASSWORD_LOCKED_AND_INVALID,
+                };
+
+                r = putsgent_with_members(c, &n, gshadow);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to add new group \"%s\" to temporary gshadow file: %m",
+                                               n.sg_namp);
+
+                group_changed = true;
+        }
+
+        r = fflush_sync_and_check(gshadow);
+        if (r < 0)
+                return log_error_errno(r, "Failed to flush %s: %m", gshadow_tmp);
+
+        if (group_changed) {
+                *ret_tmpfile = TAKE_PTR(gshadow);
+                *ret_tmpfile_path = TAKE_PTR(gshadow_tmp);
+        }
+#endif
+        return 0;
+}
+
+static int write_files(Context *c) {
+        _cleanup_fclose_ FILE *passwd = NULL, *group = NULL, *shadow = NULL, *gshadow = NULL;
+        _cleanup_(unlink_and_freep) char *passwd_tmp = NULL, *group_tmp = NULL, *shadow_tmp = NULL, *gshadow_tmp = NULL;
+        int r;
+
+        const char
+                *passwd_path = prefix_roota(arg_root, "/etc/passwd"),
+                *shadow_path = prefix_roota(arg_root, "/etc/shadow"),
+                *group_path = prefix_roota(arg_root, "/etc/group"),
+                *gshadow_path = prefix_roota(arg_root, "/etc/gshadow");
+
+        assert(c);
+
+        r = write_temporary_group(c, group_path, &group, &group_tmp);
+        if (r < 0)
+                return r;
+
+        r = write_temporary_gshadow(c, gshadow_path, &gshadow, &gshadow_tmp);
+        if (r < 0)
+                return r;
+
+        r = write_temporary_passwd(c, passwd_path, &passwd, &passwd_tmp);
+        if (r < 0)
+                return r;
+
+        r = write_temporary_shadow(c, shadow_path, &shadow, &shadow_tmp);
+        if (r < 0)
+                return r;
+
+        /* Make a backup of the old files */
+        if (group) {
+                r = make_backup("/etc/group", group_path);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to backup %s: %m", group_path);
+        }
+        if (gshadow) {
+                r = make_backup("/etc/gshadow", gshadow_path);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to backup %s: %m", gshadow_path);
+        }
+
+        if (passwd) {
+                r = make_backup("/etc/passwd", passwd_path);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to backup %s: %m", passwd_path);
+        }
+        if (shadow) {
+                r = make_backup("/etc/shadow", shadow_path);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to backup %s: %m", shadow_path);
+        }
+
+        /* And make the new files count */
+        if (group) {
+                r = rename_and_apply_smack_floor_label(group_tmp, group_path);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to rename %s to %s: %m",
+                                               group_tmp, group_path);
+                group_tmp = mfree(group_tmp);
+
+                if (!arg_root && !arg_image)
+                        (void) nscd_flush_cache(STRV_MAKE("group"));
+        }
+        if (gshadow) {
+                r = rename_and_apply_smack_floor_label(gshadow_tmp, gshadow_path);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to rename %s to %s: %m",
+                                               gshadow_tmp, gshadow_path);
+
+                gshadow_tmp = mfree(gshadow_tmp);
+        }
+
+        if (passwd) {
+                r = rename_and_apply_smack_floor_label(passwd_tmp, passwd_path);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to rename %s to %s: %m",
+                                               passwd_tmp, passwd_path);
+
+                passwd_tmp = mfree(passwd_tmp);
+
+                if (!arg_root && !arg_image)
+                        (void) nscd_flush_cache(STRV_MAKE("passwd"));
+        }
+        if (shadow) {
+                r = rename_and_apply_smack_floor_label(shadow_tmp, shadow_path);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to rename %s to %s: %m",
+                                               shadow_tmp, shadow_path);
+
+                shadow_tmp = mfree(shadow_tmp);
+        }
+
+        return 0;
+}
+
+static int uid_is_ok(
+                Context *c,
+                uid_t uid,
+                const char *name,
+                bool check_with_gid) {
+
+        assert(c);
+
+        /* Let's see if we already have assigned the UID a second time */
+        if (ordered_hashmap_get(c->todo_uids, UID_TO_PTR(uid)))
+                return 0;
+
+        /* Try to avoid using uids that are already used by a group
+         * that doesn't have the same name as our new user. */
+        if (check_with_gid) {
+                Item *i;
+
+                i = ordered_hashmap_get(c->todo_gids, GID_TO_PTR(uid));
+                if (i && !streq(i->name, name))
+                        return 0;
+        }
+
+        /* Let's check the files directly */
+        if (hashmap_contains(c->database_by_uid, UID_TO_PTR(uid)))
+                return 0;
+
+        if (check_with_gid) {
+                const char *n;
+
+                n = hashmap_get(c->database_by_gid, GID_TO_PTR(uid));
+                if (n && !streq(n, name))
+                        return 0;
+        }
+
+        /* Let's also check via NSS, to avoid UID clashes over LDAP and such, just in case */
+        if (!arg_root) {
+                struct passwd *p;
+                struct group *g;
+
+                errno = 0;
+                p = getpwuid(uid);
+                if (p)
+                        return 0;
+                if (!IN_SET(errno, 0, ENOENT))
+                        return -errno;
+
+                if (check_with_gid) {
+                        errno = 0;
+                        g = getgrgid((gid_t) uid);
+                        if (g) {
+                                if (!streq(g->gr_name, name))
+                                        return 0;
+                        } else if (!IN_SET(errno, 0, ENOENT))
+                                return -errno;
+                }
+        }
+
+        return 1;
+}
+
+static int root_stat(const char *p, struct stat *st) {
+        const char *fix;
+
+        fix = prefix_roota(arg_root, p);
+        return RET_NERRNO(stat(fix, st));
+}
+
+static int read_id_from_file(Item *i, uid_t *ret_uid, gid_t *ret_gid) {
+        struct stat st;
+        bool found_uid = false, found_gid = false;
+        uid_t uid = 0;
+        gid_t gid = 0;
+
+        assert(i);
+
+        /* First, try to get the GID directly */
+        if (ret_gid && i->gid_path && root_stat(i->gid_path, &st) >= 0) {
+                gid = st.st_gid;
+                found_gid = true;
+        }
+
+        /* Then, try to get the UID directly */
+        if ((ret_uid || (ret_gid && !found_gid))
+            && i->uid_path
+            && root_stat(i->uid_path, &st) >= 0) {
+
+                uid = st.st_uid;
+                found_uid = true;
+
+                /* If we need the gid, but had no success yet, also derive it from the UID path */
+                if (ret_gid && !found_gid) {
+                        gid = st.st_gid;
+                        found_gid = true;
+                }
+        }
+
+        /* If that didn't work yet, then let's reuse the GID as UID */
+        if (ret_uid && !found_uid && i->gid_path) {
+
+                if (found_gid) {
+                        uid = (uid_t) gid;
+                        found_uid = true;
+                } else if (root_stat(i->gid_path, &st) >= 0) {
+                        uid = (uid_t) st.st_gid;
+                        found_uid = true;
+                }
+        }
+
+        if (ret_uid) {
+                if (!found_uid)
+                        return 0;
+
+                *ret_uid = uid;
+        }
+
+        if (ret_gid) {
+                if (!found_gid)
+                        return 0;
+
+                *ret_gid = gid;
+        }
+
+        return 1;
+}
+
+static int add_user(Context *c, Item *i) {
+        void *z;
+        int r;
+
+        assert(c);
+        assert(i);
+
