Android Recovery 模式

Recovery简介

Android利用Recovery模式，进行恢复出厂设置，OTA升级，patch升级及firmware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。如果对该包内容感兴趣，可以从http://forum.xda-developers.com/showthread.php?t=442480下载JF升级包来看看。

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

G1中的三种启动模式

MAGIC KEY:

camera + power：bootloader模式，ADP里则可以使用fastboot模式
home + power：recovery模式
正常启动

Bootloader正常启动，又有三种方式，按照BCB（Bootloader Control Block, 下节介绍）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 status[32]; // 未知用途
  
  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）

用户选择“恢复出厂设置”
设置系统将"--wipe_data"命令写入/cache/recovery/command
系统重启，并进入recover模式（/sbin/recovery）
get_args() 将 "boot-recovery"和"--wipe_data"写入BCB
erase_root() 格式化（擦除）DATA分区
erase_root() 格式化（擦除）CACHE分区
finish_recovery() 擦除BCB
重启系统

OTA INSTALL（OTA升级）

升级系统下载 OTA包到/cache/some-filename.zip
升级系统写入recovery命令"--update_package=CACHE:some-filename.zip"
重启，并进入recovery模式
get_args() 将"boot-recovery" 和 "--update_package=..." 写入BCB
install_package() 作升级
finish_recovery() 擦除 BCB
** 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
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
main() 调用 reboot() 重启系统

Recovery模式流程

/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) 2^nd 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()

Recovery模式流程图

以下流程图绘制了系统从启动加载bootloader后的行为流程。