Refactor module and node implementation

native/src/core/bootstages.cpp

@@ -290,7 +290,7 @@ static void post_fs_data() {
 
 early_abort:
     // We still do magic mount because root itself might need it
-    magic_mount();
+    load_modules();
     boot_state |= FLAG_POST_FS_DATA_DONE;
 }
 

native/src/core/core.hpp

@@ -14,7 +14,7 @@ std::string read_certificate(int fd, int version = -1);
 
 // Module stuffs
 void handle_modules();
-void magic_mount();
+void load_modules();
 void disable_modules();
 void remove_modules();
 void exec_module_scripts(const char *stage);

native/src/core/module.cpp

@@ -9,28 +9,12 @@
 #include <resetprop.hpp>
 
 #include "core.hpp"
+#include "node.hpp"
 
 using namespace std;
 
 #define VLOGD(tag, from, to) LOGD("%-8s: %s <- %s\n", tag, to, from)
 
-#define TYPE_MIRROR  (1 << 0)    /* mount from mirror */
-#define TYPE_INTER   (1 << 1)    /* intermediate node */
-#define TYPE_TMPFS   (1 << 2)    /* replace with tmpfs */
-#define TYPE_MODULE  (1 << 3)    /* mount from module */
-#define TYPE_ROOT    (1 << 4)    /* partition root */
-#define TYPE_CUSTOM  (1 << 5)    /* custom node type overrides all */
-#define TYPE_DIR     (TYPE_INTER|TYPE_TMPFS|TYPE_ROOT)
-
-class node_entry;
-class dir_node;
-class inter_node;
-class mirror_node;
-class tmpfs_node;
-class module_node;
-class root_node;
-
-template<class T> static bool isa(node_entry *node);
 static int bind_mount(const char *reason, const char *from, const char *to) {
     int ret = xmount(from, to, nullptr, MS_BIND | MS_REC, nullptr);
     if (ret == 0)
@@ -38,349 +22,22 @@ static int bind_mount(const char *reason, const char *from, const char *to) {
     return ret;
 }
 