+        /* Check the database directly */
+        z = hashmap_get(c->database_by_username, i->name);
+        if (z) {
+                log_debug("User %s already exists.", i->name);
+                i->uid = PTR_TO_UID(z);
+                i->uid_set = true;
+                return 0;
+        }
+
+        if (!arg_root) {
+                struct passwd *p;
+
+                /* Also check NSS */
+                errno = 0;
+                p = getpwnam(i->name);
+                if (p) {
+                        log_debug("User %s already exists.", i->name);
+                        i->uid = p->pw_uid;
+                        i->uid_set = true;
+
+                        r = free_and_strdup(&i->description, p->pw_gecos);
+                        if (r < 0)
+                                return log_oom();
+
+                        return 0;
+                }
+                if (!errno_is_not_exists(errno))
+                        return log_error_errno(errno, "Failed to check if user %s already exists: %m", i->name);
+        }
+
+        /* Try to use the suggested numeric UID */
+        if (i->uid_set) {
+                r = uid_is_ok(c, i->uid, i->name, !i->id_set_strict);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to verify UID " UID_FMT ": %m", i->uid);
+                if (r == 0) {
+                        log_info("Suggested user ID " UID_FMT " for %s already used.", i->uid, i->name);
+                        i->uid_set = false;
+                }
+        }
+
+        /* If that didn't work, try to read it from the specified path */
+        if (!i->uid_set) {
+                uid_t candidate;
+
+                if (read_id_from_file(i, &candidate, NULL) > 0) {
+
+                        if (candidate <= 0 || !uid_range_contains(c->uid_range, candidate))
+                                log_debug("User ID " UID_FMT " of file not suitable for %s.", candidate, i->name);
+                        else {
+                                r = uid_is_ok(c, candidate, i->name, true);
+                                if (r < 0)
+                                        return log_error_errno(r, "Failed to verify UID " UID_FMT ": %m", i->uid);
+                                else if (r > 0) {
+                                        i->uid = candidate;
+                                        i->uid_set = true;
+                                } else
+                                        log_debug("User ID " UID_FMT " of file for %s is already used.", candidate, i->name);
+                        }
+                }
+        }
+
+        /* Otherwise, try to reuse the group ID */
+        if (!i->uid_set && i->gid_set) {
+                r = uid_is_ok(c, (uid_t) i->gid, i->name, true);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to verify UID " UID_FMT ": %m", i->uid);
+                if (r > 0) {
+                        i->uid = (uid_t) i->gid;
+                        i->uid_set = true;
+                }
+        }
+
+        /* And if that didn't work either, let's try to find a free one */
+        if (!i->uid_set) {
+                maybe_emit_login_defs_warning(c);
+
+                for (;;) {
+                        r = uid_range_next_lower(c->uid_range, &c->search_uid);
+                        if (r < 0)
+                                return log_error_errno(r, "No free user ID available for %s.", i->name);
+
+                        r = uid_is_ok(c, c->search_uid, i->name, true);
+                        if (r < 0)
+                                return log_error_errno(r, "Failed to verify UID " UID_FMT ": %m", i->uid);
+                        else if (r > 0)
+                                break;
+                }
+
+                i->uid_set = true;
+                i->uid = c->search_uid;
+        }
+
+        r = ordered_hashmap_ensure_put(&c->todo_uids, NULL, UID_TO_PTR(i->uid), i);
+        if (r == -EEXIST)
+                return log_error_errno(r, "Requested user %s with UID " UID_FMT " and gid" GID_FMT " to be created is duplicated "
+                                       "or conflicts with another user.", i->name, i->uid, i->gid);
+        if (r == -ENOMEM)
+                return log_oom();
+        if (r < 0)
+                return log_error_errno(r, "Failed to store user %s with UID " UID_FMT " and GID " GID_FMT " to be created: %m",
+                                       i->name, i->uid, i->gid);
+
+        i->todo_user = true;
+        log_info("Creating user '%s' (%s) with UID " UID_FMT " and GID " GID_FMT ".",
+                 i->name, strna(i->description), i->uid, i->gid);
+
+        return 0;
+}
+
+static int gid_is_ok(
+                Context *c,
+                gid_t gid,
+                const char *groupname,
+                bool check_with_uid) {
+
+        struct group *g;
+        struct passwd *p;
+        Item *user;
+        char *username;
+
+        assert(c);
+        assert(groupname);
+
+        if (ordered_hashmap_get(c->todo_gids, GID_TO_PTR(gid)))
+                return 0;
+
+        /* Avoid reusing gids that are already used by a different user */
+        if (check_with_uid) {
+                user = ordered_hashmap_get(c->todo_uids, UID_TO_PTR(gid));
+                if (user && !streq(user->name, groupname))
+                        return 0;
+        }
+
+        if (hashmap_contains(c->database_by_gid, GID_TO_PTR(gid)))
+                return 0;
+
+        if (check_with_uid) {
+                username = hashmap_get(c->database_by_uid, UID_TO_PTR(gid));
+                if (username && !streq(username, groupname))
+                        return 0;
+        }
+
+        if (!arg_root) {
+                errno = 0;
+                g = getgrgid(gid);
+                if (g)
+                        return 0;
+                if (!IN_SET(errno, 0, ENOENT))
+                        return -errno;
+
+                if (check_with_uid) {
+                        errno = 0;
+                        p = getpwuid((uid_t) gid);
+                        if (p)
+                                return 0;
+                        if (!IN_SET(errno, 0, ENOENT))
+                                return -errno;
+                }
+        }
+
+        return 1;
+}
+
+static int get_gid_by_name(
+                Context *c,
+                const char *name,
+                gid_t *ret_gid) {
+
+        void *z;
+
+        assert(c);
+        assert(ret_gid);
+
+        /* Check the database directly */
+        z = hashmap_get(c->database_by_groupname, name);
+        if (z) {
+                *ret_gid = PTR_TO_GID(z);
+                return 0;
+        }
+
+        /* Also check NSS */
+        if (!arg_root) {
+                struct group *g;
+
+                errno = 0;
+                g = getgrnam(name);
+                if (g) {
+                        *ret_gid = g->gr_gid;
+                        return 0;
+                }
+                if (!errno_is_not_exists(errno))
+                        return log_error_errno(errno, "Failed to check if group %s already exists: %m", name);
+        }
+
+        return -ENOENT;
+}
+
+static int add_group(Context *c, Item *i) {
+        int r;
+
+        assert(c);
+        assert(i);
+
+        r = get_gid_by_name(c, i->name, &i->gid);
+        if (r != -ENOENT) {
+                if (r < 0)
+                        return r;
+                log_debug("Group %s already exists.", i->name);
+                i->gid_set = true;
+                return 0;
+        }
+
+        /* Try to use the suggested numeric GID */
+        if (i->gid_set) {
+                r = gid_is_ok(c, i->gid, i->name, false);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to verify GID " GID_FMT ": %m", i->gid);
+                if (i->id_set_strict) {
+                        /* If we require the GID to already exist we can return here:
+                         * r > 0: means the GID does not exist -> fail
+                         * r == 0: means the GID exists -> nothing more to do.
