Android 的recovery模式分析

Recovery Binary：

　　Recovery Binary 是 Android 进入 Recovery 模式所运行的程序，实现了 Recovery 模式下的功能。它由目录 bootable/recovery 下的源代码编译生成。头文件 bootable/recovery/recovery_ui.h 定义了 Recovery UI 的接口，bootable/recovery/default_recovery_ui.c 是其默认实现，每个设备可以有自己不同的实现，然后通过变量 TARGET_RECOVERY_UI_LIB 来指定，否则使用默认实现。

# bootable/recovery/Android.mk

ifeq ($(TARGET_RECOVERY_UI_LIB),)
  LOCAL_SRC_FILES +=default_recovery_ui.c
else
  LOCAL_STATIC_LIBRARIES +=$(TARGET_RECOVERY_UI_LIB)
endif

• Recovery Image：

　　Recovery Image 的生成规则在文件 build/core/Makefile 中定义，具体分析如下：

# build/core/Makefile

# -----------------------------------------------------------------
# Recovery image

# If neither TARGET_NO_KERNEL nor TARGET_NO_RECOVERY are true
ifeq (,$(filter true, $(TARGET_NO_KERNEL) $(TARGET_NO_RECOVERY) $(BUILD_TINY_ANDROID)))

INSTALLED_RECOVERYIMAGE_TARGET:= $(PRODUCT_OUT)/recovery.img

recovery_initrc :=$(call include-path-for, recovery)/etc/init.rc
recovery_kernel :=$(INSTALLED_KERNEL_TARGET)# same as a non-recovery system
recovery_ramdisk :=$(PRODUCT_OUT)/ramdisk-recovery.img
recovery_build_prop :=$(INSTALLED_BUILD_PROP_TARGET)
recovery_binary :=$(call intermediates-dir-for,EXECUTABLES,recovery)/recovery
recovery_resources_common :=$(call include-path-for, recovery)/res
recovery_resources_private :=$(strip$(wildcard$(TARGET_DEVICE_DIR)/recovery/res))
recovery_resource_deps :=$(shell find$(recovery_resources_common)\
  $(recovery_resources_private)-type f)
recovery_fstab :=$(strip$(wildcard$(TARGET_DEVICE_DIR)/recovery.fstab))
recovery_mmc_fstab :=$(strip$(wildcard$(TARGET_DEVICE_DIR)/recovery_mmc.fstab))

ifeq ($(recovery_resources_private),)
  $(info No private recovery resourcesfor TARGET_DEVICE$(TARGET_DEVICE))
endif

ifeq ($(recovery_fstab),)
  $(info No recovery.fstabfor TARGET_DEVICE$(TARGET_DEVICE))
endif

INTERNAL_RECOVERYIMAGE_ARGS :=\
    $(addprefix --second ,$(INSTALLED_2NDBOOTLOADER_TARGET))\
    --kernel $(recovery_kernel)\
    --ramdisk $(recovery_ramdisk)

# Assumes this has already been stripped
ifdef BOARD_KERNEL_CMDLINE
  INTERNAL_RECOVERYIMAGE_ARGS +=--cmdline "$(BOARD_KERNEL_CMDLINE)"
endif
ifdef BOARD_KERNEL_BASE
  INTERNAL_RECOVERYIMAGE_ARGS +=--base $(BOARD_KERNEL_BASE)
endif
BOARD_KERNEL_PAGESIZE :=$(strip$(BOARD_KERNEL_PAGESIZE))
ifdef BOARD_KERNEL_PAGESIZE
  INTERNAL_RECOVERYIMAGE_ARGS +=--pagesize $(BOARD_KERNEL_PAGESIZE)
endif

INSTALLED_BOOTIMAGE_TARGET :=$(PRODUCT_OUT)/boot.img
kernel: $(INSTALLED_BOOTIMAGE_TARGET)
.PHONY: kernel

# Keys authorized to sign OTA packages this build will accept.  The
# build always uses test-keys for this; release packaging tools will
# substitute other keys for this one.
OTA_PUBLIC_KEYS :=$(SRC_TARGET_DIR)/product/security/testkey.x509.pem

