Logical Resizable Android Partitions support

The way how logical partition, or "Logical Resizable Android Partitions"
as they say in AOSP source code, is setup makes it impossible to early
mount the partitions from the shared super partition with just
a few lines of code; in fact, AOSP has a whole "fs_mgr" folder which
consist of multiple complex libraries, with 15K lines of code just
to deal with the device mapper shenanigans.

In order to keep the already overly complicated MagiskInit more
managable, I chose NOT to go the route of including fs_mgr directly
into MagiskInit. Luckily, starting from Android Q, Google decided to
split init startup into 3 stages, with the first stage doing _only_
early mount. This is great news, because we can simply let the stock
init do its own thing for us, and we intercept the bootup sequence.

So the workflow can be visualized roughly below:

Magisk First Stage --> First Stage Mount --> Magisk Second Stage --+
   (MagiskInit)         (Original Init)         (MagiskInit)       +
                                                                   +
                                                                   +
     ...Rest of the boot... <-- Second Stage <-- Selinux Setup  <--+
      (__________________ Original Init ____________________)

The catch here is that after doing all the first stage mounting, /init
will pivot /system as root directory (/), leaving us impossible to
regain control after we hand it over. So the solution here is to patch
fstab in /first_stage_ramdisk on-the-fly to redirect /system to
/system_root, making the original init do all the hard work for
us and mount required early mount partitions, but skips the step of
switching root directory. It will also conveniently hand over execution
back to MagiskInit, which we will reuse the routine for patching
root directory in normal system-as-root situations.

native/jni/init/early_mount.cpp

@@ -159,8 +159,15 @@ 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);
+		// Skip root and self
+		if (me->mnt_dir == "/"sv || me->mnt_dir == path)
+			return true;
+		// Do not include subtrees
+		for (const auto &m : mounts) {
+			if (strncmp(me->mnt_dir, m.data(), m.length()) == 0)
+				return true;
+		}
+		mounts.emplace_back(me->mnt_dir);
 		return true;
 	});
 	for (auto &dir : mounts) {
@@ -202,6 +209,28 @@ void SARInit::early_mount() {
 	mount_root(odm);
 }
 
+void SecondStageInit::early_mount() {
+	// Early mounts should already be done by first stage init
+
+	full_read("/system/bin/init", &self.buf, &self.sz);
+	full_read("/.backup/.magisk", &config.buf, &config.sz);
+	rm_rf("/system");
+	rm_rf("/.backup");
+
+	// Find system_dev
+	parse_mnt("/proc/mounts", [&](mntent *me) -> bool {
+		if (me->mnt_dir == "/system_root"sv) {
+			struct stat st;
+			stat(me->mnt_fsname, &st);
+			system_dev = st.st_rdev;
+			return false;
+		}
+		return true;
+	});
+
+	switch_root("/system_root");
+}
+
 #define umount_root(name) \
 if (mnt_##name) \
 	umount("/" #name);

native/jni/init/getinfo.cpp

@@ -112,6 +112,8 @@ void load_kernel_info(cmdline *cmd) {
 			strcpy(cmd->slot + 1, value);
 		} else if (key == "skip_initramfs") {
 			cmd->system_as_root = true;
+		} else if (key == "androidboot.force_normal_boot") {
+			cmd->force_normal_boot = value[0] == '1';
 		} else if (key == "androidboot.android_dt_dir") {
 			strcpy(cmd->dt_dir, value);
 		} else if (key == "enter_recovery") {
@@ -147,6 +149,7 @@ void load_kernel_info(cmdline *cmd) {
 		strcpy(cmd->dt_dir, DEFAULT_DT_DIR);
 
 	LOGD("system_as_root=[%d]\n", cmd->system_as_root);
+	LOGD("force_normal_boot=[%d]\n", cmd->force_normal_boot);
 	LOGD("slot=[%s]\n", cmd->slot);
 	LOGD("dt_dir=[%s]\n", cmd->dt_dir);
 }

native/jni/init/init.cpp

@@ -141,12 +141,19 @@ void RootFSInit::start() {
 	re_exec_init();
 }
 
-void SARInit::start() {
+void SARCommon::start() {
 	early_mount();
 	patch_rootdir();
 	re_exec_init();
 }
 