+                         */
+                        if (r > 0)
+                                return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
+                                                       "Failed to create %s: please create GID " GID_FMT,
+                                                       i->name, i->gid);
+                        if (r == 0)
+                                return 0;
+                }
+                if (r == 0) {
+                        log_info("Suggested group ID " GID_FMT " for %s already used.", i->gid, i->name);
+                        i->gid_set = false;
+                }
+        }
+
+        /* Try to reuse the numeric uid, if there's one */
+        if (!i->gid_set && i->uid_set) {
+                r = gid_is_ok(c, (gid_t) i->uid, i->name, true);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to verify GID " GID_FMT ": %m", i->gid);
+                if (r > 0) {
+                        i->gid = (gid_t) i->uid;
+                        i->gid_set = true;
+                }
+        }
+
+        /* If that didn't work, try to read it from the specified path */
+        if (!i->gid_set) {
+                gid_t candidate;
+
+                if (read_id_from_file(i, NULL, &candidate) > 0) {
+
+                        if (candidate <= 0 || !uid_range_contains(c->uid_range, candidate))
+                                log_debug("Group ID " GID_FMT " of file not suitable for %s.", candidate, i->name);
+                        else {
+                                r = gid_is_ok(c, candidate, i->name, true);
+                                if (r < 0)
+                                        return log_error_errno(r, "Failed to verify GID " GID_FMT ": %m", i->gid);
+                                else if (r > 0) {
+                                        i->gid = candidate;
+                                        i->gid_set = true;
+                                } else
+                                        log_debug("Group ID " GID_FMT " of file for %s already used.", candidate, i->name);
+                        }
+                }
+        }
+
+        /* And if that didn't work either, let's try to find a free one */
+        if (!i->gid_set) {
+                maybe_emit_login_defs_warning(c);
+
+                for (;;) {
+                        /* We look for new GIDs in the UID pool! */
+                        r = uid_range_next_lower(c->uid_range, &c->search_uid);
+                        if (r < 0)
+                                return log_error_errno(r, "No free group ID available for %s.", i->name);
+
+                        r = gid_is_ok(c, c->search_uid, i->name, true);
+                        if (r < 0)
+                                return log_error_errno(r, "Failed to verify GID " GID_FMT ": %m", i->gid);
+                        else if (r > 0)
+                                break;
+                }
+
+                i->gid_set = true;
+                i->gid = c->search_uid;
+        }
+
+        r = ordered_hashmap_ensure_put(&c->todo_gids, NULL, GID_TO_PTR(i->gid), i);
+        if (r == -EEXIST)
+                return log_error_errno(r, "Requested group %s with GID "GID_FMT " to be created is duplicated or conflicts with another user.", i->name, i->gid);
+        if (r == -ENOMEM)
+                return log_oom();
+        if (r < 0)
+                return log_error_errno(r, "Failed to store group %s with GID " GID_FMT " to be created: %m", i->name, i->gid);
+
+        i->todo_group = true;
+        log_info("Creating group '%s' with GID " GID_FMT ".", i->name, i->gid);
+
+        return 0;
+}
+
+static int process_item(Context *c, Item *i) {
+        int r;
+
+        assert(c);
+        assert(i);
+
+        switch (i->type) {
+
+        case ADD_USER: {
+                Item *j = NULL;
+
+                if (!i->gid_set)
+                        j = ordered_hashmap_get(c->groups, i->group_name ?: i->name);
+
+                if (j && j->todo_group) {
+                        /* When a group with the target name is already in queue,
+                         * use the information about the group and do not create
+                         * duplicated group entry. */
+                        i->gid_set = j->gid_set;
+                        i->gid = j->gid;
+                        i->id_set_strict = true;
+                } else if (i->group_name) {
+                        /* When a group name was given instead of a GID and it's
+                         * not in queue, then it must already exist. */
+                        r = get_gid_by_name(c, i->group_name, &i->gid);
+                        if (r < 0)
+                                return log_error_errno(r, "Group %s not found.", i->group_name);
+                        i->gid_set = true;
+                        i->id_set_strict = true;
+                } else {
+                        r = add_group(c, i);
+                        if (r < 0)
+                                return r;
+                }
+
+                return add_user(c, i);
+        }
+
+        case ADD_GROUP:
+                return add_group(c, i);
+
+        default:
+                assert_not_reached();
+        }
+}
+
+static Item* item_free(Item *i) {
+        if (!i)
+                return NULL;
+
+        free(i->name);
+        free(i->group_name);
+        free(i->uid_path);
+        free(i->gid_path);
+        free(i->description);
+        free(i->home);
+        free(i->shell);
+        free(i->filename);
+        return mfree(i);
+}
+
+DEFINE_TRIVIAL_CLEANUP_FUNC(Item*, item_free);
+DEFINE_PRIVATE_HASH_OPS_WITH_VALUE_DESTRUCTOR(item_hash_ops, char, string_hash_func, string_compare_func, Item, item_free);
+