# Generate a file containing the keys that will be read by the
# recovery binary.
RECOVERY_INSTALL_OTA_KEYS :=\
    $(call intermediates-dir-for,PACKAGING,ota_keys)/keys
DUMPKEY_JAR :=$(HOST_OUT_JAVA_LIBRARIES)/dumpkey.jar
$(RECOVERY_INSTALL_OTA_KEYS): PRIVATE_OTA_PUBLIC_KEYS :=$(OTA_PUBLIC_KEYS)
$(RECOVERY_INSTALL_OTA_KEYS):$(OTA_PUBLIC_KEYS)$(DUMPKEY_JAR)
    @echo "DumpPublicKey: $@ <= $(PRIVATE_OTA_PUBLIC_KEYS)"
    @rm -rf $@
    @mkdir -p $(dir $@)
    java -jar $(DUMPKEY_JAR)$(PRIVATE_OTA_PUBLIC_KEYS)> $@

$(INSTALLED_RECOVERYIMAGE_TARGET):$(MKBOOTFS)$(MKBOOTIMG)$(MINIGZIP)\
        $(INSTALLED_RAMDISK_TARGET)\
        $(INSTALLED_BOOTIMAGE_TARGET)\
        $(recovery_binary)\
        $(recovery_initrc)$(recovery_kernel)\
        $(INSTALLED_2NDBOOTLOADER_TARGET)\
        $(recovery_build_prop)$(recovery_resource_deps)\
        $(recovery_fstab)\
        $(RECOVERY_INSTALL_OTA_KEYS)

    /* 以正常系统的根文件系统为基础构建 Recovery 的根文件系统 */
    @echo ----- Making recovery image ------
    rm -rf $(TARGET_RECOVERY_OUT)
    mkdir -p $(TARGET_RECOVERY_OUT)
    mkdir -p $(TARGET_RECOVERY_ROOT_OUT)
    mkdir -p $(TARGET_RECOVERY_ROOT_OUT)/etc
    mkdir -p $(TARGET_RECOVERY_ROOT_OUT)/tmp
    echo Copying baseline ramdisk...
    cp -R $(TARGET_ROOT_OUT)$(TARGET_RECOVERY_OUT)

    /* 删除所有的 Init 脚本，使用 Recovery 特定的 Init 脚本 */
    rm $(TARGET_RECOVERY_ROOT_OUT)/init*.rc
    echo Modifying ramdisk contents...
    cp -f $(recovery_initrc)$(TARGET_RECOVERY_ROOT_OUT)/

    /* 添加 Recovery Binary */
    cp -f $(recovery_binary)$(TARGET_RECOVERY_ROOT_OUT)/sbin/

    /* 添加通用的和设备特定的 Recovery 资源 */
    cp -rf $(recovery_resources_common)$(TARGET_RECOVERY_ROOT_OUT)/
    $(foreach item,$(recovery_resources_private),\
      cp -rf $(item) $(TARGET_RECOVERY_ROOT_OUT)/)

    /* 添加设备特定的文件系统表 */
    $(foreach item,$(recovery_fstab),\
      cp -f $(item)$(TARGET_RECOVERY_ROOT_OUT)/etc/recovery.fstab)
    $(foreach item,$(recovery_mmc_fstab),\
      cp -f $(item)$(TARGET_RECOVERY_ROOT_OUT)/etc/recovery_mmc.fstab)

    /* 内嵌验证签名的公钥 */
    cp $(RECOVERY_INSTALL_OTA_KEYS)$(TARGET_RECOVERY_ROOT_OUT)/res/keys

    /* 生成 Recovery 模式的默认属性文件 */
    cat $(INSTALLED_DEFAULT_PROP_TARGET)$(recovery_build_prop)\
            > $(TARGET_RECOVERY_ROOT_OUT)/default.prop

    /* 生成 Recovery 的根文件系统 ramdisk-recovery.img */
    $(MKBOOTFS)$(TARGET_RECOVERY_ROOT_OUT) |$(MINIGZIP)> $(recovery_ramdisk)

    /* 把正常系统的内核跟 ramdisk-recovery.img 打包生成 Recovery Image */
    $(MKBOOTIMG)$(INTERNAL_RECOVERYIMAGE_ARGS)--output $@
    @echo ----- Made recovery image --------$@

    /* 验证生成的 Recovery Image 有没有超出 Recovery 分区的大小 */
    $(hide)$(call assert-max-image-size,$@,$(BOARD_RECOVERYIMAGE_PARTITION_SIZE),raw)

else
INSTALLED_RECOVERYIMAGE_TARGET :=
endif

.PHONY: recoveryimage
recoveryimage: $(INSTALLED_RECOVERYIMAGE_TARGET)

• Recovery Init Script：

　　从上面的分析可以看出 recovery.img 和 boot.img 的区别不大，主要是 init 脚本不一样，recovery 的 init 脚本相对简单，系统起来后只运行 ueventd、recovery、adbd 三个服务。