-template<class T> uint8_t type_id() { return TYPE_CUSTOM; }
-template<> uint8_t type_id<dir_node>() { return TYPE_DIR; }
-template<> uint8_t type_id<inter_node>() { return TYPE_INTER; }
-template<> uint8_t type_id<mirror_node>() { return TYPE_MIRROR; }
-template<> uint8_t type_id<tmpfs_node>() { return TYPE_TMPFS; }
-template<> uint8_t type_id<module_node>() { return TYPE_MODULE; }
-template<> uint8_t type_id<root_node>() { return TYPE_ROOT; }
-
-class node_entry {
-public:
-    virtual ~node_entry() = default;
-
-    // Node info
-    bool is_dir() { return file_type() == DT_DIR; }
-    bool is_lnk() { return file_type() == DT_LNK; }
-    bool is_reg() { return file_type() == DT_REG; }
-    uint8_t type() { return node_type; }
-    const string &name() { return _name; }
-
-    // Don't call the following two functions before prepare
-    const string &node_path();
-    string mirror_path() { return mirror_dir + node_path(); }
-
-    // Tree methods
-    dir_node *parent() { return _parent; }
-    void merge(node_entry *other);
-
-    virtual void mount() = 0;
-
-    static string module_mnt;
-    static string mirror_dir;
-
-protected:
-    template<class T>
-    node_entry(const char *name, uint8_t file_type, T*)
-    : _name(name), _file_type(file_type), node_type(type_id<T>()) {}
-
-    template<class T>
-    explicit node_entry(T*) : _file_type(0), node_type(type_id<T>()) {}
-
-    void create_and_mount(const char *reason, const string &src);
-
-    // Use top bit of _file_type for node exist status
-    bool exist() { return static_cast<bool>(_file_type & (1 << 7)); }
-    void set_exist(bool b) { if (b) _file_type |= (1 << 7); else _file_type &= ~(1 << 7); }
-    uint8_t file_type() { return static_cast<uint8_t>(_file_type & ~(1 << 7)); }
-
-private:
-    friend class dir_node;
-
-    static bool should_be_tmpfs(node_entry *child);
-
-    // Node properties
-    string _name;
-    uint8_t _file_type;
-    uint8_t node_type;
-
-    dir_node *_parent = nullptr;
-
-    // Cache, it should only be used within prepare
-    string _node_path;
-};
-
-class dir_node : public node_entry {
-public:
-    friend void node_entry::merge(node_entry *other);
-
-    using map_type = map<string_view, node_entry *>;
-    using iterator = map_type::iterator;
-
-    ~dir_node() override {
-        for (auto &it : children)
-            delete it.second;
-        children.clear();
-    }
-
-    void collect_files(const char *module, int dfd);
-
-    // Return true to indicate need to upgrade to skeleton
-    bool prepare(bool should_skip_mirror=false);
-
-    // Default directory mount logic
-    void mount() override {
-        for (auto &pair : children)
-            pair.second->mount();
-    }
-
-    /***************
-     * Tree Methods
-     ***************/
-
-    bool is_empty() { return children.empty(); }
-
-    template<class T>
-    T *child(string_view name) { return iterator_to_node<T>(children.find(name)); }
-
-    // Lazy val
-    root_node *root() {
-        if (!_root)
-            _root = _parent->root();
-        return _root;
-    }
-
-    // Return child with name or nullptr
-    node_entry *extract(string_view name);
-
-    // Return false if rejected
-    bool insert(node_entry *node) {
-        auto fn = [=](auto) { return node; };
-        return node && iterator_to_node(insert(node->_name, node->node_type, fn));
-    }
-
-    // Return inserted node or null if rejected
-    template<class T, class ...Args>
-    T *emplace(string_view name, Args &&...args) {
-        auto fn = [&](auto) { return new T(std::forward<Args>(args)...); };
-        return iterator_to_node<T>(insert(name, type_id<T>(), fn));
-    }
-
-    // Return inserted node, existing node with same rank, or null if rejected
-    template<class T, class ...Args>
-    T *emplace_or_get(string_view name, Args &&...args) {
-        auto fn = [&](auto) { return new T(std::forward<Args>(args)...); };
-        return iterator_to_node<T>(insert(name, type_id<T>(), fn, true));
-    }
-
-    // Return upgraded node or null if rejected
-    template<class T, class ...Args>
-    T *upgrade(string_view name, Args &...args) {
-        return iterator_to_node<T>(upgrade<T>(children.find(name), args...));
-    }
-
-    bool skip_mirror() const { return _skip_mirror; }
-    void set_skip_mirror(bool b) { _skip_mirror = b; }
-
-protected:
-    template<class T>
-    dir_node(const char *name, uint8_t file_type, T *self) : node_entry(name, file_type, self) {
-        if constexpr (std::is_same_v<T, root_node>)
-            _root = self;
-    }
-
-    template<class T>
-    dir_node(node_entry *node, T *self) : node_entry(self) {
-        merge(node);
-        if constexpr (std::is_same_v<T, root_node>)
-            _root = self;
-    }
-
-    template<class T>
-    dir_node(const char *name, T *self) : dir_node(name, DT_DIR, self) {}
-
-    template<class T = node_entry>
-    T *iterator_to_node(iterator it) {
-        return static_cast<T*>(it == children.end() ? nullptr : it->second);
-    }
-
-    // Emplace insert a new node, or upgrade if the requested type has a higher rank.
-    // Return iterator to new node or end() if insertion is rejected.
-    // If get_same is true and a node with the same rank exists, it will return that node instead.
-    // fn is the node construction callback. Signature: (node_ent *&) -> node_ent *
-    // fn gets a reference to the existing node pointer and returns a new node object.
-    // Input is null when there is no existing node. If returns null, the insertion is rejected.
-    // If fn consumes the input, it should set the reference to null.
-    template<typename Func>
-    iterator insert(iterator it, uint8_t type, const Func &fn, bool get_same);
-
-    template<typename Func>
-    iterator insert(string_view name, uint8_t type, const Func &fn, bool get_same = false) {
-        return insert(children.find(name), type, fn, get_same);
-    }
-
-    template<class To, class From = node_entry, class ...Args>
-    iterator upgrade(iterator it, Args &&...args);
-
-    // dir nodes host children
-    map_type children;
-
-    // Root node lookup cache
-    root_node *_root = nullptr;
-
-    // Skip binding mirror for this directory
-    bool _skip_mirror = false;
-};
-
-class root_node : public dir_node {
-public:
-    explicit root_node(const char *name) : dir_node(name, this), prefix("") {
-        set_exist(true);
-    }
-    explicit root_node(node_entry *node) : dir_node(node, this), prefix("/system") {
-        set_exist(true);
-    }
-    const char * const prefix;
-};
-
-class inter_node : public dir_node {
-public:
-    inter_node(const char *name, const char *module) : dir_node(name, this), module(module) {}
-private:
-    const char *module;
-    friend class module_node;
-};
-
-class module_node : public node_entry {
-public:
-    module_node(const char *module, dirent *entry)
-    : node_entry(entry->d_name, entry->d_type, this), module(module) {}
-
-    module_node(node_entry *node, const char *module) : node_entry(this), module(module) {
-        merge(node);
-    }
-
-    explicit module_node(inter_node *node) : module_node(node, node->module) {}
-
-    void mount() override;
-private:
-    const char *module;
-};
-
-// Don't create the following two nodes before prepare
-class mirror_node : public node_entry {
-public:
-    explicit mirror_node(dirent *entry) : node_entry(entry->d_name, entry->d_type, this) {}
-    void mount() override {
-        create_and_mount("mirror", mirror_path());
-    }
-};
-
-class tmpfs_node : public dir_node {
-public:
-    explicit tmpfs_node(node_entry *node);
-    void mount() override;
-};
-
-// Poor man's dynamic cast without RTTI
-template<class T>
-static bool isa(node_entry *node) {
-    return node && (node->type() & type_id<T>());
-}
-template<class T>
-static T *dyn_cast(node_entry *node) {
-    return isa<T>(node) ? static_cast<T*>(node) : nullptr;
-}
-
 string node_entry::module_mnt;
 string node_entry::mirror_dir;
 
