fstab-generator: Support root on tmpfs (or other deviceless FS)

This allows for stateless systems.

src/fstab-generator/fstab-generator.c

@@ -397,15 +397,20 @@ static int add_root_mount(void) {
         _cleanup_free_ char *what = NULL;
         const char *opts;
 
-        if (isempty(arg_root_what)) {
+        if (fstype_is_deviceless(arg_root_fstype)) {
-                log_debug("Could not find a root= entry on the kernel command line.");
+                if (free_and_strdup(&what, arg_root_what) < 0)
-                return 0;
+                        return log_oom();
-        }
+        } else {
+                if (isempty(arg_root_what)) {
+                        log_debug("Could not find a root= entry on the kernel command line.");
+                        return 0;
+                }
 
-        what = fstab_node_to_udev_node(arg_root_what);
+                what = fstab_node_to_udev_node(arg_root_what);
-        if (!path_is_absolute(what)) {
+                if (!path_is_absolute(what)) {
-                log_debug("Skipping entry what=%s where=/sysroot type=%s", what, strna(arg_root_fstype));
+                        log_debug("Skipping entry what=%s where=/sysroot type=%s", what, strna(arg_root_fstype));
-                return 0;
+                        return 0;
+                }
         }
 
         if (!arg_root_options)

src/shared/util.c

@@ -1713,6 +1713,35 @@ bool fstype_is_network(const char *fstype) {
         return nulstr_contains(table, fstype);
 }
 
+bool fstype_is_deviceless(const char *fstype) {
+        static const char table[] =
+                "autofs\0"
+                "bdev\0"
+                "cgroup\0"
+                "configfs\0"
+                "cpuset\0"
+                "debugfs\0"
+                "devpts\0"
+                "devtmpfs\0"
+                "efivarfs\0"
+                "hugetlbfs\0"
+                "mqueue\0"
+                "overlayfs\0"
+                "pipefs\0"
+                "proc\0"
+                "pstore\0"
+                "ramfs\0"
+                "rootfs\0"
+                "rpc_pipefs\0"
+                "securityfs\0"
+                "sockfs\0"
+                "sysfs\0"
+                "tmpfs\0";
+
+        return !isempty(fstype) && (
+                nulstr_contains(table, fstype) || fstype_is_network(fstype));
+}
+
 int chvt(int vt) {
         _cleanup_close_ int fd;
 

src/shared/util.h

@@ -409,6 +409,7 @@ int fd_cloexec(int fd, bool cloexec);
 int close_all_fds(const int except[], unsigned n_except);
 
 bool fstype_is_network(const char *fstype);
+bool fstype_is_deviceless(const char *fstype);
 
 int chvt(int vt);