Android recovery分析

Recovery的作用

Android利用Recovery模式，进行恢复出厂设置，全量包OTA升级，增量包升级。

升级一般通过运行升级包中的META-INF/com/google/android/update-script脚本来执行自定义升级，脚本中是一组recovery系统能识别的UI控制，文件系统操作命令，例如write_raw_image（写FLASH分区），copy_dir（复制目录）。该包一般被下载至SDCARD和CACHE分区下。

升级中还涉及到包的数字签名，签名方式和普通JAR文件签名差不错。公钥会被硬编译入recovery，编译时生成在：out/target/product/XX/obj/PACKAGING/ota_keys_inc_intermediates/keys.inc

下面先上图，说明recovery的总体流程，流程图绘制了系统从启动加载bootloader后的行为流程。

Bootloader正常启动，有三种方式，按照BCB（Bootloader Control Block）中的command分类：

• command == ‘boot-recovery‘ → 启动recovery.img。recovery模式

• command == ‘update-radio/hboot‘ → 更新firmware（bootloader)

• 其他 → 启动boot.img

Recovery涉及到的其他系统及文件

• CACHE分区文件

Recovery 工具通过NAND cache分区上的三个文件和主系统打交道。主系统（包括恢复出厂设置和OTA升级）可以写入recovery所需的命令，读出recovery过程中的LOG和intent。

• /cache/recovery/command： recovery命令，由主系统写入。所有命令如下：

  • --send_intent=anystring - write the text out to recovery.intent

  • --update_package=root:path - verify install an OTA package file

  • --wipe_data - erase user data (and cache), then reboot

  • --wipe_cache - wipe cache (but not user data), then reboot

• /cache/recovery/log：recovery过程日志，由主系统读出

• /cache/recovery/intent：recovery输出的intent

• MISC分区内容

  Bootloader Control Block (BCB) 存放recovery bootloader message。结构如下：

  struct bootloader_message {

  char command[32];

  char recovery[1024];

  };

  • command可以有以下两个值

    “boot-recovery”：标示recovery正在进行，或指示bootloader应该进入recovery mode

    “update-hboot/radio”：指示bootloader更新firmware

  • recovery内容

    “recovery\n

    <recovery command>\n

    <recovery command>”

    其中recovery command为CACHE:/recovery/command命令

两种Recovery Case

• FACTORY RESET（恢复出厂设置）

1. 用户选择“恢复出厂设置”

2. 设置系统将"--wipe_data"命令写入/cache/recovery/command

3. 系统重启，并进入recover模式（/sbin/recovery）

4. get_args() 将 "boot-recovery"和"--wipe_data"写入BCB

5. erase_root() 格式化（擦除）DATA分区

6. erase_root() 格式化（擦除）CACHE分区

7. finish_recovery() 擦除BCB

8. 重启系统

• OTA INSTALL（OTA升级）

1. 升级系统下载 OTA包到/cache/some-filename.zip

2. 升级系统写入recovery命令"--update_package=CACHE:some-filename.zip"

3. 重启，并进入recovery模式

4. get_args() 将"boot-recovery" 和 "--update_package=..." 写入BCB

5. install_package() 作升级

6. finish_recovery() 擦除 BCB

7. ** 如果安装包失败 ** prompt_and_wait() 等待用户操作，选择ALT+S或ALT+W 升级或恢复出厂设置

8. main() 调用 maybe_install_firmware_update()

   1. 如果包里有hboot/radio的firmware则继续，否则返回

   2. 将 "boot-recovery" 和 "--wipe_cache" 写入BCB

   3. 将 firmware image写入cache分区

   4. 将 "update-radio/hboot" 和 "--wipe_cache" 写入BCB

   5. 重启系统

   6. bootloader自身更新firmware

   7. bootloader 将 "boot-recovery" 写入BCB

   8. erase_root() 擦除CACHE分区

   9. 清除 BCB

9. main() 调用 reboot() 重启系统

Recovery模式流程的英文文档

/init → init.rc → /sbin/recovery →

main():recovery.c

• ui_init():ui.c ［UI initialize］

  • gr_init():minui/graphics.c ［set tty0 to graphic mode, open fb0]

  • ev_init():minui/events.c [open /dev/input/event*]

  • res_create_surface:minui/resource.c [create surfaces for all bitmaps used later, include icons, bmps]

  • create 2 threads: progress/input_thread [create progress show and input event handler thread]

• get_args():recovery.c

  • get_bootloader_message():bootloader.c [read mtdblock0(misc partition) 2nd page for commandline]

  • check if nand misc partition has boot message. If yes, fill argc/argv.

  • If no, get arguments from /cache/recovery/command, and fill argc/argv.

  • set_bootloader_message():bootloader.c [set bootloader message back to mtdblock0]

• Parser argv[] filled above

• register_update_commands():commands.c [ register all commands with name and hook function ]

  • registerCommand():commands.c

    • Register command with name, hook, type, cookie.

    • Commands, e.g: assert, delete, copy_dir, symlink, write_raw_image.

  • registerFunction():commands.c

    • Register function with name, hook, cookie.

    • Function, e.g: get_mark, matches, getprop, file_contains

• install_package():

  • translate_root_path():roots.c [ "SYSTEM:lib" and turns it into a string like "/system/lib", translate the updater.zip path ]

  • mzOpenZipArchive():zip.c [ open updater.zip file (uncompass) ]

  • handle_update_package():install.c

    • verify_jar_signature():verifier.c [ verify signature with keys.inc key; verify manifest and zip package archive ]

      • verifySignature() [ verify the signature file: CERT.sf/rsa. ]

        • digestEntry():verifier.c [ get SHA-1 digest of CERT.sf file ]

        • RSA_verify(public key:keys.inc, signature:CERT.rsa, CERT.sf‘s digest):libc/rsa.c [ Verify a 2048 bit RSA PKCS1.5 signature against an expected SHA-1 hash. Use public key to decrypt the CERT.rsa to get original SHA digest, then compare to digest of CERT.sf ]

      • verifyManifest() [ Get manifest SHA1-Digest from CERT.sf. Then do digest to MANIFEST.MF. Compare them ]

      • verifyArchive() [ verify all the files in update.zip with digest listed in MANIFEST.MF ]

    • find_update_script():install.c [ find META-INF/com/google/android/update-script updater script ]

    • handle_update_script():install.c [ read cmds from script file, and do parser, exec ]

      • parseAmendScript():amend.c [ call yyparse() to parse to command ]

      • exeCommandList():install.c

        • exeCommand():execute.c [ call command hook function ]

• erase DATA/CACHE partition

• prompt_and_wait():recovery.c [ wait for user input: 1) reboot 2) update.zip 3) wipe data ]

  • ui_key_xxx get ALT+x keys

  • 1) do nothing

  • 2) install_package(‘SDCARD:update.zip‘)

  • 3) erase_root() → format_root_device() DATA/CACHE

• may_install_firmware_update():firmware.c [ remember_firmware_update() is called by write_hboot/radio_image command, it stores the bootloader image to CACHE partition, and write update-hboot/radio command to MISC partition for bootloader message to let bootloader update itself after reboot ]

  • set_bootloader_message()

  • write_update_for_bootloader():bootloader.c [ write firmware image into CACHE partition with update_header, busyimage and failimage ]

• finish_recovery():recovery.c [ clear the recovery command and prepare to boot a (hopefully working) system, copy our log file to cache as well (for the system to read), and record any intent we were asked to communicate back to the system. ]

• reboot()