Boot MagiskInit as actual system-as-root

WIP, no customization. DO NOT USE YET!

native/jni/Android.mk

@@ -106,7 +106,7 @@ LOCAL_C_INCLUDES := \
 LOCAL_SRC_FILES := \
 	init/init.cpp \
 	init/early_mount.cpp \
-	init/rootfs.cpp \
+	init/rootdir.cpp \
 	init/getinfo.cpp \
 	magiskpolicy/api.cpp \
 	magiskpolicy/magiskpolicy.cpp \

native/jni/init/early_mount.cpp

@@ -53,17 +53,20 @@ static void collect_devices() {
 	closedir(dir);
 }
 
-static void setup_block(const char *partname, char *block_dev) {
+static void setup_block(const char *partname, char *block_dev = nullptr) {
 	if (dev_list.empty())
 		collect_devices();
 	for (;;) {
 		for (auto &dev : dev_list) {
 			if (strcasecmp(dev.partname, partname) == 0) {
-				sprintf(block_dev, "/dev/block/%s", dev.devname);
-				LOGD("Found %s: [%s] (%d, %d)\n", dev.partname, dev.devname, dev.major, dev.minor);
 				xmkdir("/dev", 0755);
-				xmkdir("/dev/block", 0755);
+				if (block_dev) {
-				mknod(block_dev, S_IFBLK | 0600, makedev(dev.major, dev.minor));
+					sprintf(block_dev, "/dev/block/%s", dev.devname);
+					xmkdir("/dev/block", 0755);
+				}
+				LOGD("Found %s: [%s] (%d, %d)\n", dev.partname, dev.devname, dev.major, dev.minor);
+				mknod(block_dev ? block_dev : "/dev/root", S_IFBLK | 0600,
+						makedev(dev.major, dev.minor));
 				return;
 			}
 		}
@@ -148,6 +151,36 @@ void SARCompatInit::early_mount() {
 	mount_root(odm);
 }
 
+static void switch_root(const string &path) {
+	LOGD("Switch root to %s\n", path.data());
+	vector<string> mounts;
+	parse_mnt("/proc/mounts", [&](mntent *me) {
+		if (me->mnt_dir != "/"sv && me->mnt_dir != path)
+			mounts.emplace_back(me->mnt_dir);
+		return true;
+	});
+	for (auto &dir : mounts) {
+		auto new_path = path + dir;
+		mkdir(new_path.data(), 0755);
+		xmount(dir.data(), new_path.c_str(), nullptr, MS_MOVE, nullptr);
+	}
+	chdir(path.data());
+	xmount(path.data(), "/", nullptr, MS_MOVE, nullptr);
+	chroot(".");
+}
+
+void SARInit::early_mount() {
+	char partname[32];
+
+	LOGD("Early mount system_root\n");
+	sprintf(partname, "system%s", cmd->slot);
+	setup_block(partname);
+	xmkdir("/system_root", 0755);
+	if (xmount("/dev/root", "/system_root", "ext4", MS_RDONLY, nullptr))
+		xmount("/dev/root", "/system_root", "erofs", MS_RDONLY, nullptr);
+	switch_root("/system_root");
+}
+
 #define umount_root(name) \
 if (mnt_##name) \
 	umount("/" #name);

native/jni/init/init.cpp

@@ -132,6 +132,14 @@ void LegacyInit::preset() {
 	rename("/.backup/init", "/init");
 }
 
+void SARInit::preset() {
+	full_read("/init", &self.buf, &self.sz);
+
+	LOGD("Cleaning rootfs\n");
+	root = open("/", O_RDONLY | O_CLOEXEC);
+	frm_rf(root, { "proc", "sys" });
+}
+
 void SARCompatInit::preset() {
 	full_read("/init", &self.buf, &self.sz);
 
@@ -140,23 +148,19 @@ void SARCompatInit::preset() {
 	frm_rf(root, { "overlay", "proc", "sys" });
 }
 
-void MagiskInit::start() {
+void RootFSInit::start() {
-	// Prevent file descriptor confusion
-	mknod("/null", S_IFCHR | 0666, makedev(1, 3));
-	int null = open("/null", O_RDWR | O_CLOEXEC);
-	unlink("/null");
-	xdup3(null, STDIN_FILENO, O_CLOEXEC);
-	xdup3(null, STDOUT_FILENO, O_CLOEXEC);
-	xdup3(null, STDERR_FILENO, O_CLOEXEC);
-	if (null > STDERR_FILENO)
-		close(null);
-
 	preset();
 	early_mount();
 	setup_rootfs();
 	re_exec_init();
 }
 
