firstboot: Add --reset option

This can be used to prepare an image for firstboot by removing all
files that systemd knows about that contain machine specific
information.

src/firstboot/firstboot.c

@@ -72,6 +72,7 @@ static bool arg_force = false;
 static bool arg_delete_root_password = false;
 static bool arg_root_password_is_hashed = false;
 static bool arg_welcome = true;
+static bool arg_reset = false;
 
 STATIC_DESTRUCTOR_REGISTER(arg_root, freep);
 STATIC_DESTRUCTOR_REGISTER(arg_image, freep);
@@ -1089,6 +1090,48 @@ static int process_kernel_cmdline(int rfd) {
         return 0;
 }
 
+static int reset_one(int rfd, const char *path) {
+        _cleanup_close_ int pfd = -EBADF;
+        _cleanup_free_ char *f = NULL;
+
+        assert(rfd >= 0);
+        assert(path);
+
+        pfd = chase_and_open_parent_at(rfd, path, CHASE_AT_RESOLVE_IN_ROOT|CHASE_WARN|CHASE_NOFOLLOW, &f);
+        if (pfd == -ENOENT)
+                return 0;
+        if (pfd < 0)
+                return log_error_errno(pfd, "Failed to resolve %s: %m", path);
+
+        if (unlinkat(pfd, f, 0) < 0)
+                return errno == ENOENT ? 0 : log_error_errno(errno, "Failed to remove %s: %m", path);
+
+        log_info("Removed %s", path);
+        return 0;
+}
+
+static int process_reset(int rfd) {
+        int r;
+
+        assert(rfd >= 0);
+
+        if (!arg_reset)
+                return 0;
+
+        FOREACH_STRING(p,
+                       "/etc/locale.conf",
+                       "/etc/vconsole.conf",
+                       "/etc/hostname",
+                       "/etc/machine-id",
+                       "/etc/kernel/cmdline") {
+                r = reset_one(rfd, p);
+                if (r < 0)
+                        return r;
+        }
+
+        return 0;
+}
+
 static int help(void) {
         _cleanup_free_ char *link = NULL;
         int r;
@@ -1130,6 +1173,7 @@ static int help(void) {
                "     --force                      Overwrite existing files\n"
                "     --delete-root-password       Delete root password\n"
                "     --welcome=no                 Disable the welcome text\n"
+               "     --reset                      Remove existing files\n"
                "\nSee the %s for details.\n",
                program_invocation_short_name,
                link);
@@ -1171,6 +1215,7 @@ static int parse_argv(int argc, char *argv[]) {
                 ARG_FORCE,
                 ARG_DELETE_ROOT_PASSWORD,
                 ARG_WELCOME,
+                ARG_RESET,
         };
 
         static const struct option options[] = {
@@ -1206,6 +1251,7 @@ static int parse_argv(int argc, char *argv[]) {
                 { "force",                   no_argument,       NULL, ARG_FORCE                   },
                 { "delete-root-password",    no_argument,       NULL, ARG_DELETE_ROOT_PASSWORD    },
                 { "welcome",                 required_argument, NULL, ARG_WELCOME                 },
+                { "reset",                   no_argument,       NULL, ARG_RESET                   },
                 {}
         };
 
@@ -1408,6 +1454,10 @@ static int parse_argv(int argc, char *argv[]) {
                         arg_welcome = r;
                         break;
 
+                case ARG_RESET:
+                        arg_reset = true;
+                        break;
+
                 case '?':
                         return -EINVAL;
 
@@ -1497,6 +1547,10 @@ static int run(int argc, char *argv[]) {
                         return r;
         }
 
+        r = process_reset(rfd);
+        if (r < 0)
+                return r;
+
         r = process_locale(rfd);
         if (r < 0)
                 return r;