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()


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