Android Recovery具体工作原理

【什么是Recovery模式？ 什么是ClockworkMod Recovery？请参考：http://blog.csdn.net/wu070815/article/details/8198995】

Android系统利用Recovery模式，进行恢复出厂设置，OTA升级，patch升级及firmware升级，系统备份等。Recovery模式功能的强弱跟Recovery Image软件版本有关系，也就是在制作Recovery Image时，开发者给Recovery添加功能的多少，这是一个完全可以裁减的工作。

一、Recovery升级简介
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

二、Android系统三种启动模式
这里为了了解Recovery机制，我们以G1手机为例，后续的Android版本可能有些细微差别，但是大同小异。
G1手机的Magic Key操作有：
(1) camera +power：bootloader模式，ADP里则可以使用fastboot模式
(2) home + power：recovery模式
G1手机有三种正常启动模式，或者说三种Bootloader正常启动。按照BCB（Bootloader Control Block, 下节介绍）中的command分类： 
(1)  command == 'boot-recovery'   → 启动recovery.img。recovery模式
(2)  command =='update-radio/hboot' → 更新firmware（bootloader)
(3)  其他 → 启动boot.img  

三、Recovery涉及到的其他系统及文件

1、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。
       1）/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
       2）/cache/recovery/log：recovery过程日志，由主系统读出
       3）/cache/recovery/intent：recovery输出的intent

2、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：
        1）boot-recovery：标示recovery正在进行，或指示bootloader应该进入recovery mode
        2）update-hboot/radio：指示bootloader更新firmware
       recovery的值：
        1）<recovery command>
       其中recovery command为CACHE:/recovery/command命令。

四、两种操作分析

    1、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(Bootloader Control Block)
       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

    2、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()
          8.1) 如果包里有hboot/radio的firmware则继续，否则返回
          8.2) 将 "boot-recovery" 和 "--wipe_cache" 写入BCB
          8.3) 将 firmware image写入cache分区
          8.4) 将 "update-radio/hboot" 和 "--wipe_cache" 写入BCB
          8.5) 重启系统
          8.6) bootloader自身更新firmware
          8.7）bootloader 将 "boot-recovery" 写入BCB
          8.8）erase_root() 擦除CACHE分区
          8.9）清除 BCB
       9）main() 调用 reboot() 重启系统

五、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) 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()


六、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。