# bootable/recovery/etc/init.rc

on early-init
    start ueventd

on init
    export PATH /sbin
    export ANDROID_ROOT /system
    export ANDROID_DATA /data
    export EXTERNAL_STORAGE /sdcard

    symlink /system/etc /etc

    mkdir /sdcard
    mkdir /system
    mkdir /data
    mkdir /cache
  
    mount /tmp /tmp tmpfs

on boot
    ifup lo
    hostname localhost
    domainname localdomain

    class_start default

service ueventd /sbin/ueventd
    critical

service recovery /sbin/recovery

service adbd /sbin/adbd recovery
    disabled

on property:persist.service.adb.enable=1
    start adbd

on property:persist.service.adb.enable=0
    stop adbd

• Android <----> Recovery Binary <----> Bootloader：

　　有时候 Android 需要不同的模式互相协助来完成一项任务，这样不同模式之间就要有一种机制来交换信息。Recovery Binary 和 Bootloader 之间是通过 misc 分区来传递信息的，如果是 MTD 设备，则使用 misc 分区的第二个页面，如果是块设备，则使用 misc 分区的第一块，交换的信息通过如下结构体封装。Recovery Binary 和 Android 之间是通过 cache 分区下的如下几个固定文件来传递信息的。

/* Recovery Binary <----> Bootloader */

struct bootloader_message{
    char command[32];
    char status[32];
    char recovery[1024];
};

/* Recovery Binary <----> Android */

/cache/recovery/command
/cache/recovery/intent
/cache/recovery/log
/cache/recovery/last_log

• Updater Binary：

　　Updater Binary 是 OTA package 的安装程序，被打包到 OTA package 中一起发布。Updater Binary 的源代码位于目录 bootable/recovery/updater 中。每个设备都可以为 Updater Binary 添加自己特定的扩展，然后通过变量TARGET_RECOVERY_UPDATER_LIBS 和 TARGET_RECOVERY_UPDATER_EXTRA_LIBS 来指定。

# bootable/recovery/updater/Android.mk

LOCAL_STATIC_LIBRARIES+= $(TARGET_RECOVERY_UPDATER_LIBS) \
                          $(TARGET_RECOVERY_UPDATER_EXTRA_LIBS)
LOCAL_STATIC_LIBRARIES +=libapplypatch libedify libmtdutils libminzip libz
LOCAL_STATIC_LIBRARIES +=libmincrypt libbz
LOCAL_STATIC_LIBRARIES +=libcutils libstdc++ libc
LOCAL_C_INCLUDES +=$(LOCAL_PATH)/..

# Each library in TARGET_RECOVERY_UPDATER_LIBS should have a function
# named "Register_<libname>()".  Here we emit a little C function that
# gets #included by updater.c.  It calls all those registration
# functions.

# Devices can also add libraries to TARGET_RECOVERY_UPDATER_EXTRA_LIBS.
# These libs are also linked in with updater, but we don't try to call
# any sort of registration function for these.  Use this variable for
# any subsidiary static libraries required for your registered
# extension libs.

inc :=$(call intermediates-dir-for,PACKAGING,updater_extensions)/register.inc

# During the first pass of reading the makefiles, we dump the list of
# extension libs to a temp file, then copy that to the ".list" file if
# it is different than the existing .list (if any).  The register.inc
# file then uses the .list as a prerequisite, so it is only rebuilt
# (and updater.o recompiled) when the list of extension libs changes.

junk :=$(shell mkdir -p$(dir$(inc));\
            echo $(TARGET_RECOVERY_UPDATER_LIBS)> $(inc).temp;\
            diff -q $(inc).temp$(inc).list ||cp -f $(inc).temp$(inc).list)

$(inc): libs :=$(TARGET_RECOVERY_UPDATER_LIBS)
$(inc): $(inc).list
    $(hide)mkdir -p $(dir$@)
    $(hide)echo "" > $@
    $(hide)$(foreach lib,$(libs),echo"extern void Register_$(lib)(void);">> $@)
    $(hide)echo "void RegisterDeviceExtensions() {" >> $@
    $(hide)$(foreach lib,$(libs),echo"  Register_$(lib)();">> $@)
    $(hide)echo "}" >> $@

$(call intermediates-dir-for,EXECUTABLES,updater)/updater.o :$(inc)
LOCAL_C_INCLUDES +=$(dir$(inc))