sysusers: allow admin/runtime overrides to command-line config

When used in a package installation script, we want to invoke systemd-sysusers
before that package is installed (so it can contain files owned by the newly
created user), so the configuration to use is specified on the command
line. This should be a copy of the configuration that will be installed as
/usr/lib/sysusers.d/package.conf. We still want to obey any overrides in
/etc/sysusers.d or /run/sysusers.d in the usual fashion. Otherwise, we'd get a
different result when systemd-sysusers is run with a copy of the new config on
the command line and when systemd-sysusers is run at boot after package
instalation. In the second case any files in /etc or /run have higher priority,
so the same should happen when the configuration is given on the command line.
More generally, we want the behaviour in this special case to be as close to
the case where the file is finally on disk as possible, so we have to read all
configuration files, since they all might contain overrides and additional
configuration that matters. Even files that have lower priority might specify
additional groups for the user we are creating. Thus, we need to read all
configuration, but insert our new configuration somewhere with the right
priority.

If --target=/path/to/file.conf is given on the command line, we gather the list
of files, and pretend that the command-line config is read from
/path/to/file.conf (doesn't matter if the file on disk actually exists or
not). All package scripts should use this option to obtain consistent and
idempotent behaviour.

The corner case when --target= is specified and there are no positional
arguments is disallowed.

v1:
- version with --config-name=
v2:
- disallow --config-name= and no positional args
v3:
- remove --config-name=
v4:
- add --target= and rework the code completely
v5:
- fix argcounting bug and add example in man page
v6:
- rename --target to --replace

1 files changed, 89 insertions, 29 deletions

diff --git a/src/sysusers/sysusers.c b/src/sysusers/sysusers.c
index e2a3b9968c..af21e3b854 100644
--- a/src/sysusers/sysusers.c
+++ b/src/sysusers/sysusers.c
@@ -75,6 +75,7 @@ typedef struct Item {
 } Item;
 
 static char *arg_root = NULL;
+static const char *arg_replace = NULL;
 static bool arg_inline = false;
 
 static const char conf_file_dirs[] = CONF_PATHS_NULSTR("sysusers.d");
@@ -1746,6 +1747,7 @@ static void help(void) {
                "  -h --help                 Show this help\n"
                "     --version              Show package version\n"
                "     --root=PATH            Operate on an alternate filesystem root\n"
+               "     --replace=PATH         Treat arguments as replacement for PATH\n"
                "     --inline               Treat arguments as configuration lines\n"
                , program_invocation_short_name);
 }
@@ -1755,6 +1757,7 @@ static int parse_argv(int argc, char *argv[]) {
         enum {
                 ARG_VERSION = 0x100,
                 ARG_ROOT,
+                ARG_REPLACE,
                 ARG_INLINE,
         };
 
@@ -1762,6 +1765,7 @@ static int parse_argv(int argc, char *argv[]) {
                 { "help",    no_argument,       NULL, 'h'         },
                 { "version", no_argument,       NULL, ARG_VERSION },
                 { "root",    required_argument, NULL, ARG_ROOT    },
+                { "replace", required_argument, NULL, ARG_REPLACE },
                 { "inline",  no_argument,       NULL, ARG_INLINE  },
                 {}
         };
@@ -1788,6 +1792,16 @@ static int parse_argv(int argc, char *argv[]) {
                                 return r;
                         break;
 
+                case ARG_REPLACE:
+                        if (!path_is_absolute(optarg) ||
+                            !endswith(optarg, ".conf")) {
+                                log_error("The argument to --replace= must an absolute path to a config file");
+                                return -EINVAL;
+                        }
+
+                        arg_replace = optarg;
+                        break;
+
                 case ARG_INLINE:
                         arg_inline = true;
                         break;
@@ -1799,14 +1813,76 @@ static int parse_argv(int argc, char *argv[]) {
                         assert_not_reached("Unhandled option");
                 }
 
+        if (arg_replace && optind >= argc) {
+                log_error("When --replace= is given, some configuration items must be specified");
+                return -EINVAL;
+        }
+
         return 1;
 }
 
+static int parse_arguments(char **args) {
+        char **arg;
+        unsigned pos = 1;
+        int r;
+
+        STRV_FOREACH(arg, args) {
+                if (arg_inline)
+                        /* Use (argument):n, where n==1 for the first positional arg */
+                        r = parse_line("(argument)", pos, *arg);
+                else
+                        r = read_config_file(*arg, false);
+                if (r < 0)
+                        return r;
+
+                pos++;
+        }
+
+        return 0;
+}
+
+static int read_config_files(const char* dirs, char **args) {
+        _cleanup_strv_free_ char **files = NULL;
+        _cleanup_free_ char *p = NULL;
+        char **f;
+        int r;
+
+        r = conf_files_list_nulstr(&files, ".conf", arg_root, 0, dirs);
+        if (r < 0)
+                return log_error_errno(r, "Failed to enumerate sysusers.d files: %m");
+
+        if (arg_replace) {
+                r = conf_files_insert(&files, arg_root, dirs, arg_replace);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to extend sysusers.d file list: %m");
+
+                p = path_join(arg_root, arg_replace, NULL);
+                if (!p)
+                        return log_oom();
+        }
+
+        STRV_FOREACH(f, files)
+                if (p && path_equal(*f, p)) {
+                        log_debug("Parsing arguments at position \"%s\"…", *f);
+
+                        r = parse_arguments(args);
+                        if (r < 0)
+                                return r;
+                } else {
+                        log_debug("Reading config file \"%s\"…", *f);
+
+                        /* Just warn, ignore result otherwise */
+                        (void) read_config_file(*f, true);
+                }
+
+        return 0;
+}
+
 int main(int argc, char *argv[]) {
 
         _cleanup_close_ int lock = -1;
         Iterator iterator;
-        int r, k;
+        int r;
         Item *i;
         char *n;
 
@@ -1826,34 +1902,18 @@ int main(int argc, char *argv[]) {
                 goto finish;
         }
 
-        if (optind < argc) {
-                int j;
-
-                for (j = optind; j < argc; j++) {
-                        if (arg_inline)
-                                /* Use (argument):n, where n==1 for the first positional arg */
-                                r = parse_line("(argument)", j - optind + 1, argv[j]);
-                        else
-                                r = read_config_file(argv[j], false);
-                        if (r < 0)
-                                goto finish;
-                }
-        } else {
-                _cleanup_strv_free_ char **files = NULL;
-                char **f;
-
-                r = conf_files_list_nulstr(&files, ".conf", arg_root, 0, conf_file_dirs);
-                if (r < 0) {
-                        log_error_errno(r, "Failed to enumerate sysusers.d files: %m");
-                        goto finish;
-                }
-
-                STRV_FOREACH(f, files) {
-                        k = read_config_file(*f, true);
-                        if (k < 0 && r == 0)
-                                r = k;
-                }
-        }
+        /* 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(conf_file_dirs, argv + optind);
+        else
+                r = parse_arguments(argv + optind);
+        if (r < 0)
+                goto finish;
 
         /* 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