Android recovery分析

Recovery的作用

Android利用Recovery模式，進行恢復出廠設置，全量包OTA升級，增量包升級。

升級一般通過運行升級包中的META-INF/com/google/android/update-script腳本來執行自定義升級，腳本中是一組recovery系統能識別的UI控制，文件系統操作命令，例如write_raw_image（寫FLASH分區），copy_dir（復制目錄）。該包一般被下載至SDCARD和CACHE分區下。

升級中還涉及到包的數字簽名，簽名方式和普通JAR文件簽名差不錯。公鑰會被硬編譯入recovery，編譯時生成在：out/target/product/XX/obj/PACKAGING/ota_keys_inc_intermediates/keys.inc

下面先上圖，說明recovery的總體流程，流程圖繪制了系統從啟動加載bootloader後的行為流程。

Bootloader正常啟動，有三種方式，按照BCB（Bootloader Control Block）中的command分類：

command == "boot-recovery' → 啟動recovery.img。recovery模式

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

其他 → 啟動boot.img

Recovery涉及到的其他系統及文件

CACHE分區文件

Recovery 工具通過NAND cache分區上的三個文件和主系統打交道。主系統（包括恢復出廠設置和OTA升級）可以寫入recovery所需的命令，讀出recovery過程中的LOG和intent。

/cache/recovery/command： recovery命令，由主系統寫入。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過程日志，由主系統讀出

/cache/recovery/intent：recovery輸出的intent

MISC分區內容

Bootloader Control Block (BCB) 存放recovery bootloader message。結構如下：

struct bootloader_message {

char command[32];

char recovery[1024];

};

command可以有以下兩個值

“boot-recovery”：標示recovery正在進行，或指示bootloader應該進入recovery mode

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

recovery內容

“recovery\n

\n

”

其中recovery command為CACHE:/recovery/command命令

兩種Recovery Case

FACTORY RESET（恢復出廠設置）

用戶選擇“恢復出廠設置”

設置系統將"--wipe_data"命令寫入/cache/recovery/command

系統重啟，並進入recover模式（/sbin/recovery）

get_args() 將 "boot-recovery"和"--wipe_data"寫入BCB

erase_root() 格式化（擦除）DATA分區

erase_root() 格式化（擦除）CACHE分區

finish_recovery() 擦除BCB

重啟系統

OTA INSTALL（OTA升級）

升級系統下載 OTA包到/cache/some-filename.zip

升級系統寫入recovery命令"--update_package=CACHE:some-filename.zip"

重啟，並進入recovery模式

get_args() 將"boot-recovery" 和 "--update_package=..." 寫入BCB

install_package() 作升級

finish_recovery() 擦除 BCB

** 如果安裝包失敗 ** prompt_and_wait() 等待用戶操作，選擇ALT+S或ALT+W 升級或恢復出廠設置

main() 調用 maybe_install_firmware_update()

如果包裡有hboot/radio的firmware則繼續，否則返回

將 "boot-recovery" 和 "--wipe_cache" 寫入BCB

將 firmware image寫入cache分區

將 "update-radio/hboot" 和 "--wipe_cache" 寫入BCB

重啟系統

bootloader自身更新firmware

bootloader 將 "boot-recovery" 寫入BCB

erase_root() 擦除CACHE分區

清除 BCB

main() 調用 reboot() 重啟系統

Recovery模式流程的英文文檔

/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()