-// other will be deleted
-void node_entry::merge(node_entry *other) {
-    _name.swap(other->_name);
-    _file_type = other->_file_type;
-    _parent = other->_parent;
-
-    // Merge children if both is dir
-    if (auto a = dyn_cast<dir_node>(this)) {
-        if (auto b = dyn_cast<dir_node>(other)) {
-            a->_skip_mirror = b->_skip_mirror;
-            a->children.merge(b->children);
-            for (auto &pair : a->children)
-                pair.second->_parent = a;
-        }
-    }
-    delete other;
-}
-
-const string &node_entry::node_path() {
-    if (_parent && _node_path.empty())
-        _node_path = _parent->node_path() + '/' + _name;
-    return _node_path;
-}
-
 /*************************
  * Node Tree Construction
  *************************/
 
-template<typename Func>
-dir_node::iterator dir_node::insert(iterator it, uint8_t type, const Func &fn, bool get_same) {
-    node_entry *node = nullptr;
-    if (it != children.end()) {
-        // Upgrade existing node only if higher rank
-        if (it->second->node_type < type) {
-            node = fn(it->second);
-            if (!node)
-                return children.end();
-            if (it->second)
-                node->merge(it->second);
-            it = children.erase(it);
-            // Minor optimization to make insert O(1) by using hint
-            if (it == children.begin())
-                it = children.emplace(node->_name, node).first;
-            else
-                it = children.emplace_hint(--it, node->_name, node);
-        } else {
-            if (get_same && it->second->node_type != type)
-                return children.end();
-            return it;
-        }
-    } else {
-        node = fn(node);
-        if (!node)
-            return children.end();
-        node->_parent = this;
-        it = children.emplace(node->_name, node).first;
-    }
-    return it;
-}
-
-template<class To, class From, class... Args>
-dir_node::iterator dir_node::upgrade(iterator it, Args &&... args) {
-    return insert(it, type_id<To>(), [&](node_entry *&ex) -> node_entry * {
-        if (!ex)
-            return nullptr;
-        if constexpr (!std::is_same_v<From, node_entry>) {
-            // Type check if type is specified
-            if (!isa<From>(ex))
-                return nullptr;
-        }
-        auto node = new To(static_cast<From *>(ex), std::forward<Args>(args)...);
-        ex = nullptr;
-        return node;
-    }, false);
-}
-
-node_entry* dir_node::extract(string_view name) {
-    auto it = children.find(name);
-    if (it != children.end()) {
-        auto ret = it->second;
-        children.erase(it);
-        return ret;
-    }
-    return nullptr;
-}
-
 tmpfs_node::tmpfs_node(node_entry *node) : dir_node(node, this) {
+    if (!skip_mirror) {
         string mirror = mirror_path();
-    if (!skip_mirror()) {
         if (auto dir = open_dir(mirror.data())) {
             set_exist(true);
             for (dirent *entry; (entry = xreaddir(dir.get()));) {
                 if (entry->d_type == DT_DIR) {
                     // create a dummy inter_node to upgrade later
-                    emplace<inter_node>(entry->d_name, entry->d_name, "mirror");
+                    emplace<inter_node>(entry->d_name, entry->d_name, "");
                 } else {
                     // Insert mirror nodes
                     emplace<mirror_node>(entry->d_name, entry);
@@ -396,34 +53,16 @@ tmpfs_node::tmpfs_node(node_entry *node) : dir_node(node, this) {
     }
 }
 