+void FirstStageInit::start() {
+	patch_fstab();
+	cleanup();
+	execv("/system/bin/init", argv);
+	exit(1);
+}
+
 class RecoveryInit : public BaseInit {
 public:
 	RecoveryInit(char *argv[], cmdline *cmd) : BaseInit(argv, cmd) {};
@@ -208,6 +215,11 @@ int main(int argc, char *argv[]) {
 			return dump_manager(argv[3], 0644);
 	}
 
+	if (argc > 1 && argv[1] == "selinux_setup"sv) {
+		auto init = make_unique<SecondStageInit>(argv);
+		init->start();
+	}
+
 #ifdef MAGISK_DEBUG
 	bool run_test = getenv("INIT_TEST") != nullptr;
 #else
@@ -228,6 +240,8 @@ int main(int argc, char *argv[]) {
 	unique_ptr<BaseInit> init;
 	if (run_test) {
 		init = make_unique<TestInit>(argv, &cmd);
+	} else if (cmd.force_normal_boot) {
+		init = make_unique<FirstStageInit>(argv, &cmd);
 	} else if (cmd.system_as_root) {
 		if (access("/overlay", F_OK) == 0)  /* Compatible mode */
 			init = make_unique<SARCompatInit>(argv, &cmd);
@@ -243,4 +257,5 @@ int main(int argc, char *argv[]) {
 
 	// Run the main routine
 	init->start();
+	exit(1);
 }

native/jni/init/init.h

@@ -2,6 +2,7 @@
 
 struct cmdline {
 	bool system_as_root;
+	bool force_normal_boot;
 	char slot[3];
 	char dt_dir[128];
 };
@@ -11,10 +12,15 @@ struct raw_data {
 	size_t sz;
 };
 
+/* *************
+ * Base classes
+ * *************/
+
 class BaseInit {
 protected:
 	cmdline *cmd;
 	char **argv;
+
 	void re_exec_init();
 	virtual void cleanup();
 public:
@@ -39,18 +45,6 @@ public:
 	MagiskInit(char *argv[], cmdline *cmd) : BaseInit(argv, cmd) {};
 };
 
-class SARInit : public MagiskInit {
-protected:
-	raw_data config{};
-	dev_t system_dev;
-
-	void early_mount() override;
-	void patch_rootdir();
-public:
-	SARInit(char *argv[], cmdline *cmd) : MagiskInit(argv, cmd) {};
-	void start() override;
-};
-
 class RootFSInit : public MagiskInit {
 protected:
 	int root = -1;
@@ -61,6 +55,52 @@ public:
 	void start() override;
 };
 
+class SARCommon : public MagiskInit {
+protected:
+	raw_data config{};
+	dev_t system_dev;
+
+	void patch_rootdir();
+public:
+	SARCommon(char *argv[], cmdline *cmd) : MagiskInit(argv, cmd) {};
+	void start() override;
+};
+
+/* *******************
+ * Logical Partitions
+ * *******************/
+
+class FirstStageInit : public BaseInit {
+protected:
+	void patch_fstab();
+public:
+	FirstStageInit(char *argv[], cmdline *cmd) : BaseInit(argv, cmd) {};
+	void start() override;
+};
+
+class SecondStageInit : public SARCommon {
+protected:
+	void early_mount() override;
+	void cleanup() override { /* Do not do any cleanup */ }
+public:
+	SecondStageInit(char *argv[]) : SARCommon(argv, nullptr) {};
+};
+
+/* ***********
+ * Normal SAR
+ * ***********/
+
+class SARInit : public SARCommon {
+protected:
+	void early_mount() override;
+public:
+	SARInit(char *argv[], cmdline *cmd) : SARCommon(argv, cmd) {};
+};
+
+/* **********
+ * Initramfs
+ * **********/
+
 class LegacyInit : public RootFSInit {
 protected:
 	void early_mount() override;
@@ -68,6 +108,10 @@ public:
 	LegacyInit(char *argv[], cmdline *cmd) : RootFSInit(argv, cmd) {};
 };
 
