Use /data as tmpfs mount point in 2SI setup

Design credit to @yujincheng08
Close #5146. Fix #5491, fix #3752

Previously, Magisk changes the mount point from /system to /system_root
by patching fstab to prevent the original init from changing root.
The reason why we want to prevent the original init from switching the
root directory is because it will then be read-only, making patching
and injecting magiskinit into the boot chain difficult.

This commit (ab)uses the fact that the /data folder will never be part
of early mount (because it is handled very late in the boot by vold),
so that we can use it as the mount point of tmpfs to store files.

Some advantages of this method:

- No need to switch root manually
- No need to modify fstab, which significantly improves compatibility
  e.g. avoid hacks for weird devices like those using oplus.fstab,
  and avoid hacking init to bypass fstab in device trees
- Supports skip_mount.cfg
- Support DSU

native/jni/Android.mk

@@ -63,13 +63,11 @@ LOCAL_SRC_FILES := \
     init/rootdir.cpp \
     init/getinfo.cpp \
     init/twostage.cpp \
-    core/socket.cpp \
     magiskpolicy/sepolicy.cpp \
     magiskpolicy/magiskpolicy.cpp \
     magiskpolicy/rules.cpp \
     magiskpolicy/policydb.cpp \
-    magiskpolicy/statement.cpp \
-    magiskboot/pattern.cpp
+    magiskpolicy/statement.cpp
 
 LOCAL_LDFLAGS := -static
 include $(BUILD_EXECUTABLE)

native/jni/init/init.hpp

@@ -34,7 +34,6 @@ struct fstab_entry {
     fstab_entry(fstab_entry &&) = default;
     fstab_entry &operator=(const fstab_entry&) = delete;
     fstab_entry &operator=(fstab_entry&&) = default;
-    void to_file(FILE *fp);
 };
 
 #define INIT_SOCKET "MAGISKINIT"
@@ -44,7 +43,6 @@ extern std::vector<std::string> mount_list;
 
 bool unxz(int fd, const uint8_t *buf, size_t size);
 void load_kernel_info(BootConfig *config);
-bool is_dsu();
 bool check_two_stage();
 void setup_klog();
 const char *backup_init();
@@ -107,7 +105,6 @@ class SARBase : virtual public MagiskInit {
 class FirstStageInit : public BaseInit {
 private:
     void prepare();
-    void get_default_fstab(char *buf, size_t len);
 public:
     FirstStageInit(char *argv[], BootConfig *cmd) : BaseInit(argv, cmd) {
         LOGD("%s\n", __FUNCTION__);
@@ -124,17 +121,14 @@ class FirstStageInit : public BaseInit {
 
 class SARInit : public SARBase {
 private:
-    bool is_two_stage;
-
-    void early_mount();
+    bool early_mount();
     void first_stage_prep();
 public:
-    SARInit(char *argv[], BootConfig *cmd) : MagiskInit(argv, cmd), is_two_stage(false) {
+    SARInit(char *argv[], BootConfig *cmd) : MagiskInit(argv, cmd) {
         LOGD("%s\n", __FUNCTION__);
     };
     void start() override {
-        early_mount();
-        if (is_two_stage)
+        if (early_mount())
             first_stage_prep();
         else
             patch_rootdir();
@@ -178,8 +172,10 @@ class SecondStageInit : public RootFSBase, public SARBase {
     };
 
     void start() override {
-        if (prepare()) patch_rootfs();
-        else patch_rootdir();
+        if (prepare())
+            patch_rootfs();
+        else
+            patch_rootdir();
         exec_init();
     }
 };

native/jni/init/mount.cpp

@@ -205,27 +205,6 @@ static void switch_root(const string &path) {
     frm_rf(root);
 }
 