-// We need to upgrade to tmpfs node if any child:
+bool dir_node::prepare() {
-// - Target does not exist
+    bool to_tmpfs = skip_mirror;
-// - Source or target is a symlink (since we cannot bind mount link)
-bool node_entry::should_be_tmpfs(node_entry *child) {
-    struct stat st{};
-    if (lstat(child->node_path().data(), &st) != 0) {
-        return true;
-    } else {
-        child->set_exist(true);
-        if (child->is_lnk() || S_ISLNK(st.st_mode))
-            return true;
-    }
-    return false;
-}
-
-bool dir_node::prepare(bool should_skip_mirror) {
-    if (!skip_mirror() && should_skip_mirror) {
-        set_skip_mirror(true);
-    }
-    bool to_tmpfs = false;
     if (!exist()) {
         // If not exist, we need to create it by mounting tmpfs
         to_tmpfs = true;
         set_exist(true);
     }
     for (auto it = children.begin(); it != children.end();) {
-        if (should_be_tmpfs(it->second)) {
+        if (require_tmpfs_upgrade(it->second)) {
-            if (node_type > type_id<tmpfs_node>()) {
+            if (_node_type > type_id<tmpfs_node>()) {
                 // Upgrade will fail, remove the unsupported child node
                 LOGW("Unable to add: %s, skipped\n", it->second->node_path().data());
                 delete it->second;
@@ -432,7 +71,7 @@ bool dir_node::prepare(bool should_skip_mirror) {
             }
             // Tell parent to upgrade self to tmpfs
             to_tmpfs = true;
-            // If child is inter_node and it does not (need to) exist, upgrade to module
+            // If child is inter_node and it does not exist, upgrade to module
             if (auto dn = dyn_cast<inter_node>(it->second)) {
                 if (!dn->exist()) {
                     if (auto nit = upgrade<module_node, inter_node>(it); nit != children.end()) {
@@ -442,25 +81,30 @@ bool dir_node::prepare(bool should_skip_mirror) {
                 }
             }
         }
-        if (auto dn = dyn_cast<dir_node>(it->second); dn && dn->prepare(skip_mirror())) {
+        if (auto dn = dyn_cast<dir_node>(it->second)) {
+            if (skip_mirror) {
+                dn->skip_mirror = true;
+            }
+            if (dn->prepare()) {
                 // Upgrade child to tmpfs
                 it = upgrade<tmpfs_node>(it);
             }
+        }
 next_node:
         ++it;
     }
-    return to_tmpfs || skip_mirror();
+    return to_tmpfs;
 }
 
-void dir_node::collect_files(const char *module, int dfd) {
+void dir_node::collect_module_files(const char *module, int dfd) {
-    auto dir = xopen_dir(xopenat(dfd, _name.data(), O_RDONLY | O_CLOEXEC));
+    auto dir = xopen_dir(xopenat(dfd, name().data(), O_RDONLY | O_CLOEXEC));
     if (!dir)
         return;
 
     for (dirent *entry; (entry = xreaddir(dir.get()));) {
         inter_node *dn;
         if (entry->d_name == ".replace"sv) {
-            set_skip_mirror(true);
+            skip_mirror = true;
             continue;
         }
 