+static Item* item_new(ItemType type, const char *name, const char *filename, unsigned line) {
+        assert(name);
+        assert(!!filename == (line > 0));
+
+        _cleanup_(item_freep) Item *new = new(Item, 1);
+        if (!new)
+                return NULL;
+
+        *new = (Item) {
+                .type = type,
+                .line = line,
+        };
+
+        if (free_and_strdup(&new->name, name) < 0 ||
+            free_and_strdup(&new->filename, filename) < 0)
+                return NULL;
+
+        return TAKE_PTR(new);
+}
+
+static int add_implicit(Context *c) {
+        char *g, **l;
+        int r;
+
+        assert(c);
+
+        /* Implicitly create additional users and groups, if they were listed in "m" lines */
+        ORDERED_HASHMAP_FOREACH_KEY(l, g, c->members) {
+                STRV_FOREACH(m, l)
+                        if (!ordered_hashmap_get(c->users, *m)) {
+                                _cleanup_(item_freep) Item *j =
+                                        item_new(ADD_USER, *m, /* filename= */ NULL, /* line= */ 0);
+                                if (!j)
+                                        return log_oom();
+
+                                r = ordered_hashmap_ensure_put(&c->users, &item_hash_ops, j->name, j);
+                                if (r == -ENOMEM)
+                                        return log_oom();
+                                if (r < 0)
+                                        return log_error_errno(r, "Failed to add implicit user '%s': %m", j->name);
+
+                                log_debug("Adding implicit user '%s' due to m line", j->name);
+                                TAKE_PTR(j);
+                        }
+
+                if (!(ordered_hashmap_get(c->users, g) ||
+                      ordered_hashmap_get(c->groups, g))) {
+                        _cleanup_(item_freep) Item *j =
+                                item_new(ADD_GROUP, g, /* filename= */ NULL, /* line= */ 0);
+                        if (!j)
+                                return log_oom();
+
+                        r = ordered_hashmap_ensure_put(&c->groups, &item_hash_ops, j->name, j);
+                        if (r == -ENOMEM)
+                                return log_oom();
+                        if (r < 0)
+                                return log_error_errno(r, "Failed to add implicit group '%s': %m", j->name);
+
+                        log_debug("Adding implicit group '%s' due to m line", j->name);
+                        TAKE_PTR(j);
+                }
+        }
+
+        return 0;
+}
+
+static int item_equivalent(Item *a, Item *b) {
+        int r;
+
+        assert(a);
+        assert(b);
+
+        if (a->type != b->type) {
+                log_syntax(NULL, LOG_DEBUG, a->filename, a->line, 0,
+                           "Item not equivalent because types differ");
+                return false;
+        }
+
+        if (!streq_ptr(a->name, b->name)) {
+                log_syntax(NULL, LOG_DEBUG, a->filename, a->line, 0,
+                           "Item not equivalent because names differ ('%s' vs. '%s')",
+                           a->name, b->name);
+                return false;
+        }
+
+        /* Paths were simplified previously, so we can use streq. */
+        if (!streq_ptr(a->uid_path, b->uid_path)) {
+                log_syntax(NULL, LOG_DEBUG, a->filename, a->line, 0,
+                           "Item not equivalent because UID paths differ (%s vs. %s)",
+                           a->uid_path ?: "(unset)", b->uid_path ?: "(unset)");
+                return false;
+        }
+
+        if (!streq_ptr(a->gid_path, b->gid_path)) {
+                log_syntax(NULL, LOG_DEBUG, a->filename, a->line, 0,
+                           "Item not equivalent because GID paths differ (%s vs. %s)",
+                           a->gid_path ?: "(unset)", b->gid_path ?: "(unset)");
+                return false;
+        }
+
+        if (!streq_ptr(a->description, b->description))  {
+                log_syntax(NULL, LOG_DEBUG, a->filename, a->line, 0,
+                           "Item not equivalent because descriptions differ ('%s' vs. '%s')",
+                           strempty(a->description), strempty(b->description));
+                return false;
+        }
+
+        if ((a->uid_set != b->uid_set) ||
+            (a->uid_set && a->uid != b->uid)) {
+                log_syntax(NULL, LOG_DEBUG, a->filename, a->line, 0,
+                           "Item not equivalent because UIDs differ (%s vs. %s)",
+                           FORMAT_UID(a->uid_set, a->uid), FORMAT_UID(b->uid_set, b->uid));
+                return false;
+        }
+
+        if ((a->gid_set != b->gid_set) ||
+            (a->gid_set && a->gid != b->gid)) {
+                log_syntax(NULL, LOG_DEBUG, a->filename, a->line, 0,
+                           "Item not equivalent because GIDs differ (%s vs. %s)",
+                           FORMAT_GID(a->gid_set, a->gid), FORMAT_GID(b->gid_set, b->gid));
+                return false;
+        }
+
+        if (!streq_ptr(a->home, b->home)) {
+                log_syntax(NULL, LOG_DEBUG, a->filename, a->line, 0,
+                           "Item not equivalent because home directories differ ('%s' vs. '%s')",
+                           strempty(a->description), strempty(b->description));
+                return false;
+        }
+
+        /* Check if the two paths refer to the same file.
+         * If the paths are equal (after normalization), it's obviously the same file.
+         * If both paths specify a nologin shell, treat them as the same (e.g. /bin/true and /bin/false).
+         * Otherwise, try to resolve the paths, and see if we get the same result, (e.g. /sbin/nologin and
+         * /usr/sbin/nologin).