-bool is_dsu() {
-    strcpy(blk_info.partname, "metadata");
-    xmkdir("/metadata", 0755);
-    if (setup_block(true) < 0 ||
-        xmount(blk_info.block_dev, "/metadata", "ext4", MS_RDONLY, nullptr)) {
-        PLOGE("Failed to mount /metadata");
-        return false;
-    } else {
-        run_finally f([]{ xumount2("/metadata", MNT_DETACH); });
-        constexpr auto dsu_status = "/metadata/gsi/dsu/install_status";
-        if (xaccess(dsu_status, F_OK) == 0) {
-            char status[PATH_MAX] = {0};
-            auto fp = xopen_file(dsu_status, "r");
-            fgets(status, sizeof(status), fp.get());
-            if (status == "ok"sv || status == "0"sv)
-                return true;
-        }
-    }
-    return false;
-}
-
 void MagiskInit::mount_rules_dir(const char *dev_base, const char *mnt_base) {
     char path[128];
     xrealpath(dev_base, blk_info.block_dev);
@@ -327,10 +306,14 @@ void RootFSInit::early_mount() {
 void SARBase::backup_files() {
     if (access("/overlay.d", F_OK) == 0)
         backup_folder("/overlay.d", overlays);
+    else if (access("/data/overlay.d", F_OK) == 0)
+        backup_folder("/data/overlay.d", overlays);
 
     self = mmap_data("/proc/self/exe");
     if (access("/.backup/.magisk", R_OK) == 0)
         magisk_config = mmap_data("/.backup/.magisk");
+    else if (access("/data/.backup/.magisk", R_OK) == 0)
+        magisk_config = mmap_data("/data/.backup/.magisk");
 }
 
 void SARBase::mount_system_root() {
@@ -367,13 +350,13 @@ void SARBase::mount_system_root() {
         xmount("/dev/root", "/system_root", "erofs", MS_RDONLY, nullptr);
 }
 
-void SARInit::early_mount() {
+bool SARInit::early_mount() {
     backup_files();
     mount_system_root();
     switch_root("/system_root");
 
     // Use the apex folder to determine whether 2SI (Android 10+)
-    is_two_stage = access("/apex", F_OK) == 0;
+    bool is_two_stage = access("/apex", F_OK) == 0;
     LOGD("is_two_stage: [%d]\n", is_two_stage);
 
     if (!is_two_stage) {
@@ -386,26 +369,8 @@ void SARInit::early_mount() {
 #endif
         mount_with_dt();
     }
-}
-
-bool SecondStageInit::prepare() {
-    backup_files();
 
-    umount2("/init", MNT_DETACH);
-    umount2("/proc/self/exe", MNT_DETACH);
-
-    // some weird devices, like meizu, embrace two stage init but still have legacy rootfs behaviour
-    bool legacy = false;
-    if (access("/system_root", F_OK) == 0) {
-        if (access("/system_root/proc", F_OK) == 0) {
-            switch_root("/system_root");
-        } else {
-            xmount("/system_root", "/system", nullptr, MS_MOVE, nullptr);
-            rmdir("/system_root");
-            legacy = true;
-        }
-    }
-    return legacy;
+    return is_two_stage;
 }
 
 void BaseInit::exec_init() {

native/jni/init/rootdir.cpp

@@ -240,7 +240,7 @@ void SARBase::patch_rootdir() {
             make_pair(SPLIT_PLAT_CIL, "xxx"), /* Force loading monolithic sepolicy */
             make_pair(MONOPOLICY, sepol)      /* Redirect /sepolicy to custom path */
          });
-        if constexpr (avd_hack) {
+        if (avd_hack) {
             // Force disable early mount on original init
             init.patch({ make_pair("android,fstab", "xxx") });
         }
@@ -276,23 +276,9 @@ void SARBase::patch_rootdir() {
     // sepolicy
     patch_sepolicy(sepol);
 
-    // Restore backup files
-    struct sockaddr_un sun;
-    int sockfd = xsocket(AF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0);
-    if (connect(sockfd, (struct sockaddr*) &sun, setup_sockaddr(&sun, INIT_SOCKET)) == 0) {
-        LOGD("ACK init daemon to write backup files\n");
-        // Let daemon know where tmp_dir is
-        write_string(sockfd, tmp_dir);
-        // Wait for daemon to finish restoring files
-        read_int(sockfd);
-    } else {
-        LOGD("Restore backup files locally\n");
-        restore_folder(ROOTOVL, overlays);
-        overlays.clear();
-    }
-    close(sockfd);
-
     // Handle overlay.d
+    restore_folder(ROOTOVL, overlays);
+    overlays.clear();
     load_overlay_rc(ROOTOVL);
     if (access(ROOTOVL "/sbin", F_OK) == 0) {
         // Move files in overlay.d/sbin into tmp_dir