Detect 2SI after system_root mount on legacy SAR

native/jni/init/init.cpp

@@ -166,18 +166,14 @@ int main(int argc, char *argv[]) {
 		// This will also mount /sys and /proc
 		load_kernel_info(&cmd);
 
-		bool two_stage = check_two_stage();
 		if (cmd.skip_initramfs) {
-			if (two_stage)
+			init = new SARInit(argv, &cmd);
-				init = new SARFirstStageInit(argv, &cmd);
-			else
-				init = new SARInit(argv, &cmd);
 		} else {
 			if (cmd.force_normal_boot)
 				init = new FirstStageInit(argv, &cmd);
 			else if (access("/sbin/recovery", F_OK) == 0 || access("/system/bin/recovery", F_OK) == 0)
 				init = new RecoveryInit(argv, &cmd);
-			else if (two_stage)
+			else if (check_two_stage())
 				init = new FirstStageInit(argv, &cmd);
 			else
 				init = new RootFSInit(argv, &cmd);

native/jni/init/init.hpp

@@ -128,28 +128,21 @@ public:
  *************/
 
 class SARInit : public SARBase {
-protected:
-	void early_mount() override;
-public:
-	SARInit(char *argv[], cmdline *cmd) : SARBase(argv, cmd) {
-		LOGD("%s\n", __FUNCTION__);
-	};
-};
-
-// Special case for legacy SAR on Android 10+
-// Should be followed by normal 2SI SecondStageInit
-class SARFirstStageInit : public SARBase {
 private:
-	void prepare();
+	bool is_two_stage;
+	void first_stage_prep();
 protected:
 	void early_mount() override;
 public:
-	SARFirstStageInit(char *argv[], cmdline *cmd) : SARBase(argv, cmd) {
+	SARInit(char *argv[], cmdline *cmd) : SARBase(argv, cmd), is_two_stage(false) {
 		LOGD("%s\n", __FUNCTION__);
 	};
 	void start() override {
 		early_mount();
-		prepare();
+		if (is_two_stage)
+			first_stage_prep();
+		else
+			patch_rootdir();
 		exec_init();
 	}
 };

native/jni/init/mount.cpp

@@ -338,23 +338,22 @@ mount_root:
 }
 
 void SARInit::early_mount() {
-	// Make dev writable
-	xmkdir("/dev", 0755);
-	xmount("tmpfs", "/dev", "tmpfs", 0, "mode=755");
-	mount_list.emplace_back("/dev");
-
-	backup_files();
-
-	mount_system_root();
-	switch_root("/system_root");
-
-	mount_with_dt();
-}
-
-void SARFirstStageInit::early_mount() {
 	backup_files();
 	mount_system_root();
 	switch_root("/system_root");
+
+	{
+		auto init = raw_data::mmap_ro("/init");
+		is_two_stage = init.contains("selinux_setup");
+	}
+
+	if (!is_two_stage) {
+		// Make dev writable
+		xmkdir("/dev", 0755);
+		xmount("tmpfs", "/dev", "tmpfs", 0, "mode=755");
+		mount_list.emplace_back("/dev");
+		mount_with_dt();
+	}
 }
 
 void SecondStageInit::early_mount() {

native/jni/init/twostage.cpp

@@ -139,7 +139,7 @@ void FirstStageInit::prepare() {
 #define INIT_PATH  "/system/bin/init"
 #define REDIR_PATH "/system/bin/am"
 
-void SARFirstStageInit::prepare() {
+void SARInit::first_stage_prep() {
 	int pid = getpid();
 
 	xmount("tmpfs", "/dev", "tmpfs", 0, "mode=755");