+/* ****************
+ * Compat-mode SAR
+ * ****************/
+
 class SARCompatInit : public RootFSInit {
 protected:
 	void early_mount() override;

native/jni/init/rootdir.cpp

@@ -34,7 +34,7 @@ static void patch_socket_name(const char *path) {
 }
 
 static void patch_init_rc(FILE *rc) {
-	file_readline("/init.rc", [&](string_view line) -> bool {
+	file_readline("/init.rc", [=](string_view line) -> bool {
 		// Do not start vaultkeeper
 		if (str_contains(line, "start vaultkeeper")) {
 			LOGD("Remove vaultkeeper\n");
@@ -197,7 +197,7 @@ static void sbin_overlay(const raw_data &self, const raw_data &config) {
 #define PATCHPOLICY "/sbin/.se"
 #define LIBSELINUX  "/system/" LIBNAME "/libselinux.so"
 
-void SARInit::patch_rootdir() {
+void SARCommon::patch_rootdir() {
 	sbin_overlay(self, config);
 
 	// Mount system_root mirror
@@ -300,6 +300,68 @@ void SARInit::patch_rootdir() {
 	xmount(ROOTOVERLAY "/init.rc", "/init.rc", nullptr, MS_BIND, nullptr);
 }
 
+#define FSR "/first_stage_ramdisk"
+
+void FirstStageInit::patch_fstab() {
+	// Find fstab
+	DIR *dir = xopendir(FSR);
+	if (!dir)
+		return;
+	dirent *de;
+	string fstab(FSR "/");
+	while ((de = readdir(dir))) {
+		if (strstr(de->d_name, "fstab")) {
+			fstab += de->d_name;
+			break;
+		}
+	}
+	closedir(dir);
+	if (fstab.length() == sizeof(FSR))
+		return;
+
+	// Patch fstab
+	string patched = fstab + ".p";
+	FILE *fp = xfopen(patched.data(), "we");
+	file_readline(fstab.data(), [=](string_view l) -> bool {
+		if (l[0] == '#' || l.length() == 1)
+			return true;
+		char *line = (char *) l.data();
+		int src0, src1, mnt0, mnt1, type0, type1, opt0, opt1, flag0, flag1;
+		sscanf(line, "%n%*s%n %n%*s%n %n%*s%n %n%*s%n %n%*s%n",
+			   &src0, &src1, &mnt0, &mnt1, &type0, &type1, &opt0, &opt1, &flag0, &flag1);
+		const char *src, *mnt, *type, *opt, *flag;
+		src = &line[src0];
+		line[src1] = '\0';
+		mnt = &line[mnt0];
+		line[mnt1] = '\0';
+		type = &line[type0];
+		line[type1] = '\0';
+		opt = &line[opt0];
+		line[opt1] = '\0';
+		flag = &line[flag0];
+		line[flag1] = '\0';
+
+		// Redirect system to system_root
+		if (mnt == "/system"sv)
+			mnt = "/system_root";
+
+		fprintf(fp, "%s %s %s %s %s\n", src, mnt, type, opt, flag);
+		return true;
+	});
+	fclose(fp);
+
+	// Replace old fstab
+	clone_attr(fstab.data(), patched.data());
+	rename(patched.data(), fstab.data());
+
+	// Move stuffs for next stage
+	xmkdir(FSR "/system", 0755);
+	xmkdir(FSR "/system/bin", 0755);
+	rename("/init", FSR "/system/bin/init");
+	xmkdir(FSR "/.backup", 0);
+	rename("/.backup/.magisk", FSR "/.backup/.magisk");
+}
+
 #ifdef MAGISK_DEBUG
 static FILE *kmsg;
 static int vprintk(const char *fmt, va_list ap) {

scripts/util_functions.sh

@@ -198,7 +198,8 @@ mount_partitions() {
     mount --move /system /system_root
     mount -o bind /system_root/system /system
   else
-    grep -qE '/dev/root|/system_root' /proc/mounts && SYSTEM_ROOT=true || SYSTEM_ROOT=false
+    grep ' / ' /proc/mounts | grep -qv 'rootfs' || grep -q ' /system_root ' /proc/mounts \
+    && SYSTEM_ROOT=true || SYSTEM_ROOT=false
   fi
   [ -L /system/vendor ] && mount_part vendor
   $SYSTEM_ROOT && ui_print "- Device is system-as-root"