+         * If we can't resolve something, treat different paths as different. */
+
+        const char *a_shell = pick_shell(a),
+                   *b_shell = pick_shell(b);
+        if (!path_equal_ptr(a_shell, b_shell) &&
+            !(is_nologin_shell(a_shell) && is_nologin_shell(b_shell))) {
+                _cleanup_free_ char *pa = NULL, *pb = NULL;
+
+                r = chase(a_shell, arg_root, CHASE_PREFIX_ROOT | CHASE_NONEXISTENT, &pa, NULL);
+                if (r < 0) {
+                        log_full_errno(ERRNO_IS_RESOURCE(r) ? LOG_ERR : LOG_DEBUG,
+                                       r, "Failed to look up path '%s%s%s': %m",
+                                       strempty(arg_root), arg_root ? "/" : "", a_shell);
+                        return ERRNO_IS_RESOURCE(r) ? r : false;
+                }
+
+                r = chase(b_shell, arg_root, CHASE_PREFIX_ROOT | CHASE_NONEXISTENT, &pb, NULL);
+                if (r < 0) {
+                        log_full_errno(ERRNO_IS_RESOURCE(r) ? LOG_ERR : LOG_DEBUG,
+                                       r, "Failed to look up path '%s%s%s': %m",
+                                       strempty(arg_root), arg_root ? "/" : "", b_shell);
+                        return ERRNO_IS_RESOURCE(r) ? r : false;
+                }
+
+                if (!path_equal(pa, pb)) {
+                        log_syntax(NULL, LOG_DEBUG, a->filename, a->line, 0,
+                                   "Item not equivalent because shells differ ('%s' vs. '%s')",
+                                   pa, pb);
+                        return false;
+                }
+        }
+
+        return true;
+}
+
+static int parse_line(
+                Context *c,
+                const char *fname,
+                unsigned line,
+                const char *buffer) {
+
+        _cleanup_free_ char *action = NULL,
+                *name = NULL, *resolved_name = NULL,
+                *id = NULL, *resolved_id = NULL,
+                *description = NULL, *resolved_description = NULL,
+                *home = NULL, *resolved_home = NULL,
+                *shell = NULL, *resolved_shell = NULL;
+        _cleanup_(item_freep) Item *i = NULL;
+        Item *existing;
+        OrderedHashmap *h;
+        int r;
+        const char *p;
+
+        assert(c);
+        assert(fname);
+        assert(line >= 1);
+        assert(buffer);
+
+        /* Parse columns */
+        p = buffer;
+        r = extract_many_words(&p, NULL, EXTRACT_UNQUOTE,
+                               &action, &name, &id, &description, &home, &shell, NULL);
+        if (r < 0)
+                return log_syntax(NULL, LOG_ERR, fname, line, r, "Syntax error.");
+        if (r < 2)
+                return log_syntax(NULL, LOG_ERR, fname, line, SYNTHETIC_ERRNO(EINVAL),
+                                  "Missing action and name columns.");
+        if (!isempty(p))
+                return log_syntax(NULL, LOG_ERR, fname, line, SYNTHETIC_ERRNO(EINVAL),
+                                  "Trailing garbage.");
+
+        /* Verify action */
+        if (strlen(action) != 1)
+                return log_syntax(NULL, LOG_ERR, fname, line, SYNTHETIC_ERRNO(EINVAL),
+                                  "Unknown modifier '%s'.", action);
+
+        if (!IN_SET(action[0], ADD_USER, ADD_GROUP, ADD_MEMBER, ADD_RANGE))
+                return log_syntax(NULL, LOG_ERR, fname, line, SYNTHETIC_ERRNO(EBADMSG),
+                                  "Unknown command type '%c'.", action[0]);
+
+        /* Verify name */
+        if (empty_or_dash(name))
+                name = mfree(name);
+
+        if (name) {
+                r = specifier_printf(name, NAME_MAX, system_and_tmp_specifier_table, arg_root, NULL, &resolved_name);
+                if (r < 0)
+                        return log_syntax(NULL, LOG_ERR, fname, line, r, "Failed to replace specifiers in '%s': %m", name);
+
+                if (!valid_user_group_name(resolved_name, 0))
+                        return log_syntax(NULL, LOG_ERR, fname, line, SYNTHETIC_ERRNO(EINVAL),
+                                          "'%s' is not a valid user or group name.", resolved_name);
+        }
+
+        /* Verify id */
+        if (empty_or_dash(id))
+                id = mfree(id);
+
+        if (id) {
+                r = specifier_printf(id, PATH_MAX-1, system_and_tmp_specifier_table, arg_root, NULL, &resolved_id);
+                if (r < 0)
+                        return log_syntax(NULL, LOG_ERR, fname, line, r,
+                                          "Failed to replace specifiers in '%s': %m", name);
+        }
+
+        /* Verify description */
+        if (empty_or_dash(description))
+                description = mfree(description);
+
+        if (description) {
+                r = specifier_printf(description, LONG_LINE_MAX, system_and_tmp_specifier_table, arg_root, NULL, &resolved_description);
+                if (r < 0)
+                        return log_syntax(NULL, LOG_ERR, fname, line, r,
+                                          "Failed to replace specifiers in '%s': %m", description);
+
+                if (!valid_gecos(resolved_description))
+                        return log_syntax(NULL, LOG_ERR, fname, line, SYNTHETIC_ERRNO(EINVAL),
+                                          "'%s' is not a valid GECOS field.", resolved_description);
+        }
+
+        /* Verify home */
+        if (empty_or_dash(home))
+                home = mfree(home);
+
+        if (home) {
+                r = specifier_printf(home, PATH_MAX-1, system_and_tmp_specifier_table, arg_root, NULL, &resolved_home);
+                if (r < 0)
+                        return log_syntax(NULL, LOG_ERR, fname, line, r,
+                                          "Failed to replace specifiers in '%s': %m", home);
+
+                path_simplify(resolved_home);
+
+                if (!valid_home(resolved_home))
+                        return log_syntax(NULL, LOG_ERR, fname, line, SYNTHETIC_ERRNO(EINVAL),
+                                          "'%s' is not a valid home directory field.", resolved_home);
+        }
+
+        /* Verify shell */
+        if (empty_or_dash(shell))
+                shell = mfree(shell);
+
+        if (shell) {
+                r = specifier_printf(shell, PATH_MAX-1, system_and_tmp_specifier_table, arg_root, NULL, &resolved_shell);
+                if (r < 0)
+                        return log_syntax(NULL, LOG_ERR, fname, line, r,
+                                          "Failed to replace specifiers in '%s': %m", shell);
+
+                path_simplify(resolved_shell);
+
+                if (!valid_shell(resolved_shell))
+                        return log_syntax(NULL, LOG_ERR, fname, line, SYNTHETIC_ERRNO(EINVAL),
+                                          "'%s' is not a valid login shell field.", resolved_shell);
+        }
+
+        switch (action[0]) {
+
+        case ADD_RANGE:
+                if (resolved_name)
+                        return log_syntax(NULL, LOG_ERR, fname, line, SYNTHETIC_ERRNO(EINVAL),
+                                          "Lines of type 'r' don't take a name field.");
+
+                if (!resolved_id)
+                        return log_syntax(NULL, LOG_ERR, fname, line, SYNTHETIC_ERRNO(EINVAL),
+                                          "Lines of type 'r' require an ID range in the third field.");
+
+                if (description || home || shell)
+                        return log_syntax(NULL, LOG_ERR, fname, line, SYNTHETIC_ERRNO(EINVAL),
+                                          "Lines of type '%c' don't take a %s field.",