+void SARInit::start() {
+	preset();
+	early_mount();
+	re_exec_init();
+}
+
 class RecoveryInit : public BaseInit {
 public:
 	RecoveryInit(char *argv[], cmdline *cmd) : BaseInit(argv, cmd) {};
@@ -168,13 +172,12 @@ public:
 	}
 };
 
-class TestInit : public SARCompatInit {
+class TestInit : public SARInit {
 public:
-	TestInit(char *argv[], cmdline *cmd) : SARCompatInit(argv, cmd) {};
+	TestInit(char *argv[], cmdline *cmd) : SARInit(argv, cmd) {};
 	void start() override {
 		preset();
 		early_mount();
-		setup_rootfs();
 		cleanup();
 	}
 };
@@ -210,6 +213,16 @@ int main(int argc, char *argv[]) {
 		if (getpid() != 1)
 			return 1;
 		setup_klog();
+
+		// Prevent file descriptor confusion
+		mknod("/null", S_IFCHR | 0666, makedev(1, 3));
+		int null = open("/null", O_RDWR | O_CLOEXEC);
+		unlink("/null");
+		xdup3(null, STDIN_FILENO, O_CLOEXEC);
+		xdup3(null, STDOUT_FILENO, O_CLOEXEC);
+		xdup3(null, STDERR_FILENO, O_CLOEXEC);
+		if (null > STDERR_FILENO)
+			close(null);
 	}
 
 	cmdline cmd{};
@@ -219,7 +232,10 @@ int main(int argc, char *argv[]) {
 	if (run_test) {
 		init = make_unique<TestInit>(argv, &cmd);
 	} else if (cmd.system_as_root) {
-		init = make_unique<SARCompatInit>(argv, &cmd);
+		if (access("/overlay", F_OK) == 0)  /* Compatible mode */
+			init = make_unique<SARCompatInit>(argv, &cmd);
+		else
+			init = make_unique<SARInit>(argv, &cmd);
 	} else {
 		decompress_ramdisk();
 		if (access("/sbin/recovery", F_OK) == 0)

native/jni/init/init.h

@@ -35,29 +35,44 @@ protected:
 
 	virtual void preset() = 0;
 	virtual void early_mount() = 0;
-	void setup_rootfs();
 	bool read_dt_fstab(const char *name, char *partname, char *fstype);
 	bool patch_sepolicy();
 	void cleanup() override;
 public:
 	MagiskInit(char *argv[], cmdline *cmd) : BaseInit(argv, cmd) {};
+};
+
+class SARInit : public MagiskInit {
+protected:
+	void preset() override;
+	void early_mount() override;
+public:
+	SARInit(char *argv[], cmdline *cmd) : MagiskInit(argv, cmd) {};
 	void start() override;
 };
 
-class LegacyInit : public MagiskInit {
+class RootFSInit : public MagiskInit {
 protected:
-	void preset() override;
+	void setup_rootfs();
-	void early_mount() override;
 public:
-	LegacyInit(char *argv[], cmdline *cmd) : MagiskInit(argv, cmd) {};
+	RootFSInit(char *argv[], cmdline *cmd) : MagiskInit(argv, cmd) {};
+	void start() override;
 };
 
-class SARCompatInit : public MagiskInit {
+class LegacyInit : public RootFSInit {
 protected:
 	void preset() override;
 	void early_mount() override;
 public:
-	SARCompatInit(char *argv[], cmdline *cmd) : MagiskInit(argv, cmd) {};
+	LegacyInit(char *argv[], cmdline *cmd) : RootFSInit(argv, cmd) {};
+};
+
+class SARCompatInit : public RootFSInit {
+protected:
+	void preset() override;
+	void early_mount() override;
+public:
+	SARCompatInit(char *argv[], cmdline *cmd) : RootFSInit(argv, cmd) {};
 };
 
 static inline bool is_lnk(const char *name) {

native/jni/init/rootdir.cpp

@@ -39,7 +39,7 @@ constexpr const char wrapper[] =
 "exec /sbin/magisk.bin \"$0\" \"$@\"\n"
 ;
 
-void MagiskInit::setup_rootfs() {
+void RootFSInit::setup_rootfs() {
 	bool patch_init = patch_sepolicy();
 
 	if (cmd->system_as_root) {