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/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 @@ mount_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() {