+                                          action[0],
+                                          description ? "GECOS" : home ? "home directory" : "login shell");
+
+                r = uid_range_add_str(&c->uid_range, resolved_id);
+                if (r < 0)
+                        return log_syntax(NULL, LOG_ERR, fname, line, SYNTHETIC_ERRNO(EINVAL),
+                                          "Invalid UID range %s.", resolved_id);
+
+                return 0;
+
+        case ADD_MEMBER: {
+                /* Try to extend an existing member or group item */
+                if (!name)
+                        return log_syntax(NULL, LOG_ERR, fname, line, SYNTHETIC_ERRNO(EINVAL),
+                                          "Lines of type 'm' require a user name in the second field.");
+
+                if (!resolved_id)
+                        return log_syntax(NULL, LOG_ERR, fname, line, SYNTHETIC_ERRNO(EINVAL),
+                                          "Lines of type 'm' require a group name in the third field.");
+
+                if (!valid_user_group_name(resolved_id, 0))
+                        return log_syntax(NULL, LOG_ERR, fname, line, SYNTHETIC_ERRNO(EINVAL),
+                                               "'%s' is not a valid user or group name.", resolved_id);
+
+                if (description || home || shell)
+                        return log_syntax(NULL, LOG_ERR, fname, line, SYNTHETIC_ERRNO(EINVAL),
+                                          "Lines of type '%c' don't take a %s field.",
+                                          action[0],
+                                          description ? "GECOS" : home ? "home directory" : "login shell");
+
+                r = string_strv_ordered_hashmap_put(&c->members, resolved_id, resolved_name);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to store mapping for %s: %m", resolved_id);
+
+                return 0;
+        }
+
+        case ADD_USER:
+                if (!name)
+                        return log_syntax(NULL, LOG_ERR, fname, line, SYNTHETIC_ERRNO(EINVAL),
+                                          "Lines of type 'u' require a user name in the second field.");
+
+                r = ordered_hashmap_ensure_allocated(&c->users, &item_hash_ops);
+                if (r < 0)
+                        return log_oom();
+
+                i = item_new(ADD_USER, resolved_name, fname, line);
+                if (!i)
+                        return log_oom();
+
+                if (resolved_id) {
+                        if (path_is_absolute(resolved_id))
+                                i->uid_path = path_simplify(TAKE_PTR(resolved_id));
+                        else {
+                                _cleanup_free_ char *uid = NULL, *gid = NULL;
+                                if (split_pair(resolved_id, ":", &uid, &gid) == 0) {
+                                        r = parse_gid(gid, &i->gid);
+                                        if (r < 0) {
+                                                if (valid_user_group_name(gid, 0))
+                                                        i->group_name = TAKE_PTR(gid);
+                                                else
+                                                        return log_syntax(NULL, LOG_ERR, fname, line, r,
+                                                                          "Failed to parse GID: '%s': %m", id);
+                                        } else {
+                                                i->gid_set = true;
+                                                i->id_set_strict = true;
+                                        }
+                                        free_and_replace(resolved_id, uid);
+                                }
+                                if (!streq(resolved_id, "-")) {
+                                        r = parse_uid(resolved_id, &i->uid);
+                                        if (r < 0)
+                                                return log_syntax(NULL, LOG_ERR, fname, line, r,
+                                                                  "Failed to parse UID: '%s': %m", id);
+                                        i->uid_set = true;
+                                }
+                        }
+                }
+
+                i->description = TAKE_PTR(resolved_description);
+                i->home = TAKE_PTR(resolved_home);
+                i->shell = TAKE_PTR(resolved_shell);
+
+                h = c->users;
+                break;
+
+        case ADD_GROUP:
+                if (!name)
+                        return log_syntax(NULL, LOG_ERR, fname, line, SYNTHETIC_ERRNO(EINVAL),
+                                          "Lines of type 'g' require a user name in the second field.");
+
+                if (description || home || shell)
+                        return log_syntax(NULL, LOG_ERR, fname, line, SYNTHETIC_ERRNO(EINVAL),
+                                          "Lines of type '%c' don't take a %s field.",
+                                          action[0],
+                                          description ? "GECOS" : home ? "home directory" : "login shell");
+
+                r = ordered_hashmap_ensure_allocated(&c->groups, &item_hash_ops);
+                if (r < 0)
+                        return log_oom();
+
+                i = item_new(ADD_GROUP, resolved_name, fname, line);
+                if (!i)
+                        return log_oom();
+
+                if (resolved_id) {
+                        if (path_is_absolute(resolved_id))
+                                i->gid_path = path_simplify(TAKE_PTR(resolved_id));
+                        else {
+                                r = parse_gid(resolved_id, &i->gid);
+                                if (r < 0)
+                                        return log_syntax(NULL, LOG_ERR, fname, line, r,
+                                                          "Failed to parse GID: '%s': %m", id);
+
+                                i->gid_set = true;
+                        }
+                }
+
+                h = c->groups;
+                break;
+
+        default:
+                assert_not_reached();
+        }
+
+        existing = ordered_hashmap_get(h, i->name);
+        if (existing) {
+                /* Two functionally-equivalent items are fine */
+                r = item_equivalent(i, existing);
+                if (r < 0)
+                        return r;
+                if (r == 0) {
+                        if (existing->filename)
+                                log_syntax(NULL, LOG_WARNING, fname, line, 0,
+                                           "Conflict with earlier configuration for %s '%s' in %s:%u, ignoring line.",
+                                           item_type_to_string(i->type),
+                                           i->name,
+                                           existing->filename, existing->line);
+                        else
+                                log_syntax(NULL, LOG_WARNING, fname, line, 0,
+                                           "Conflict with earlier configuration for %s '%s', ignoring line.",
+                                           item_type_to_string(i->type),
+                                           i->name);
+                }
+
+                return 0;
+        }
+
+        r = ordered_hashmap_put(h, i->name, i);
+        if (r < 0)
+                return log_oom();
+
+        i = NULL;
+        return 0;
+}
+
