Support GKIs without ramdisk

Fix #5819

native/jni/boot/bootimg.cpp

@@ -398,6 +398,13 @@ void boot_img::parse_image(uint8_t *addr, format_t type) {
         }
         fprintf(stderr, "%-*s [%s]\n", PADDING, "RAMDISK_FMT", fmt2name[r_fmt]);
     }
+    if (!hdr->is_vendor && hdr->ramdisk_size() == 0 && hdr->header_version() == 4) {
+        // When a v4 boot image does not contain ramdisk, we will have to create
+        // the CPIO from scratch. However, since this ramdisk will have to be merged
+        // with other vendor ramdisks, it has to use the exact same compression method.
+        // v4 GKIs are required to use lz4 (legacy), so hardcode it here.
+        r_fmt = LZ4_LEGACY;
+    }
     if (auto size = hdr->extra_size()) {
         e_fmt = check_fmt_lg(extra, size);
         fprintf(stderr, "%-*s [%s]\n", PADDING, "EXTRA_FMT", fmt2name[e_fmt]);

native/jni/init/twostage.cpp

@@ -17,7 +17,15 @@ void FirstStageInit::prepare() {
     cp_afc("/init" /* magiskinit */, REDIR_PATH);
 
     unlink("/init");
-    xrename(backup_init(), "/init");
+    const char *orig_init = backup_init();
+    if (access(orig_init, F_OK) == 0) {
+        xrename(orig_init, "/init");
+    } else {
+        // If the backup init is missing, this means that the boot ramdisk
+        // was created from scratch, and the real init is in a separate CPIO,
+        // which is guaranteed to be placed at /system/bin/init.
+        xsymlink(INIT_PATH, "/init");
+    }
 
     {
         auto init = mmap_data("/init", true);

scripts/boot_patch.sh

@@ -111,7 +111,7 @@ if [ -e ramdisk.cpio ]; then
   ./magiskboot cpio ramdisk.cpio test
   STATUS=$?
 else
-  # Stock A only system-as-root
+  # Stock A only legacy SAR, or some Android 13 GKIs
   STATUS=0
 fi
 case $((STATUS & 3)) in