@@ -471,7 +115,7 @@ void dir_node::collect_files(const char *module, int dfd) {
                 dn = dyn_cast<inter_node>(it->second);
             }
             if (dn) {
-                dn->collect_files(module, dirfd(dir.get()));
+                dn->collect_module_files(module, dirfd(dir.get()));
             }
         } else {
             emplace<module_node>(entry->d_name, module, entry);
@@ -518,10 +162,9 @@ void tmpfs_node::mount() {
     else
         getattr(parent()->node_path().data(), &a);
     if (!isa<tmpfs_node>(parent())) {
-        // We don't need another layer of tmpfs if parent is skel
         auto worker_dir = MAGISKTMP + "/" WORKERDIR + dest;
         mkdirs(worker_dir.data(), 0);
-        create_and_mount(skip_mirror() ? "replace" : "tmpfs", worker_dir);
+        create_and_mount(skip_mirror ? "replace" : "tmpfs", worker_dir);
     } else {
         // We don't need another layer of tmpfs if parent is tmpfs
         mkdir(dest.data(), 0);
@@ -560,7 +203,7 @@ public:
 };
 
 static void inject_magisk_bins(root_node *system) {
-    auto bin = system->child<inter_node>("bin");
+    auto bin = system->get_child<inter_node>("bin");
     if (!bin) {
         bin = new inter_node("bin", "");
         system->insert(bin);
@@ -594,7 +237,7 @@ if (access("/system/bin/app_process" #bit, F_OK) == 0) {
     bind_mount("zygisk", zbin.data(), "/system/bin/app_process" #bit);                  \
 }
 
-void magic_mount() {
+void load_modules() {
     node_entry::mirror_dir = MAGISKTMP + "/" MIRRDIR;
     node_entry::module_mnt = MAGISKTMP + "/" MODULEMNT "/";
 
@@ -628,7 +271,7 @@ void magic_mount() {
         LOGI("%s: loading mount files\n", module);
         b[-1] = '\0';
         int fd = xopen(buf, O_RDONLY | O_CLOEXEC);
-        system->collect_files(module, fd);
+        system->collect_module_files(module, fd);
         close(fd);
     }
     if (MAGISKTMP != "/sbin" || !str_contains(getenv("PATH") ?: "", "/sbin")) {
@@ -662,6 +305,10 @@ void magic_mount() {
     }
 }
 
+/************************
+ * Filesystem operations
+ ************************/
+
 static void prepare_modules() {
     // Upgrade modules
     if (auto dir = open_dir(MODULEUPGRADE); dir) {