+static int read_config_file(Context *c, const char *fn, bool ignore_enoent) {
+        _cleanup_fclose_ FILE *rf = NULL;
+        _cleanup_free_ char *pp = NULL;
+        FILE *f = NULL;
+        unsigned v = 0;
+        int r = 0;
+
+        assert(c);
+        assert(fn);
+
+        if (streq(fn, "-"))
+                f = stdin;
+        else {
+                r = search_and_fopen(fn, "re", arg_root, (const char**) CONF_PATHS_STRV("sysusers.d"), &rf, &pp);
+                if (r < 0) {
+                        if (ignore_enoent && r == -ENOENT)
+                                return 0;
+
+                        return log_error_errno(r, "Failed to open '%s', ignoring: %m", fn);
+                }
+
+                f = rf;
+                fn = pp;
+        }
+
+        for (;;) {
+                _cleanup_free_ char *line = NULL;
+                int k;
+
+                k = read_stripped_line(f, LONG_LINE_MAX, &line);
+                if (k < 0)
+                        return log_error_errno(k, "Failed to read '%s': %m", fn);
+                if (k == 0)
+                        break;
+
+                v++;
+
+                if (IN_SET(line[0], 0, '#'))
+                        continue;
+
+                k = parse_line(c, fn, v, line);
+                if (k < 0 && r == 0)
+                        r = k;
+        }
+
+        if (ferror(f)) {
+                log_error_errno(errno, "Failed to read from file %s: %m", fn);
+                if (r == 0)
+                        r = -EIO;
+        }
+
+        return r;
+}
+
+static int cat_config(void) {
+        _cleanup_strv_free_ char **files = NULL;
+        int r;
+
+        r = conf_files_list_with_replacement(arg_root, CONF_PATHS_STRV("sysusers.d"), arg_replace, &files, NULL);
+        if (r < 0)
+                return r;
+
+        pager_open(arg_pager_flags);
+
+        return cat_files(NULL, files, arg_cat_flags);
+}
+
+static int help(void) {
+        _cleanup_free_ char *link = NULL;
+        int r;
+
+        r = terminal_urlify_man("systemd-sysusers.service", "8", &link);
+        if (r < 0)
+                return log_oom();
+
+        printf("%s [OPTIONS...] [CONFIGURATION FILE...]\n\n"
+               "Creates system user accounts.\n\n"
+               "  -h --help                 Show this help\n"
+               "     --version              Show package version\n"
+               "     --cat-config           Show configuration files\n"
+               "     --tldr                 Show non-comment parts of configuration\n"
+               "     --root=PATH            Operate on an alternate filesystem root\n"
+               "     --image=PATH           Operate on disk image as filesystem root\n"
+               "     --image-policy=POLICY  Specify disk image dissection policy\n"
+               "     --replace=PATH         Treat arguments as replacement for PATH\n"
+               "     --dry-run              Just print what would be done\n"
+               "     --inline               Treat arguments as configuration lines\n"
+               "     --no-pager             Do not pipe output into a pager\n"
+               "\nSee the %s for details.\n",
+               program_invocation_short_name,
+               link);
+
+        return 0;
+}
+
+static int parse_argv(int argc, char *argv[]) {
+
+        enum {
+                ARG_VERSION = 0x100,
+                ARG_CAT_CONFIG,
+                ARG_TLDR,
+                ARG_ROOT,
+                ARG_IMAGE,
+                ARG_IMAGE_POLICY,
+                ARG_REPLACE,
+                ARG_DRY_RUN,
+                ARG_INLINE,
+                ARG_NO_PAGER,
+        };
+
+        static const struct option options[] = {
+                { "help",         no_argument,       NULL, 'h'              },
+                { "version",      no_argument,       NULL, ARG_VERSION      },
+                { "cat-config",   no_argument,       NULL, ARG_CAT_CONFIG   },
+                { "tldr",         no_argument,       NULL, ARG_TLDR         },
+                { "root",         required_argument, NULL, ARG_ROOT         },
+                { "image",        required_argument, NULL, ARG_IMAGE        },
+                { "image-policy", required_argument, NULL, ARG_IMAGE_POLICY },
+                { "replace",      required_argument, NULL, ARG_REPLACE      },
+                { "dry-run",      no_argument,       NULL, ARG_DRY_RUN      },
+                { "inline",       no_argument,       NULL, ARG_INLINE       },
+                { "no-pager",     no_argument,       NULL, ARG_NO_PAGER     },
+                {}
+        };
+
+        int c, r;
+
+        assert(argc >= 0);
+        assert(argv);
+
+        while ((c = getopt_long(argc, argv, "h", options, NULL)) >= 0)
+
+                switch (c) {
+
+                case 'h':
+                        return help();
+
+                case ARG_VERSION:
+                        return version();
+
+                case ARG_CAT_CONFIG:
+                        arg_cat_flags = CAT_CONFIG_ON;
+                        break;
+
+                case ARG_TLDR:
+                        arg_cat_flags = CAT_TLDR;
+                        break;
+
+                case ARG_ROOT:
+                        r = parse_path_argument(optarg, /* suppress_root= */ false, &arg_root);
+                        if (r < 0)
+                                return r;
+                        break;
+
+                case ARG_IMAGE:
+#ifdef STANDALONE
+                        return log_error_errno(SYNTHETIC_ERRNO(EOPNOTSUPP),
+                                               "This systemd-sysusers version is compiled without support for --image=.");
+#else
+                        r = parse_path_argument(optarg, /* suppress_root= */ false, &arg_image);
+                        if (r < 0)
+                                return r;
+                        break;
+#endif
+
+                case ARG_IMAGE_POLICY:
+                        r = parse_image_policy_argument(optarg, &arg_image_policy);
+                        if (r < 0)
+                                return r;
+                        break;
+
+                case ARG_REPLACE:
+                        if (!path_is_absolute(optarg))
+                                return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
+                                                       "The argument to --replace= must be an absolute path.");
+                        if (!endswith(optarg, ".conf"))
+                                return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
+                                                       "The argument to --replace= must have the extension '.conf'.");
+
+                        arg_replace = optarg;
+                        break;
+
+                case ARG_DRY_RUN:
+                        arg_dry_run = true;
+                        break;
+
+                case ARG_INLINE:
+                        arg_inline = true;
+                        break;
+
+                case ARG_NO_PAGER:
+                        arg_pager_flags |= PAGER_DISABLE;
+                        break;
+
+                case '?':
+                        return -EINVAL;
+
+                default:
+                        assert_not_reached();
+                }
+
+        if (arg_replace && arg_cat_flags != CAT_CONFIG_OFF)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
+                                       "Option --replace= is not supported with --cat-config/--tldr.");
+
+        if (arg_replace && optind >= argc)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
