Keep mirror shared before magic mount

This allows mounting during post-fs-data be kept after magic mount

native/src/core/bootstages.cpp

@@ -20,13 +20,11 @@ bool zygisk_enabled = false;
  * Setup *
  *********/
 
-static bool mount_mirror(const std::string_view from, const std::string_view to) {
+static bool rec_mount(const std::string_view from, const std::string_view to) {
     return !xmkdirs(to.data(), 0755) &&
            // recursively bind mount to mirror dir, rootfs will fail before 3.12 kernel
            // because of MS_NOUSER
-           !mount(from.data(), to.data(), nullptr, MS_BIND | MS_REC, nullptr) &&
-           // make mirror dir as a private mount so that it won't be affected by magic mount
-           !xmount(nullptr, to.data(), nullptr, MS_PRIVATE | MS_REC, nullptr);
+           !mount(from.data(), to.data(), nullptr, MS_BIND | MS_REC, nullptr);
 }
 
 static void mount_mirrors() {
@@ -67,7 +65,7 @@ static void mount_mirrors() {
                 if (!rw) continue;
                 string preinit_dir = resolve_preinit_dir(info.target.data());
                 xmkdir(preinit_dir.data(), 0700);
-                if ((mounted = mount_mirror(preinit_dir, path))) {
+                if ((mounted = rec_mount(preinit_dir, path))) {
                     xmount(nullptr, path, nullptr, MS_UNBINDABLE, nullptr);
                     break;
                 }
@@ -83,10 +81,12 @@ static void mount_mirrors() {
     ssprintf(path, sizeof(path), "%s/" WORKERDIR, get_magisk_tmp());
     xmount("worker", path, "tmpfs", 0, "mode=755");
     xmount(nullptr, path, nullptr, MS_PRIVATE, nullptr);
-
     // Recursively bind mount / to mirror dir
-    if (auto mirror_dir = get_magisk_tmp() + "/"s MIRRDIR; !mount_mirror("/", mirror_dir)) {
+    // Keep mirror shared so that mounting during post-fs-data will be propagated
+    if (auto mirror_dir = get_magisk_tmp() + "/"s MIRRDIR; !rec_mount("/", mirror_dir)) {
         LOGI("fallback to mount subtree\n");
+        // create new a bind mount for easy make private
+        xmount(mirror_dir.data(), mirror_dir.data(), nullptr, MS_BIND, nullptr);
         // rootfs may fail, fallback to bind mount each mount point
         set<string, greater<>> mounted_dirs {{ get_magisk_tmp() }};
         for (const auto &info: self_mount_info) {
@@ -96,7 +96,7 @@ static void mount_mirrors() {
                 last_mount != mounted_dirs.end() && info.target.starts_with(*last_mount + '/')) {
                 continue;
             }
-            if (mount_mirror(info.target, mirror_dir + info.target)) {
+            if (rec_mount(info.target, mirror_dir + info.target)) {
                 LOGD("%-8s: %s <- %s\n", "rbind", (mirror_dir + info.target).data(), info.target.data());
                 mounted_dirs.insert(info.target);
             }
@@ -342,6 +342,9 @@ bool MagiskD::post_fs_data() const {
 
 early_abort:
     auto mirror_dir = get_magisk_tmp() + "/"s MIRRDIR;
+    // make mirror dir as a private mount so that it won't be affected by magic mount
+    LOGD("make %s private\n", mirror_dir.data());
+    xmount(nullptr, mirror_dir.data(), nullptr, MS_PRIVATE | MS_REC, nullptr);
     // We still do magic mount because root itself might need it
     load_modules();
     // make mirror dir as a shared mount to make magisk --stop work for other ns