Support A only System-as-root Devices

Most Chinese devices (and supposedly Galaxy S10) running Android Pie
is using system-as-root without A/B partition.

https://source.android.com/devices/bootloader/system-as-root#about-system-as-root

According to the docs above, these devices will have a ramdisk block
with size 0 in their boot images. Since magiskinit can run independently
on system-as-root devices, we simply just create an empty ramdisk with
magiskinit added as init.

Huge thanks to @vvb2060 for the heads up and original PR.
Close #980, close #1102

native/jni/magiskboot/ramdisk.cpp

@@ -22,6 +22,7 @@ static const char *MAGISK_LIST[] =
 
 class magisk_cpio : public cpio_rw {
 public:
+	magisk_cpio() = default;
 	explicit magisk_cpio(const char *filename) : cpio_rw(filename) {}
 	void patch(bool keepverity, bool keepforceencrypt);
 	int test();
@@ -140,6 +141,8 @@ void magisk_cpio::restore() {
 }
 
 void magisk_cpio::backup(const char *orig) {
+	if (access(orig, R_OK))
+		return;
 	entry_map bkup_entries;
 	string remv;
 
@@ -257,7 +260,9 @@ int cpio_commands(int argc, char *argv[]) {
 	++argv;
 	--argc;
 
-	magisk_cpio cpio(incpio);
+	magisk_cpio cpio;
+	if (access(incpio, R_OK) == 0)
+		cpio.load_cpio(incpio);
 
 	int cmdc;
 	char *cmdv[6];
@@ -305,7 +310,7 @@ int cpio_commands(int argc, char *argv[]) {
 				return 0;
 			}
 		} else if (cmdc == 3 && strcmp(cmdv[0], "mkdir") == 0) {
-			cpio.makedir(strtoul(cmdv[1], nullptr, 8), cmdv[2]);
+			cpio.mkdir(strtoul(cmdv[1], nullptr, 8), cmdv[2]);
 		} else if (cmdc == 3 && strcmp(cmdv[0], "ln") == 0) {
 			cpio.ln(cmdv[1], cmdv[2]);
 		} else if (cmdc == 4 && strcmp(cmdv[0], "add") == 0) {

native/jni/utils/cpio.cpp

@@ -149,11 +149,15 @@ void cpio::output(OutStream &out) {
 	out_align();
 }
 
-cpio_rw::cpio_rw(const char *filename) {
+cpio_rw::cpio_rw(const char *file) {
+	load_cpio(file);
+}
+
+void cpio_rw::load_cpio(const char *file) {
 	char *buf;
 	size_t sz;
-	mmap_ro(filename, buf, sz);
-	fprintf(stderr, "Loading cpio: [%s]\n", filename);
+	mmap_ro(file, buf, sz);
+	fprintf(stderr, "Loading cpio: [%s]\n", file);
 	load_cpio(buf, sz);
 	munmap(buf, sz);
 }
@@ -182,7 +186,7 @@ void cpio_rw::add(mode_t mode, const char *name, const char *file) {
 	fprintf(stderr, "Add entry [%s] (%04o)\n", name, mode);
 }
 
-void cpio_rw::makedir(mode_t mode, const char *name) {
+void cpio_rw::mkdir(mode_t mode, const char *name) {
 	insert(new cpio_entry(name, S_IFDIR | mode));
 	fprintf(stderr, "Create directory [%s] (%04o)\n", name, mode);
 }
@@ -241,9 +245,9 @@ void cpio_rw::load_cpio(char *buf, size_t sz) {
 	}
 }
 
-cpio_mmap::cpio_mmap(const char *filename) {
-	mmap_ro(filename, buf, sz);
-	fprintf(stderr, "Loading cpio: [%s]\n", filename);
+cpio_mmap::cpio_mmap(const char *file) {
+	mmap_ro(file, buf, sz);
+	fprintf(stderr, "Loading cpio: [%s]\n", file);
 	size_t pos = 0;
 	cpio_newc_header *header;
 	unique_ptr<cpio_entry_base> entry;

native/jni/utils/include/cpio.h

@@ -53,21 +53,23 @@ protected:
 
 class cpio_rw : public cpio {
 public:
-	explicit cpio_rw(const char *filename);
-	void insert(cpio_entry *e);
+	cpio_rw() = default;
+	explicit cpio_rw(const char *file);
+	void load_cpio(const char *file);
 	void add(mode_t mode, const char *name, const char *file);
-	void makedir(mode_t mode, const char *name);
+	void mkdir(mode_t mode, const char *name);
 	void ln(const char *target, const char *name);
 	bool mv(const char *from, const char *to);
 
 protected:
+	void insert(cpio_entry *e);
 	void mv(entry_map::iterator &it, const char *to);
 	void load_cpio(char *buf, size_t sz);
 };
 
 class cpio_mmap : public cpio {
 public:
-	explicit cpio_mmap(const char *filename);
+	explicit cpio_mmap(const char *file);
 	~cpio_mmap();
 private:
 	char *buf;

scripts/boot_patch.sh

@@ -87,10 +87,15 @@ esac
 # Ramdisk restores
 ##########################################################################################
 
-# Test patch status and do restore, after this section, ramdisk.cpio.orig is guaranteed to exist
+# Test patch status and do restore
 ui_print "- Checking ramdisk status"
-./magiskboot --cpio ramdisk.cpio test
-STATUS=$?
+if [ -e ramdisk.cpio ]; then
+  ./magiskboot --cpio ramdisk.cpio test
+  STATUS=$?
+else
+  # Stock A only system-as-root
+  STATUS=0
+fi
 case $((STATUS & 3)) in
   0 )  # Stock boot
     ui_print "- Stock boot image detected"
@@ -98,18 +103,24 @@ case $((STATUS & 3)) in
     SHA1=`./magiskboot --sha1 "$BOOTIMAGE" 2>/dev/null`
     STOCKDUMP=stock_boot_${SHA1}.img.gz
     ./magiskboot --compress "$BOOTIMAGE" $STOCKDUMP
-    cp -af ramdisk.cpio ramdisk.cpio.orig
+    cp -af ramdisk.cpio ramdisk.cpio.orig 2>/dev/null
     ;;
   1 )  # Magisk patched
     ui_print "- Magisk patched boot image detected"
     # Find SHA1 of stock boot image
     [ -z $SHA1 ] && SHA1=`./magiskboot --cpio ramdisk.cpio sha1 2>/dev/null`
     ./magiskboot --cpio ramdisk.cpio restore
+    if ./magiskboot --cpio ramdisk.cpio "exists init.rc"; then
+      # Normal boot image
       cp -af ramdisk.cpio ramdisk.cpio.orig
+    else
+      # A only system-as-root
+      rm -f ramdisk.cpio
+    fi
     ;;
-  2 ) # Other patched
+  2 )  # Unsupported
     ui_print "! Boot image patched by unsupported programs"
-    abort "! Please restore stock boot image"
+    abort "! Please restore back to stock boot image"
     ;;
 esac
 
@@ -127,6 +138,7 @@ echo "KEEPFORCEENCRYPT=$KEEPFORCEENCRYPT" >> config
 "add 750 init magiskinit" \
 "patch $KEEPVERITY $KEEPFORCEENCRYPT" \
 "backup ramdisk.cpio.orig" \
+"mkdir 000 .backup" \
 "add 000 .backup/.magisk config"
 
 if [ $((STATUS & 4)) -ne 0 ]; then