+                                       "When --replace= is given, some configuration items must be specified.");
+
+        if (arg_image && arg_root)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
+                                       "Use either --root= or --image=, the combination of both is not supported.");
+
+        return 1;
+}
+
+static int parse_arguments(Context *c, char **args) {
+        unsigned pos = 1;
+        int r;
+
+        assert(c);
+
+        STRV_FOREACH(arg, args) {
+                if (arg_inline)
+                        /* Use (argument):n, where n==1 for the first positional arg */
+                        r = parse_line(c, "(argument)", pos, *arg);
+                else
+                        r = read_config_file(c, *arg, /* ignore_enoent= */ false);
+                if (r < 0)
+                        return r;
+
+                pos++;
+        }
+
+        return 0;
+}
+
+static int read_config_files(Context *c, char **args) {
+        _cleanup_strv_free_ char **files = NULL;
+        _cleanup_free_ char *p = NULL;
+        int r;
+
+        assert(c);
+
+        r = conf_files_list_with_replacement(arg_root, CONF_PATHS_STRV("sysusers.d"), arg_replace, &files, &p);
+        if (r < 0)
+                return r;
+
+        STRV_FOREACH(f, files)
+                if (p && path_equal(*f, p)) {
+                        log_debug("Parsing arguments at position \"%s\"%s", *f, special_glyph(SPECIAL_GLYPH_ELLIPSIS));
+
+                        r = parse_arguments(c, args);
+                        if (r < 0)
+                                return r;
+                } else {
+                        log_debug("Reading config file \"%s\"%s", *f, special_glyph(SPECIAL_GLYPH_ELLIPSIS));
+
+                        /* Just warn, ignore result otherwise */
+                        (void) read_config_file(c, *f, /* ignore_enoent= */ true);
+                }
+
+        return 0;
+}
+
+static int read_credential_lines(Context *c) {
+        _cleanup_free_ char *j = NULL;
+        const char *d;
+        int r;
+
+        assert(c);
+
+        r = get_credentials_dir(&d);
+        if (r == -ENXIO)
+                return 0;
+        if (r < 0)
+                return log_error_errno(r, "Failed to get credentials directory: %m");
+
+        j = path_join(d, "sysusers.extra");
+        if (!j)
+                return log_oom();
+
+        (void) read_config_file(c, j, /* ignore_enoent= */ true);
+        return 0;
+}
+
+static int run(int argc, char *argv[]) {
+#ifndef STANDALONE
+        _cleanup_(loop_device_unrefp) LoopDevice *loop_device = NULL;
+        _cleanup_(umount_and_freep) char *mounted_dir = NULL;
+#endif
+        _cleanup_close_ int lock = -EBADF;
+        _cleanup_(context_done) Context c = {
+                .search_uid = UID_INVALID,
+        };
+
+        Item *i;
+        int r;
+
+        r = parse_argv(argc, argv);
+        if (r <= 0)
+                return r;
+
+        log_setup();
+
+        if (arg_cat_flags != CAT_CONFIG_OFF)
+                return cat_config();
+
+        umask(0022);
+
+        r = mac_init();
+        if (r < 0)
+                return r;
+
+#ifndef STANDALONE
+        if (arg_image) {
+                assert(!arg_root);
+
+                r = mount_image_privately_interactively(
+                                arg_image,
+                                arg_image_policy,
+                                DISSECT_IMAGE_GENERIC_ROOT |
+                                DISSECT_IMAGE_REQUIRE_ROOT |
+                                DISSECT_IMAGE_VALIDATE_OS |
+                                DISSECT_IMAGE_RELAX_VAR_CHECK |
+                                DISSECT_IMAGE_FSCK |
+                                DISSECT_IMAGE_GROWFS,
+                                &mounted_dir,
+                                /* ret_dir_fd= */ NULL,
+                                &loop_device);
+                if (r < 0)
+                        return r;
+
+                arg_root = strdup(mounted_dir);
+                if (!arg_root)
+                        return log_oom();
+        }
+#else
+        assert(!arg_image);
+#endif
+
+        /* If command line arguments are specified along with --replace, read all configuration files and
+         * insert the positional arguments at the specified place. Otherwise, if command line arguments are
+         * specified, execute just them, and finally, without --replace= or any positional arguments, just
+         * read configuration and execute it. */
+        if (arg_replace || optind >= argc)
+                r = read_config_files(&c, argv + optind);
+        else
+                r = parse_arguments(&c, argv + optind);
+        if (r < 0)
+                return r;
+
+        r = read_credential_lines(&c);
+        if (r < 0)
+                return r;
+
+        /* Let's tell nss-systemd not to synthesize the "root" and "nobody" entries for it, so that our
+         * detection whether the names or UID/GID area already used otherwise doesn't get confused. After
+         * all, even though nss-systemd synthesizes these users/groups, they should still appear in
+         * /etc/passwd and /etc/group, as the synthesizing logic is merely supposed to be fallback for cases
+         * where we run with a completely unpopulated /etc. */
+        if (setenv("SYSTEMD_NSS_BYPASS_SYNTHETIC", "1", 1) < 0)
+                return log_error_errno(errno, "Failed to set SYSTEMD_NSS_BYPASS_SYNTHETIC environment variable: %m");
+
+        if (!c.uid_range) {
+                /* Default to default range of SYSTEMD_UID_MIN..SYSTEM_UID_MAX. */
+                r = read_login_defs(&c.login_defs, NULL, arg_root);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to read %s%s: %m",
+                                               strempty(arg_root), "/etc/login.defs");
+
+                c.login_defs_need_warning = true;
+
+                /* We pick a range that very conservative: we look at compiled-in maximum and the value in
+                 * /etc/login.defs. That way the UIDs/GIDs which we allocate will be interpreted correctly,
+                 * even if /etc/login.defs is removed later. (The bottom bound doesn't matter much, since
+                 * it's only used during allocation, so we use the configured value directly). */
+                uid_t begin = c.login_defs.system_alloc_uid_min,
+                      end = MIN3((uid_t) SYSTEM_UID_MAX, c.login_defs.system_uid_max, c.login_defs.system_gid_max);
+                if (begin < end) {
+                        r = uid_range_add(&c.uid_range, begin, end - begin + 1);
+                        if (r < 0)
+                                return log_oom();
+                }
+        }
+
+        r = add_implicit(&c);
+        if (r < 0)
+                return r;
+
+        if (!arg_dry_run) {
+                lock = take_etc_passwd_lock(arg_root);
+                if (lock < 0)
+                        return log_error_errno(lock, "Failed to take /etc/passwd lock: %m");
+        }
+
+        r = load_user_database(&c);
+        if (r < 0)
+                return log_error_errno(r, "Failed to load user database: %m");
+
+        r = load_group_database(&c);
+        if (r < 0)
+                return log_error_errno(r, "Failed to read group database: %m");
+
+        ORDERED_HASHMAP_FOREACH(i, c.groups)
+                (void) process_item(&c, i);
+
+        ORDERED_HASHMAP_FOREACH(i, c.users)
+                (void) process_item(&c, i);
+
+        return write_files(&c);
+}
+
+DEFINE_MAIN_FUNCTION(run);