native/src/core/node.hpp

@@ -0,0 +1,341 @@
+#pragma once
+
+#include <sys/mount.h>
+#include <map>
+
+using namespace std;
+
+#define TYPE_MIRROR  (1 << 0)    /* mount from mirror */
+#define TYPE_INTER   (1 << 1)    /* intermediate node */
+#define TYPE_TMPFS   (1 << 2)    /* replace with tmpfs */
+#define TYPE_MODULE  (1 << 3)    /* mount from module */
+#define TYPE_ROOT    (1 << 4)    /* partition root */
+#define TYPE_CUSTOM  (1 << 5)    /* custom node type overrides all */
+#define TYPE_DIR     (TYPE_INTER|TYPE_TMPFS|TYPE_ROOT)
+
+class node_entry;
+class dir_node;
+class inter_node;
+class mirror_node;
+class tmpfs_node;
+class module_node;
+class root_node;
+
+// Poor man's dynamic cast without RTTI
+template<class T> static bool isa(node_entry *node);
+template<class T> static T *dyn_cast(node_entry *node);
+template<class T> uint8_t type_id() { return TYPE_CUSTOM; }
+template<> uint8_t type_id<dir_node>() { return TYPE_DIR; }
+template<> uint8_t type_id<inter_node>() { return TYPE_INTER; }
+template<> uint8_t type_id<mirror_node>() { return TYPE_MIRROR; }
+template<> uint8_t type_id<tmpfs_node>() { return TYPE_TMPFS; }
+template<> uint8_t type_id<module_node>() { return TYPE_MODULE; }
+template<> uint8_t type_id<root_node>() { return TYPE_ROOT; }
+
+class node_entry {
+public:
+    virtual ~node_entry() = default;
+
+    // Node info
+    bool is_dir() const { return file_type() == DT_DIR; }
+    bool is_lnk() const { return file_type() == DT_LNK; }
+    bool is_reg() const { return file_type() == DT_REG; }
+    const string &name() const { return _name; }
+    dir_node *parent() const { return _parent; }
+
+    // Don't call the following two functions before prepare
+    const string &node_path();
+    string mirror_path() { return mirror_dir + node_path(); }
+
+    virtual void mount() = 0;
+
+    static string module_mnt;
+    static string mirror_dir;
+
+protected:
+    template<class T>
+    node_entry(const char *name, uint8_t file_type, T*)
+    : _name(name), _file_type(file_type), _node_type(type_id<T>()) {}
+
+    template<class T>
+    explicit node_entry(T*) : _file_type(0), _node_type(type_id<T>()) {}
+
+    void consume(node_entry *other);
+    void create_and_mount(const char *reason, const string &src);
+
+    // Use top bit of _file_type for node exist status
+    bool exist() const { return static_cast<bool>(_file_type & (1 << 7)); }
+    void set_exist(bool b) { if (b) _file_type |= (1 << 7); else _file_type &= ~(1 << 7); }
+    uint8_t file_type() const { return static_cast<uint8_t>(_file_type & ~(1 << 7)); }
+
+private:
+    friend class dir_node;
+
+    template<class T>
+    friend bool isa(node_entry *node);
+
+    // Node properties
+    string _name;
+    uint8_t _file_type;
+    const uint8_t _node_type;
+
+    dir_node *_parent = nullptr;
+
+    // Cache, it should only be used within prepare
+    string _node_path;
+};
+
+class dir_node : public node_entry {
+public:
+    friend void node_entry::consume(node_entry *other);
+
+    using map_type = map<string_view, node_entry *>;
+    using iterator = map_type::iterator;
+
+    ~dir_node() override {
+        for (auto &it : children)
+            delete it.second;
+        children.clear();
+    }
+
+    // Traverse through module directories to generate a tree of module files
+    void collect_module_files(const char *module, int dfd);
+
+    // Traverse through the real filesystem and prepare the tree for magic mount.
+    // Return true to indicate that this node needs to be upgraded to tmpfs_node.
+    bool prepare();
+
+    // Default directory mount logic
+    void mount() override {
+        for (auto &pair : children)
+            pair.second->mount();
+    }
+
+    /***************
+     * Tree Methods
+     ***************/
+
+    bool is_empty() { return children.empty(); }
+
+    template<class T>
+    T *get_child(string_view name) { return iterator_to_node<T>(children.find(name)); }
+
+    root_node *root() {
+        if (!_root)
+            _root = _parent->root();
+        return _root;
+    }
+
+    // Return child with name or nullptr
+    node_entry *extract(string_view name) {
+        auto it = children.find(name);
+        if (it != children.end()) {
+            auto ret = it->second;
+            children.erase(it);
+            return ret;
+        }
+        return nullptr;
+    }
+
+    // Return false if rejected
+    bool insert(node_entry *node) {
+        auto fn = [=](auto) { return node; };
+        return node && iterator_to_node(insert(node->_name, node->_node_type, fn));
+    }
+
+    // Return inserted node or null if rejected
+    template<class T, class ...Args>
+    T *emplace(string_view name, Args &&...args) {
+        auto fn = [&](auto) { return new T(std::forward<Args>(args)...); };
+        return iterator_to_node<T>(insert(name, type_id<T>(), fn));
+    }
+
+    // Return upgraded node or null if rejected
+    template<class T, class ...Args>
+    T *upgrade(string_view name, Args &...args) {
+        return iterator_to_node<T>(upgrade<T>(children.find(name), args...));
+    }
+
+protected:
+    template<class T>
+    dir_node(const char *name, T *self) : node_entry(name, DT_DIR, self) {
+        if constexpr (std::is_same_v<T, root_node>)
+        _root = self;
+    }
+
+    template<class T>
+    dir_node(node_entry *node, T *self) : node_entry(self) {
+        consume(node);
+        if constexpr (std::is_same_v<T, root_node>)
+        _root = self;
+    }
+
+    template<class T = node_entry>
+    T *iterator_to_node(iterator it) {
