fstab-generator: set mode=0755 with root=tmpfs

If mode= is not set in rootflags= add mode=0755 when a tmpfs
is used on the rootfs, otherwise it will be group/world writable
as that's the default mode for tmpfs filesystems.

Follow-up for 725ad3b

(cherry picked from commit d3a57a0)
(cherry picked from commit 0e8f13faf3fafc3679131713915a56c74403f3a3)
(cherry picked from commit 69beb94574ba41b167396fdcd651e9f59f2d8d75)
(cherry picked from commit 0fc39fe)
(cherry picked from commit e1560b3)

Author: bluca

src/fstab-generator/fstab-generator.c

@@ -840,7 +840,7 @@ static int sysroot_is_nfsroot(void) {
 
 static int add_sysroot_mount(void) {
         _cleanup_free_ char *what = NULL;
-        const char *opts, *fstype;
+        const char *extra_opts = NULL, *fstype;
         bool default_rw, makefs;
         MountPointFlags flags;
         int r;
@@ -890,6 +890,9 @@ static int add_sysroot_mount(void) {
                 fstype = arg_root_fstype ?: "tmpfs"; /* tmpfs, unless overridden */
 
                 default_rw = true; /* writable, unless overridden */;
+
+                if (streq(fstype, "tmpfs") && !fstab_test_option(arg_root_options, "mode\0"))
+                        extra_opts = "mode=0755"; /* root directory should not be world/group writable, unless overridden */
         } else {
 
                 what = fstab_node_to_udev_node(arg_root_what);
@@ -901,23 +904,28 @@ static int add_sysroot_mount(void) {
                 default_rw = false; /* read-only, unless overridden */
         }
 
-        if (!arg_root_options)
-                opts = arg_root_rw > 0 || (arg_root_rw < 0 && default_rw) ? "rw" : "ro";
-        else if (arg_root_rw >= 0 ||
-                 !fstab_test_option(arg_root_options, "ro\0" "rw\0"))
-                opts = strjoina(arg_root_options, ",", arg_root_rw > 0 ? "rw" : "ro");
-        else
-                opts = arg_root_options;
+        _cleanup_free_ char *combined_options = NULL;
+        combined_options = strdup(strempty(arg_root_options));
+        if (!combined_options)
+                return log_oom();
+
+        if (arg_root_rw >= 0 || !fstab_test_option(combined_options, "ro\0" "rw\0"))
+                if (!strextend_with_separator(&combined_options, ",", arg_root_rw > 0 || (arg_root_rw < 0 && default_rw) ? "rw" : "ro"))
+                        return log_oom();
+
+        if (extra_opts)
+                if (!strextend_with_separator(&combined_options, ",", extra_opts))
+                        return log_oom();
 
-        log_debug("Found entry what=%s where=/sysroot type=%s opts=%s", what, strna(arg_root_fstype), strempty(opts));
+        log_debug("Found entry what=%s where=/sysroot type=%s opts=%s", what, strna(fstype), strempty(combined_options));
 
         if (is_device_path(what)) {
                 r = generator_write_initrd_root_device_deps(arg_dest, what);
                 if (r < 0)
                         return r;
         }
 
-        makefs = fstab_test_option(opts, "x-systemd.makefs\0");
+        makefs = fstab_test_option(combined_options, "x-systemd.makefs\0");
         flags = makefs * MOUNT_MAKEFS;
 
         return add_mount("/proc/cmdline",
@@ -926,7 +934,7 @@ static int add_sysroot_mount(void) {
                          "/sysroot",
                          NULL,
                          fstype,
-                         opts,
+                         combined_options,
                          is_device_path(what) ? 1 : 0, /* passno */
                          flags,                        /* makefs off, pcrfs off, noauto off, nofail off, automount off */
                          SPECIAL_INITRD_ROOT_FS_TARGET);

test/test-fstab-generator/test-16-tmpfs.expected/sysroot.mount

@@ -9,4 +9,4 @@ Before=initrd-root-fs.target
 What=rootfs
 Where=/sysroot
 Type=tmpfs
-Options=rw
+Options=rw,mode=0755