sysusers: look at login.defs when setting the default range to allocate users

Also, even if login.defs are not present, don't start allocating at 1, but at
SYSTEM_UID_MIN.

Fixes #9769.

The test is adjusted. Actually, it was busted before, because sysusers would
never use SYSTEM_GID_MIN, so if SYSTEM_GID_MIN was different than
SYSTEM_UID_MIN, the tests would fail. On all "normal" systems the two are
equal, so we didn't notice. Since sysusers now always uses the minimum of the
two, we only need to substitute one value.

src/sysusers/sysusers.c

@@ -26,6 +26,7 @@
 #include "strv.h"
 #include "tmpfile-util-label.h"
 #include "uid-range.h"
+#include "user-record.h"
 #include "user-util.h"
 #include "utf8.h"
 #include "util.h"
@@ -1949,10 +1950,25 @@ static int run(int argc, char *argv[]) {
                 return log_error_errno(errno, "Failed to set SYSTEMD_NSS_BYPASS_SYNTHETIC environment variable: %m");
 
         if (!uid_range) {
-                /* Default to default range of 1..SYSTEM_UID_MAX */
-                r = uid_range_add(&uid_range, &n_uid_range, 1, SYSTEM_UID_MAX);
+                /* Default to default range of SYSTEMD_UID_MIN..SYSTEM_UID_MAX. */
+                UGIDAllocationRange defs;
+
+                r = read_login_defs(&defs, NULL, arg_root);
                 if (r < 0)
-                        return log_oom();
+                        return log_error_errno(r, "Failed to read %s%s: %m",
+                                               strempty(arg_root), "/etc/login.defs");
+
+                /* 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 = defs.system_alloc_uid_min,
+                      end = MIN3((uid_t) SYSTEM_UID_MAX, defs.system_uid_max, defs.system_gid_max);
+                if (begin < end) {
+                        r = uid_range_add(&uid_range, &n_uid_range, begin, end - begin + 1);
+                        if (r < 0)
+                                return log_oom();
+                }
         }
 
         r = add_implicit();