+        return static_cast<T*>(it == children.end() ? nullptr : it->second);
+    }
+
+    template<typename Func>
+    iterator insert(string_view name, uint8_t type, const Func &fn) {
+        return insert_at(children.find(name), type, fn);
+    }
+
+    // Emplace insert a new node, or upgrade if the requested type has a higher rank.
+    // Return iterator to the new/upgraded node, or end() if insertion is rejected.
+    // fn is the node builder function. Signature: (node_entry *&) -> node_entry *
+    // fn gets a reference to the existing node pointer and returns a new node object.
+    // Input is null when there is no existing node. If returns null, the insertion is rejected.
+    // If fn consumes the input, it should set the reference to null.
+    template<typename Builder>
+    iterator insert_at(iterator it, uint8_t type, const Builder &builder) {
+        node_entry *node = nullptr;
+        if (it != children.end()) {
+            // Upgrade existing node only if higher rank
+            if (it->second->_node_type < type) {
+                node = builder(it->second);
+                if (!node)
+                    return children.end();
+                if (it->second)
+                    node->consume(it->second);
+                it = children.erase(it);
+                // Minor optimization to make insert O(1) by using hint
+                if (it == children.begin())
+                    it = children.emplace(node->_name, node).first;
+                else
+                    it = children.emplace_hint(--it, node->_name, node);
+            } else {
+                return children.end();
+            }
+        } else {
+            node = builder(node);
+            if (!node)
+                return children.end();
+            node->_parent = this;
+            it = children.emplace(node->_name, node).first;
+        }
+        return it;
+    }
+
+    template<class To, class From = node_entry, class ...Args>
+    iterator upgrade(iterator it, Args &&...args) {
+        return insert_at(it, type_id<To>(), [&](node_entry *&ex) -> node_entry * {
+            if (!ex)
+                return nullptr;
+            if constexpr (!std::is_same_v<From, node_entry>) {
+                // Type check if type is specified
+                if (!isa<From>(ex))
+                    return nullptr;
+            }
+            auto node = new To(static_cast<From *>(ex), std::forward<Args>(args)...);
+            ex = nullptr;
+            return node;
+        });
+    }
+
+    // dir nodes host children
+    map_type children;
+
+    // Skip binding mirror for this directory
+    bool skip_mirror = false;
+
+private:
+    // Root node lookup cache
+    root_node *_root = nullptr;
+
+    // We need to upgrade to tmpfs node if any child:
+    // - Target does not exist
+    // - Source or target is a symlink (since we cannot bind mount link)
+    bool require_tmpfs_upgrade(node_entry *child) {
+        struct stat st{};
+        if (lstat(child->node_path().data(), &st) != 0) {
+            return true;
+        } else {
+            child->set_exist(true);
+            if (child->is_lnk() || S_ISLNK(st.st_mode))
+                return true;
+        }
+        return false;
+    }
+};
+
+class root_node : public dir_node {
+public:
+    explicit root_node(const char *name) : dir_node(name, this), prefix("") {
+        set_exist(true);
+    }
+    explicit root_node(node_entry *node) : dir_node(node, this), prefix("/system") {
+        set_exist(true);
+    }
+    const char * const prefix;
+};
+
+class inter_node : public dir_node {
+public:
+    inter_node(const char *name, const char *module) : dir_node(name, this), module(module) {}
+private:
+    const char *module;
+    friend class module_node;
+};
+
+class module_node : public node_entry {
+public:
+    module_node(const char *module, dirent *entry)
+    : node_entry(entry->d_name, entry->d_type, this), module(module) {}
+
+    module_node(node_entry *node, const char *module) : node_entry(this), module(module) {
+        consume(node);
+    }
+
+    explicit module_node(inter_node *node) : module_node(node, node->module) {}
+
+    void mount() override;
+private:
+    const char *module;
+};
+
+// Don't create the following two nodes before prepare
+class mirror_node : public node_entry {
+public:
+    explicit mirror_node(dirent *entry) : node_entry(entry->d_name, entry->d_type, this) {}
+    void mount() override {
+        create_and_mount("mirror", mirror_path());
+    }
+};
+
+class tmpfs_node : public dir_node {
+public:
+    explicit tmpfs_node(node_entry *node);
+    void mount() override;
+};
+
+template<class T>
+static bool isa(node_entry *node) {
+    return node && (node->_node_type & type_id<T>());
+}
+template<class T>
+static T *dyn_cast(node_entry *node) {
+    return isa<T>(node) ? static_cast<T*>(node) : nullptr;
+}
+
+const string &node_entry::node_path() {
+    if (_parent && _node_path.empty())
+        _node_path = _parent->node_path() + '/' + _name;
+    return _node_path;
+}
+
+void node_entry::consume(node_entry *other) {
+    _name.swap(other->_name);
+    _file_type = other->_file_type;
+    _parent = other->_parent;
+
+    // Merge children if both is dir
+    if (auto a = dyn_cast<dir_node>(this)) {
+        if (auto b = dyn_cast<dir_node>(other)) {
+            a->skip_mirror = b->skip_mirror;
+            a->children.merge(b->children);
+            for (auto &pair : a->children)
+                pair.second->_parent = a;
+        }
+    }
+    delete other;
+}